Chemistry amino acids chemical properties. Chemical properties. Physical properties of amino acids

Lecture No. 1765

AMINO ACIDS. PEPTIDES

Receipt methods.

Chemical properties.

Peptides

Lecture No. 16

AMINO ACIDS. PEPTIDES

1. Receipt methods.
2. Chemical properties.
3. Amino acids that make up proteins.
4. Peptides

Amino acids are heterofunctional compounds containing
carboxyl and amino groups. According to the relative position of functional groups
distinguish a -, b -, g - etc. amino acids.
Amino acids containing an amino group at the end of the chain are called w -amino acids.

1. Methods of obtaining

!) Ammonolysis of halogenated acids.

a -amino acids available a -halogenated acids.

2) Stecker-Zelinsky method

Includes the stages of aminonitrile formation at
interaction of aldehyde with HCN and NH 3 followed by hydrolysis into amino acid. As
the reagent is a mixture of NaCN and NH 4 Cl.

The method is applicable for the synthesis only a-amino acids.

3) Reductive amination
oxoacids

4) Addition of ammonia to a ,b -unsaturated carboxylic acids.

The method is applicable for synthesis b-amino acids.

5) From oximes of cyclic ketones
Beckmann's rearrangement.

Method used for synthesis w-amino acids.

2. Chemical
properties

Amino acids give reactions characteristic of carboxyl
and amino groups, and, in addition, exhibit specific properties that
determined by the presence of two functional groups and their mutual
location.

2.1. Acid-base
properties

Amino acids contain acidic and basic
centers are amphoteric compounds. In a crystalline state they
exist in the form of internal salts (bipolar ions), which are formed in
as a result of intramolecular proton transfer from a weaker base
center (SOO — ) to stronger
main center (NH 2).

The ionic structure of amino acids is confirmed by their
physical properties. Amino acids are non-volatile crystalline substances with
high melting temperatures. They are insoluble in non-polar organic
solvents and soluble in water. Their molecules have large dipoles
moments.

Form of existence of amino acids in water
solutions depends on pH. In acidic solutions, amino acids add a proton and
exist predominantly in the form of cations. In an alkaline environment, a bipolar ion
donates a proton and becomes an anion.

At a certain pH value, strictly defined
for each amino acid, it exists predominantly as a bipolar ion.
This pH value is called isoelectric point (pI). IN
at the isoelectric point, an amino acid has no charge and has the smallest
solubility in water. The cationic form of an amino acid contains two acidic
center (COOH and NH 3 + ) And
characterized by two dissociation constants pK a1 and pK a2.
The pI value is determined by the equation:

2.2. Reactions by
amino group

Deamination

Amino acids contain a primary amino group and, like primary amines,
react with nitrous acid to release nitrogen. This happens
replacement of an amino group with a hydroxyl group.

RCH(NH 2)COOH + HNO 2 ® RCH(OH)COOH + N 2 + H 2 O

The reaction is used for quantitative
determination of amino acids by the volume of released nitrogen (Van-Slyke method).

Alkylation and
arylation

When amino acids interact with excess
alkyl halide, exhaustive alkylation of the amino group occurs and the formation
internal salts.

Amino acids are arylated with 2,4-dinitrofluorobenzene
(DNFB) in an alkaline environment. The reaction proceeds as a nucleophilic substitution in
activated aromatic ring.

The reaction is used to establish
amino acid sequence in peptides.

Acylation

Amino acids react with anhydrides and
acid chlorides to form N-acyl derivatives.

The reaction is used to protect the amino group in
peptide synthesis. Such protection should be easily removed, and amides, as is known,
hydrolyze under harsh conditions. When developing methods for the synthesis of peptides there were
Protecting groups have been found that are easily removed by hydrolysis or
hydrogenolysis.

Carbobenzoxy protection:

tert-Butoxycarbonyl protection
(BOK protection).

Ease of removal of protection due to stability
benzyl- and rubs-butyl cations, which are formed as
intermediates.

2.3. Reactions at carboxyl
group

Decarboxylation

During dry distillation in the presence of hydroxide
Barium amino acids are decarboxylated to form amines.

Esterification

Amino acids react with alcohols in the presence of gaseous HCl as
catalyst to form esters.

Unlike amino acids themselves, their esters
– highly volatile compounds and can be separated by distillation or
gas-liquid chromatography, which is used for the analysis and separation of mixtures
amino acids obtained from the hydrolysis of proteins.

Preparation of acid halides and
anhydrides

When acting on those protected by the amino group
Amino acids of phosphorus or sulfur halides form acid chlorides.

The reaction is used to activate the carboxyl
groups upon nucleophilic substitution. More often for this purpose mixed
anhydrides, which are more selective acylating reagents.

