Amino acids - nomenclature, preparation, chemical properties. Squirrels. Cheat sheet: Amino acids Chemistry amino acids tools php

Amino acids, proteins and peptides are examples of the compounds described below. Many biologically active molecules include several chemically different functional groups, which can interact with each other and with each other's functional groups.

Amino acids.

Amino acids- organic bifunctional compounds, which include a carboxyl group - UNS, and the amino group is N.H. 2 .

Separate α And β - amino acids:

Mostly found in nature α -acids. Proteins contain 19 amino acids and one imino acid ( C 5 H 9NO 2 ):

The simplest amino acid- glycine. The remaining amino acids can be divided into the following main groups:

1) homologues of glycine - alanine, valine, leucine, isoleucine.

Obtaining amino acids.

Chemical properties of amino acids.

Amino acids- these are amphoteric compounds, because contain 2 opposite functional groups - an amino group and a hydroxyl group. Therefore, they react with both acids and alkalis:

Acid-base transformation can be represented as:

Nomenclature of amino acids

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.

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 with a letter Greek alphabet. Example:

For a-amino acids that play exclusively important role in the life processes of animals and plants, trivial names are used.

If an amino acid molecule contains two amino groups, then the prefix is ​​used in its name diamino, three NH2 groups – triamino etc.

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

Trivial names:

1. Optical isomerism

All a-amino acids, except glycine H 2 N-CH 2 -COOH, contain an asymmetric carbon atom (a-atom) and can exist in the form of mirror antipodes.

Optical isomerism of natural a-amino acids plays an important role in the processes of protein biosynthesis.

Properties of amino acids

Physical properties. Amino acids - solid crystalline substances with high melting point, they decompose when melted. Highly soluble in water, aqueous solutions electrically conductive. These properties are explained by the fact that amino acid molecules exist in the form of internal salts, which are formed due to the transfer of a proton from the carboxyl to the amino group

Chemical properties

Amino acids exhibit the properties of bases due to the amino group and the properties of acids due to the carboxyl group, i.e. they are amphoteric compounds. Like amines, they react with acids to form ammonium salts:

H2 N–CH2 –COOH + HCl= Cl — +

As carboxylic acids they form functional derivatives:

H2 N–CH2 –COOH + NaOH= H2 N–CH2 –COO — Na+ +H2 O

b) esters

H2 N–CH2 –COOH + C2 H5 OH= H2 N–CH2 –COOC2 H5 +H2 O

H2 N–R–COOH + NH3 = H2 N–R–CONH2 +H2 O

In addition, the interaction of amino and carboxyl groups is possible both within one molecule (intramolecular reaction for g-, d-e-, etc. amino acids) and belonging to different molecules (intermolecular reaction).

Amino acids

Any compound that contains both a carboxyl and an amino group is amino acid . However, more often this term is used to refer to carboxylic acids whose amino group is in the -position to the carboxyl group.

Amino acids, as a rule, are part of polymers - proteins. Over 70 amino acids occur in nature, but only 20 play an important role in living organisms. Indispensable are called amino acids that cannot be synthesized by the body from substances supplied with food in quantities sufficient to satisfy the physiological needs of the body. Essential amino acids are given in table. 1. For patients with phenylketonuria, an essential amino acid is also tyrosine (see Table 1).

Table 1

Essential amino acidsR-CHNH2 COOH

Amino acids are usually named as substitutes for the corresponding carboxylic acids, denoting the position of the amino group with the letters of the Greek alphabet. For the simplest amino acids, trivial names are usually used (glycine, alanine, isoleucine, etc.). Amino acid isomerism is associated with the arrangement of functional groups and the structure of the hydrocarbon skeleton. An amino acid molecule may contain one or more carboxyl groups and, accordingly, amino acids vary in basicity. Also, an amino acid molecule may contain different quantities amino group

METHODS OF OBTAINING AMINO ACIDS

1. About 25 amino acids can be obtained by hydrolysis of proteins, but the resulting mixture is difficult to separate. Usually one or two acids are obtained in much larger quantities than the others, and these acids can be isolated quite easily - using ion exchange resins.

2. From halogenated acids. One of the most common synthesis methods -amino acids involve ammonolysis -halogenated acid, which is usually obtained by the Gel-Volhard-Zelinsky reaction:

This method can be modified to produce the β-bromo acid via malonic ester:

An amino group can be introduced into the ester of a -halogenated acid using potassium phthalimide ( Gabriel synthesis):

3. From carbonyl compounds ( Strecker synthesis). The Strecker synthesis of α-amino acids consists of the reaction of a carbonyl compound with a mixture of ammonium chloride and sodium cyanide (this improvement of the method was proposed by N.D. Zelinsky and G.L. Stadnikov).

Addition-elimination reactions involving ammonia and a carbonyl compound produce an imine, which reacts with hydrogen cyanide to form -aminonitrile. As a result of its hydrolysis, an -amino acid is formed.

Chemical properties of amino acids

All α-amino acids, except glycine, contain a chiral α-carbon atom and can occur as enantiomers:

It has been shown that almost all natural -amino acids have the same relative configuration at the -carbon atom. -The carbon atom of (-)-serine was conventionally assigned L-