The reaction is used to activate the amino group in
peptide synthesis.

2.4. Specific reactions
amino acids

Reactions with the simultaneous participation of carboxyl and
amino groups usually lead to the formation of products containing
thermodynamically stable 5- and 6-membered heterocycles.

Complexation

a -Amino acids
form strong chelate complexes with transition metal ions (Cu, Ni, Co, Cr
etc.).

Ratio of amino acids to
heating

Transformations of amino acids when heated depend on the relative position
carboxyl and amino groups and are determined by the possibility of formation
thermodynamically stable 5-6-membered rings

a -Amino acids
enter into an intermolecular self-acylation reaction. In this case,
cyclic amides – diketopiperazines.

b -Amino acids at
when heated they move a ,b -unsaturated acids.

g - and d -Amino acids undergo
intramolecular acylation with the formation of cyclic amides – lactams .

Ninhydrin reaction

When interacting a -amino acids with triketone – ninhydrin there is a simultaneous oxidative
deamination and decarboxylation to form aldehyde and colored
condensation product.

The reaction is used for quantitative analysis
amino acids by photometry.

1. a -Amino acids,
   constituents of proteins

3.1. Structure and
classification

Natural amino acids correspond to the general formula RCH(NH 2 )COOH and differ in the structure of the radical R. Formulas and
the trivial names of the most important amino acids are given in the table. For
biological functioning of amino acids in proteins is decisive
is the polarity of the radical R. On this basis, amino acids are divided into
the following main groups (see table).

Table. The most important a -amino acids
RCH(NH2)COOH

Formula	Name	Designation	pI
Amino acids containing non-polar radical R
	Glycine	Gly	5,97
	Alanin	Ala	6,0
	Valin	Val	5,96
	Leucine	Leu	5,98
	Isoleucine	Ile	6,02
	Phenylalanine	Phe	5,48
	Tryptophan	Trp	5,89
	Proline	Pro	6,30
	Methionine	Met	5,74
	Cystine	(Cys) 2	5,0
nonionic radical R
	Serin	Ser	5,68
	Threonine		5,60
	Hydroxyproline	Hyp	5,8
	Aspargine	Asn	5,41
	Glutamine	Gln	5,65
Amino acids containing polar positively charged radical R
	Lysine	Lys	9,74
	5-Hydroxylysine		9,15
	Arginine	Arg	10,76
	Histidine	His	7,59
Amino acids containing polar negatively charged radical R
	Aspartic acid	Asp	2,77
	Glutamic acid	Gly	3,22
	Tyrosine	Tyr	5,66
	Cysteine	Cys	5,07

Amino acids containing a non-polar radical
R. Such groupslocated inside
protein molecules and cause hydrophobic interactions.

Amino acids containing polar
nonionic radical R. Amino acids of this type have polar groups in the side radical, not
capable of ionization in an aqueous environment (alcoholic hydroxyl, amide group).
Such groups can be located both inside and on the surface of the molecule.
squirrel. They participate in the formation of hydrogen bonds with other polar
in groups.

Amino acids containing radical R, capable of
to ionization in an aqueous environment with the formation of positive or negative
charged groups. Such amino acids contain a side radical
additional basic or acidic center, which in aqueous solution can
add or donate a proton, respectively.

In proteins, the ionogenic groups of these amino acids
are located, as a rule, on the surface of the molecule and determine
electrostatic interactions.

3.2.
Stereoisomerism.

All natural a -amino acids (except glycine)
are chiral compounds. According to the configuration of the chiral center in position
2 amino acids belong to the D- or L-series.

Natural amino acids are
L-row.

Most amino acids contain one chiral
center and have two stereoisomers. Amino acids isoleucine, threonine,
hydroxyproline, 5-hydroxylysine and cystine contain two chiral centers and have
(except cystine) 4 stereoisomers, of which only one is found in the composition
proteins.

Thus, of the 4 stereoisomers of threonine in
Only (2S,3R)-2-amino-3-hydroxybutanoic acid occurs in nature.

Use for building proteins only
one type of stereoisomer is important for their formation
spatial structure and ensuring biological activity.

a -Amino acids,
obtained synthetically are racemic mixtures, which
needs to be separated. The most preferred is the enzymatic method
separation using acylase enzymes capable of hydrolyzing N-acetyl
derivatives only L- a -amino acids. Enzymatic digestion is carried out using
the following diagram.

First, the racemic amino acid is acylated
acetic anhydride:

Then a racemic mixture of acetyl derivatives
subjected to enzymatic treatment. This hydrolyzes the acetyl
L-amino acid derivative only:

The mixture obtained after the enzymatic process is easily
is separated, since the free L-amino acid dissolves in both acids and
alkalis, and acylated - only in alkalis.

3.3. Acid-base
properties.

According to the acid-base properties of amino acids
divided into three groups.

Neutral amino acids do not contain
radical R additional acidic or basic sites capable of ionization
in an aquatic environment. In an acidic environment they exist as a singly charged cation and
are dibasic acids according to Brønsted. As can be seen with alanine,
the isoelectric point of neutral amino acids is not equal to 7, but lies in the range
5,5 – 6,3.

pI=1/2(2.34+9.69)=6.01

Essential amino acids contain in
radical R has an additional basic center. These include lysine, histidine and
arginine In an acidic environment they exist as a dication and are tribasic
acids. Isoelectric point of basic amino acids, as seen in the example
lysine, lies in the pH region above 7.

pI= 1/2(9.0+10.05)=9.74

Acidic amino acids contain in
the R radical has an additional acid center. These include aspartic acid and
glutamic acid. In an acidic environment they exist as a cation and are
tribasic acids. The isoelectric point of these amino acids lies in the region
pH is much lower than 7.

pI= 1/2(2.09+3.86)=2.77

Tyrosine and cysteine ​​are contained in side radicals
weak acid sites capable of ionization at high pH values.

It is important that when
physiological pH value (~7), not a single amino acid is in
isoelectric point. In the body, all amino acids are ionized, which
provides them with good solubility in water.

Difference in acid-base properties
used for the separation of amino acids by electrophoresis and ion exchange
chromatography. At a given pH value, different amino acids may have different
magnitude and sign of the electric charge. For example, at pH6, lysine has a charge of +1 and
moves towards the cathode, aspartic acid has a charge of –1 and moves towards the anode, and
Alanine is at the isoelectric point and does not move to electric field. Thus, at pH6 they can be
separated by electrophoresis.

For the separation of amino acids by ion exchange
chromatography uses cation exchange resins (sulfonated polystyrene).
The process is carried out in an acidic environment when the amino acids are cationic
form.

The rate of movement of amino acids through
chromatographic column depends on the strength of their electrostatic and hydrophobic
interactions with resin. The main ones bind most firmly to the resin.
amino acids that have the greatest positive charge are the least strongly acidic
amino acids. Amino acids have the greatest hydrophobic binding to the resin
with non-polar side radicals, especially aromatic ones. Thus,
The order of elution of amino acids is as follows. Acidic acids elute more easily than others.
amino acids (Asp and Glu), followed by amino acids containing polar
nonionic groups (Ser, Thr, Asn, Gln), then washed out of the column
amino acids with non-polar side radicals (Phe, Trp, Ile, etc.) and in
Basic amino acids (His, Lys, Arg) are eluted last.

3.4. Amino acid reactions in
vivo

Reductive amination – method
synthesis a-amino acids from a -oxoacids with the participation of the coenzyme NADN as
reducing reagent.

Trasamination – basic
amino acid biosynthesis pathway. During transamination, there is an interchange of two
functional groups - amine and carbonyl between amino acid and keto acid.
In this case, amino acid 1, necessary for the body, is synthesized from amino acid 2,
available in cells in excess. The reaction is carried out with the participation
transaminase enzymes and coenzyme pyridoxal phosphate.

Decarboxylation

Amino acids are decarboxylated by
decarboxylase enzymes with the participation of the coenzyme pyridoxal phosphate. At the same time
biogenic amines are formed.

Biogenic amines have a pronounced
biological activity. The most important of them are colamine (precursor
in the synthesis of choline and the neurotransmitter acetylcholine), histamine (provides
allergic reactions of the body), g -aminobutyric acid (neurotransmitter), adrenaline
(adrenal hormone, neurotransmitter)

Deamination

Nonoxidative deamination occurs by
elimination of ammonia under the action of enzymes to form a ,b -unsaturated acids.

Oxidative deamination is happening
with the participation of oxidase enzymes and coenzyme NAD + , which acts as an oxidizing agent. As a result
ammonia is released and the corresponding keto acid is formed.

With the help of deamination reactions it is reduced
excess amino acids in the body.

4. Peptides

Peptides are polyamides made from a -amino acids. According to the number of amino acid residues in
peptide molecule is distinguished dipeptides, tripeptides, tetrapeptides etc.
Peptides containing up to 10 amino acid residues are called oligopeptides, more than 10 amino acid residues – polypeptides.
Natural polypeptides containing more than 100 amino acid residues are called proteins .

4.1. Structure
peptides

Formally, peptides can be considered as polycondensation products
amino acids.

Amino acid residues in a peptide are linked
amide ( peptide) connections. One end of the chain, which is
an amino acid with a free amino group is called N-end. The other end
on which there is an amino acid with a free carboxyl group is called C-end. Peptides are usually written and named starting with
N-terminus.

The name of the peptide is based on trivial
the names of the amino acid residues included in its composition, which list,
starting from the N-terminus. Moreover, in the names of all amino acids with the exception of
The C-terminal suffix “in” is replaced with the suffix “il”. For shorthand notation
peptides use three-letter designations for their constituents
amino acids.

The peptide is characterized amino acid
composition And amino acid sequence.

The amino acid composition of the peptide can be
established by complete hydrolysis of the peptide (cleavage to amino acids) with
subsequent qualitative and quantitative analysis of the resulting amino acids
by ion exchange chromatography or GLC analysis of amino acid esters.
Complete hydrolysis of peptides is carried out in an acidic environment by boiling them with 6N.
HCl.

Same amino acid composition
several peptides respond. So, from 2 different amino acids it can be built
2 dipeptides, from three different amino acids – 6 tripeptides, from n different amino acids
n! peptides of the same composition. For example, the composition Gly:Ala:Val=1:1:1 corresponds to
the following 6 tripeptides.

Gly-Ala-Val Gly-Val-Ala Val-Gly-Ala Val-Ala-Gly Ala-Gly-Val
Ala-Val-Glu

Thus, to fully characterize the peptide
it is necessary to know its amino acid composition and amino acid
subsequence.

4.2. Determination of amino acid
sequences

To determine amino acid
sequences use a combination of two methods: terminal detection
amino acids and partial hydrolysis.

Determination of N-terminal
amino acids.

Segner method. The peptide is treated with 2,4-dinitrophrobenzene (DNPB), and
then completely hydrolyzed. Isolate and identify from hydrolyzate
DNP is a derivative of the N-terminal amino acid.

Edman method consists of
interaction of the N-terminal amino acid with phenyl isothiocyanate in an alkaline environment.
Upon further treatment with a weak acid without heating, separation occurs from
a chain of “labeled” terminal amino acid in the form of phenylhydantoin (PHG)
derivative.

The advantage of this method is that when
cleavage of the N-terminal amino acid does not destroy the peptide and the operation
cleavage can be repeated. The Edman method is used in an automatic device -
a sequencer, with the help of which 40–50 stages of cleavage can be carried out,
identifying the FTG derivatives obtained at each stage using the gas-liquid method
chromatography.

Partial hydrolysis of polypeptides

During partial hydrolysis, peptides are broken down
the formation of shorter chains. Partial hydrolysis is carried out using
enzymes that hydrolyze peptide bonds selectively, for example, only with
N-terminal ( aminopeptidases) or only from the C-terminus ( carboxypeptidases).
There are enzymes that cleave peptide bonds only between certain
amino acids. By changing the hydrolysis conditions, it is possible to break the peptide into different
fragments that overlap in their constituent amino acid residues.
Analysis of partial hydrolysis products makes it possible to reconstruct the structure of the original
peptide. Let's consider the simplest example of establishing the structure of a tripeptide.
Partial hydrolysis in two different directions of a tripeptide of unknown structure
gives the products shown in the diagram.

The only tripeptide whose structure is not
contradicts the products of partial hydrolysis - Gly-Ala-Phe.

Establishing the amino acid sequence
peptides containing several dozen amino acid residues are more complex
a task that requires a combination of different methods.

4.3. Synthesis
petids

Synthesis of a peptide with a given amino acid
consistency is an extremely difficult task. In the simplest case of synthesis
dipeptide from 2 different amino acids, it is possible to form 4 different
products.

A synthesis strategy has now been developed
peptides based on the use of methods activation And protection functional groups at the appropriate stages of synthesis. Synthesis process
dipeptide includes the following stages:

1. protection of amino group N-terminal
   amino acids;
2. activation of the N-terminal carboxyl group
   amino acids;
3. condensation of modified
   amino acids
4. deprotection

Thus, sequentially connecting
amino acids build up the polypeptide chain step by step. This synthesis is very
long, labor-intensive and gives a low yield of the final product. Major losses
are associated with the need to isolate and purify products at each stage.

The currently used
time solid phase peptide synthesis. At the first stage, protected by
amino group, the C-terminal amino acid is fixed to a solid polymer carrier
(polystyrene modified by introducing –CH groups 2 Cl). After deprotection, acylation is carried out
amino group of an amino acid attached to the carrier by another amino acid that
contains an activated carboxyl and protected amino group. After removal
protection, the next acylation step is carried out. Washing the product from impurities
carried out directly on the carrier and only after completion of synthesis the polypeptide is removed from
carrier by the action of hydrobromic acid. Solid phase synthesis
automated and carried out using devices - automatic
synthesizers.

;

Using the solid-phase synthesis method, a large amount of
number of peptides containing 50 or more amino acid residues, including
insulin (51 amino acid residues) and ribonuclease (124 amino acid residues)
remainder).

The chemical behavior of amino acids is determined by two functional groups -NH 2 and –COOH. Amino acids are characterized by reactions at the amino group, carboxyl group and at the radical part, and depending on the reagent, the interaction of substances can occur through one or more reaction centers.

Amphoteric nature of amino acids. Having both an acidic and a basic group in the molecule, amino acids in aqueous solutions behave like typical amphoteric compounds. In acidic solutions they exhibit basic properties, reacting as bases, in alkaline solutions - as acids, respectively forming two groups of salts:

Due to their amphoteric nature in a living organism, amino acids play the role of buffer substances that maintain a certain concentration of hydrogen ions. Buffer solutions obtained by the interaction of amino acids with strong bases are widely used in bioorganic and chemical practice. Salts of amino acids with mineral acids are more soluble in water than free amino acids. Salts with organic acids are sparingly soluble in water and are used to identify and separate amino acids.

Reactions caused by the amino group. With the participation of the amino group, amino acids form ammonium salts with acids, are acylated, alkylated , react with nitrous acid and aldehydes according to the following scheme:

Alkylation is carried out with the participation of R-Ha1 or Ar-Hal:

In the acylation reaction, acid chlorides or acid anhydrides are used (acetyl chloride, acetic anhydride, benzyloxycarbonyl chloride):

Acylation and alkylation reactions are used to protect the NH 2 group of amino acids during the synthesis of peptides.

Reactions caused by a carboxyl group. With the participation of the carboxyl group, amino acids form salts, esters, amides, and acid chlorides in accordance with the scheme presented below:

If at the -carbon atom in a hydrocarbon radical there is an electron-withdrawing substituent (NO 2, CC1 3, COOH, COR, etc.), polarizing the CCOOH bond, then carboxylic acids easily undergo decarboxylation reactions. Decarboxylation of α-amino acids containing a + NH 3 group as a substituent leads to the formation of biogenic amines. In a living organism, this process occurs under the action of the enzyme decarboxylase and vitamin pyridoxal phosphate.

In laboratory conditions, the reaction is carried out by heating the α-amino acid in the presence of CO 2 absorbers, for example, Ba(OH) 2.

Decarboxylation of -phenyl--alanine, lysine, serine and histidine produces phenamine, 1,5-diaminopentane (cadaverine), 2-aminoethanol-1 (colamine) and tryptamine, respectively.

Reactions of amino acids involving a side group. When the amino acid tyrosine is nitrated with nitric acid, a dinitro derivative compound is formed, colored orange (xanthoprotein test):

Redox transitions take place in the cysteine–cystine system:

2 NS CH 2 CH(NH 2)COOH  HOOCCH(NH 2)CH 2 S–S CH2CH(NH2)COOH

HOOCCH(NH 2)CH 2 S– S CH 2 CH(NH 2)COOH  2 NS CH2CH(NH2)COOH

In some reactions, amino acids react on both functional groups simultaneously.

Formation of complexes with metals. Almost all α-amino acids form complexes with divalent metal ions. The most stable are complex internal copper salts (chelate compounds), formed as a result of interaction with copper (II) hydroxide and colored blue:

Action of nitrous acid to aliphatic amino acids leads to the formation of hydroxy acids, and aromatic - diazo compounds.

Formation of hydroxy acids:

Diazotization reaction:

with the release of molecular nitrogen N 2:

2. without the release of molecular nitrogen N2:

The chromophore group of azobenzene -N=N in azo compounds causes yellow, yellow, orange or other colors of substances when absorbed in the visible region of light (400-800 nm). Auxochrome group

COOH changes and enhances the color due to π, π - conjugation with the π - electronic system of the main group of the chromophore.

Relation of amino acids to heat. When heated, amino acids decompose to form different products depending on their type. When heated  -amino acids as a result of intermolecular dehydration, cyclic amides are formed - diketopiperazines :

valine (Val) diisopropyl derivative

diketopiperazine

When heated  -amino acids Ammonia is split off from them to form α, β-unsaturated acids with a conjugated system of double bonds:

β-aminovaleric acid penten-2-oic acid

(3-aminopentanoic acid)

Heating  - And  -amino acids accompanied by intramolecular dehydration and the formation of internal cyclic amides – lactams:

γ-aminoisovaleric acid γ-aminoisovaleric lactam

(4-amino-3-methylbutanoic acid) acids

>> Chemistry: Amino acids

The general formula of the simplest amino acids can be written as follows:

H2N-CH-COOH
I
R

Because amino acids contain two different functional groups that influence each other, their reactions differ from the characteristic properties of carboxylic acids and amines.

Receipt

Amino acids can be obtained from carboxylic acids by replacing the hydrogen atom in their radical with a halogen, and then with an amino group when reacting with ammonia. A mixture of amino acids is usually obtained by acid hydrolysis of proteins.

Properties

The amino group -NH2 determines the basic properties of amino acids, since it is capable of attaching a hydrogen cation to itself via a donor-acceptor mechanism due to the presence of a free electron pair at the nitrogen atom.

The -COOH group (carboxyl group) determines the acidic properties of these compounds. Therefore, amino acids are amphoteric organic compounds.

They react with alkalis as acids. With strong acids - like amine bases.

In addition, the amino group in the amino acid molecule interacts with the carboxyl group included in its composition, forming an internal salt:

Since amino acids in aqueous solutions behave like typical amphoteric compounds, in living organisms they play the role of buffer substances that maintain a certain concentration of hydrogen ions.

Amino acids are colorless crystalline substances that melt and decompose at temperatures above 200 °C. They are soluble in water and insoluble in ether. Depending on the composition of the R- radical, they can be sweet, bitter or tasteless.

Amino acids are optically active because they contain carbon atoms (asymmetric atoms) linked to four different substituents (the exception is amino-acetic acid - glycine). An asymmetric carbon atom is indicated by an asterisk.

As you already know, optically active substances occur in the form of pairs of antipode-isomers, the physical and chemical properties of which are the same, with the exception of one thing - the ability to rotate the plane of a polarized beam in opposite directions. The direction of rotation of the plane of polarization is indicated by the sign (+) - right rotation, (-) - left rotation.

There are D-amino acids and L-amino acids. The location of the NH2 amino group in the projection formula on the left corresponds to the L-configuration, and on the right - to the D-configuration. The sign of rotation is not related to whether the connection belongs to the L- or D-series. Thus, L-ce-rin has a rotation sign (-), and L-alanine has a rotation sign (+).

Amino acids are divided into natural (found in living organisms) and synthetic. Among natural amino acids (about 150), proteinogenic amino acids (about 20) are distinguished, which are part of proteins. They are L-shapes. About half of these amino acids are considered essential, as they are not synthesized in the human body. Essential amino acids are valine, leucine, isoleucine, phenylalaline, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food (Table 7). If their quantity in food is insufficient, the normal development and functioning of the human body is disrupted. In certain diseases, the body is unable to synthesize some other amino acids. Thus, in phenylketonuria, tyrosine is not synthesized.

The most important property of amino acids is the ability to enter into molecular condensation with the release of water and the formation of an amide group -NH-CO-, for example:

H2N-(CH2)5-COOH + H-NH-(CH2)5-COOH ->
aminocaproic acid

H2N-(CH2)5-CO-NH-(CH2)5-COOH + H20

The high-molecular compounds obtained as a result of this reaction contain a large number of amide fragments and are therefore called polyamides.

These, in addition to the synthetic fiber nylon mentioned above, include, for example, enant, formed during the polycondensation of aminoenanthic acid. Amino acids with amino and carboxyl groups at the ends of the molecules are suitable for producing synthetic fibers (think about why).

Table 7. Daily requirement of the human body for amino acids

Polyamides of a-amino acids are called peptides. Depending on the number of amino acid residues, dipeptides, tripeptides, and polypeptides are distinguished. In such compounds, the -NP-CO- groups are called peptide groups.

Isomerism and nomenclature

Amino acid isomerism is determined by the different structure of the carbon chain and the position of the amino group. The names of amino acids in which the positions of the amino group are designated by letters of the Greek alphabet are also widespread. Thus, 2-aminobutanoic acid can also be called a-aminobutyric acid:

20 amino acids are involved in protein biosynthesis in living organisms, for which historical names are often used. These names and the Russian and Latin letter designations adopted for them are given in Table 8.

1. Write down the equations for the reactions of aminopropionic acid; you with sulfuric acid and sodium hydroxide, as well as methyl alcohol. Give all substances names according to the international nomenclature.

2. Why are amino acids heterofunctional compounds?

3. What structural features should the amino acids used for the synthesis of fibers and the amino acids involved in the biosynthesis of proteins in the cells of living organisms have?

4. How do polycondensation reactions differ from polymerization reactions? What are their similarities?

5. How are amino acids obtained? Write down the reaction equations for producing aminopropionic acid from propane.

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Among nitrogen-containing organic substances there are compounds with dual functions. Particularly important of them are amino acids.

About 300 different amino acids are found in the cells and tissues of living organisms, but only 20 ( α-amino acids ) of them serve as units (monomers) from which peptides and proteins of all organisms are built (therefore they are called protein amino acids). The sequence of location of these amino acids in proteins is encoded in the nucleotide sequence of the corresponding genes. The remaining amino acids are found both in the form of free molecules and in bound form. Many of the amino acids are found only in certain organisms, and there are others that are found only in one of the great variety of described organisms. Most microorganisms and plants synthesize the amino acids they need; Animals and humans are not capable of producing the so-called essential amino acids obtained from food. Amino acids are involved in the metabolism of proteins and carbohydrates, in the formation of compounds important for organisms (for example, purine and pyrimidine bases, which are an integral part of nucleic acids), they are part of hormones, vitamins, alkaloids, pigments, toxins, antibiotics, etc.; Some amino acids serve as intermediaries in the transmission of nerve impulses.

Amino acids- organic amphoteric compounds, which include carboxyl groups - COOH and amino groups -NH 2 .

Amino acids can be considered as carboxylic acids, in the molecules of which the hydrogen atom in the radical is replaced by an amino group.

CLASSIFICATION

Amino acids are classified according to their structural characteristics.

1. Depending on the relative position of the amino and carboxyl groups, amino acids are divided into α-, β-, γ-, δ-, ε- etc.

2. Depending on the number of functional groups, acidic, neutral and basic groups are distinguished.

3. Based on the nature of the hydrocarbon radical, they distinguish aliphatic(fat), aromatic, sulfur-containing And heterocyclic amino acids. The amino acids listed above belong to the fatty series.

An example of an aromatic amino acid is para-aminobenzoic acid:

An example of a heterocyclic amino acid is tryptophan, an essential α-amino acid.

NOMENCLATURE

According to systematic nomenclature, the names of amino acids are formed from the names of the corresponding acids by adding the prefix amino and indicating the location of the amino group in relation to the carboxyl group. Numbering of the carbon chain from the carbon atom of the carboxyl group.

For example:

Another method of constructing the names of amino acids is also often used, according to which the prefix is ​​added to the trivial name of the carboxylic acid amino indicating the position of the amino group by a letter of the Greek alphabet.

Example:

For α-amino acidsR-CH(NH2)COOH

Which play an extremely important role in the life processes of animals and plants, trivial names are used.

Table.

Amino acid	Abbreviated designation	Structure of the radical (R)
Glycine	Gly	H-
Alanin	Ala (Ala)	CH 3 -
Valin	Val	(CH 3) 2 CH -
Leucine	Leu (Lei)	(CH 3) 2 CH – CH 2 -
Serin	Ser	OH-CH2-
Tyrosine	Tyr (Shooting Range)	HO – C 6 H 4 – CH 2 -
Aspartic acid	Asp (Asp)	HOOC – CH 2 -
Glutamic acid	Glu	HOOC – CH 2 – CH 2 -
Cysteine	Cys (Cis)	HS – CH 2 -
Asparagine	Asn (Asn)	O = C – CH 2 – │ NH 2
Lysine	Lys (Liz)	NH 2 – CH 2 - CH 2 – CH 2 -
Phenylalanine	Phen	C 6 H 5 – CH 2 -

If an amino acid molecule contains two amino groups, then the prefix is ​​used in its namediamino-, three NH 2 groups – triamino- etc.

Example:

The presence of two or three carboxyl groups is reflected in the name by the suffix –diovy or -triic acid:

ISOMERIA

1. Isomerism of the carbon skeleton

2. Isomerism of the position of functional groups

3. Optical isomerism

α-amino acids, except glycine NH 2 -CH 2 -COOH.

PHYSICAL PROPERTIES

Amino acids are crystalline substances with high (above 250°C) melting points, which differ little among individual amino acids and are therefore uncharacteristic. Melting is accompanied by decomposition of the substance. Amino acids are highly soluble in water and insoluble in organic solvents, which makes them similar to inorganic compounds. Many amino acids have a sweet taste.

RECEIVING

3. Microbiological synthesis. Microorganisms are known that during their life processes produce α - amino acids of proteins.

CHEMICAL PROPERTIES

Amino acids are amphoteric organic compounds; they are characterized by acid-base properties.

I . General properties

1. Intramolecular neutralization → a bipolar zwitterion is formed:

Aqueous solutions are electrically conductive. These properties are explained by the fact that amino acid molecules exist in the form of internal salts, which are formed by the transfer of a proton from the carboxyl to the amino group:

zwitterion

Aqueous solutions of amino acids have a neutral, acidic or alkaline environment depending on the number of functional groups.

APPLICATION

1) amino acids are widely distributed in nature;

2) amino acid molecules are the building blocks from which all plant and animal proteins are built; amino acids necessary for building body proteins are obtained by humans and animals as part of food proteins;

3) amino acids are prescribed for severe exhaustion, after severe operations;

4) they are used to feed the sick;

5) amino acids are necessary as a therapeutic agent for certain diseases (for example, glutamic acid is used for nervous diseases, histidine for stomach ulcers);

6) some amino acids are used in agriculture to feed animals, which has a positive effect on their growth;

7) have technical significance: aminocaproic and aminoenanthic acids form synthetic fibers - capron and enanth.

ABOUT THE ROLE OF AMINO ACIDS

Occurrence in nature and biological role of amino acids

Finding in nature and the biological role of amino acids

Amino acids are organic compounds containing functional groups in the molecule: amino and carboxyl.

Nomenclature of amino acids. According to systematic nomenclature, the names of amino acids are formed from the names of the corresponding carboxylic acids and the addition of the word “amino”. The position of the amino group is indicated by numbers. The counting is from the carbon of the carboxyl group.

Isomerism of amino acids. Their structural isomerism is determined by the position of the amino group and the structure of the carbon radical. Depending on the position of the NH 2 group, -, - and -amino acids are distinguished.

Protein molecules are built from α-amino acids.

They are also characterized by isomerism of the functional group (interclass isomers of amino acids can be esters of amino acids or amides of hydroxy acids). For example, for 2-aminopropanoic acid CH 3 – CH(NH) 2 – COOH the following isomers are possible

Physical properties of α-amino acids

Amino acids are colorless crystalline substances, non-volatile (low saturated vapor pressure), melting with decomposition at high temperatures. Most of them are highly soluble in water and poorly soluble in organic solvents.

Aqueous solutions of monobasic amino acids have a neutral reaction. -Amino acids can be considered as internal salts (bipolar ions): + NH 3  CH 2  COO  . In an acidic environment they behave like cations, in an alkaline environment they behave like anions. Amino acids are amphoteric compounds that exhibit both acidic and basic properties.

Methods for obtaining α-amino acids

1. The effect of ammonia on salts of chlorinated acids.

Cl  CH 2  COONH 4 + NH 3
NH 2  CH2COOH

2. The effect of ammonia and hydrocyanic acid on aldehydes.

3. Protein hydrolysis produces 25 different amino acids. Separating them is not a very easy task.

Methods for obtaining -amino acids

1. Addition of ammonia to unsaturated carboxylic acids.

CH 2 = CH  COOH + 2NH 3  NH 2  CH 2  CH 2  COONH 4.

2. Synthesis based on dibasic malonic acid.

Chemical properties of amino acids

1. Reactions on the carboxyl group.

1.1. Formation of ethers by the action of alcohols.

2. Reactions at the amino group.

2.1. Interaction with mineral acids.

NH 2  CH 2 COOH + HCl  H 3 N +  CH 2  COOH + Cl 

2.2. Interaction with nitrous acid.

NH 2  CH 2 COOH + HNO 2  HO  CH 2  COOH + N 2 + H 2 O

3. Conversion of amino acids when heated.

3.1.-amino acids form cyclic amides.

3.2.-amino acids remove the amino group and the hydrogen atom of the y-carbon atom.

Individual representatives

Glycine NH 2 CH 2 COOH (glycocol). One of the most common amino acids found in proteins. Under normal conditions - colorless crystals with Tm = 232236С. Easily soluble in water, insoluble in absolute alcohol and ether. Hydrogen index of aqueous solution6.8; pK a = 1.510  10; рК в = 1.710  12.

α-alanine – aminopropionic acid

Widely distributed in nature. It is found free in blood plasma and in most proteins. T pl = 295296С, highly soluble in water, poorly soluble in ethanol, insoluble in ether. pK a (COOH) = 2.34; pK a (NH ) = 9,69.

-alanine NH 2 CH 2 CH 2 COOH – small crystals with melting temperature = 200°C, highly soluble in water, poorly in ethanol, insoluble in ether and acetone. pK a (COOH) = 3.60; pK a (NH ) = 10.19; absent in proteins.

Complexons. This term is used to name a series of α-amino acids containing two or three carboxyl groups. The simplest:

N The most common complexone is ethylenediaminetetraacetic acid.

Its disodium salt, Trilon B, is extremely widely used in analytical chemistry.

The bonds between α-amino acid residues are called peptide bonds, and the resulting compounds themselves are called peptides.

Two α-amino acid residues form a dipeptide, three - a tripeptide. Many residues form polypeptides. Polypeptides, like amino acids, are amphoteric; each has its own isoelectric point. Proteins are polypeptides.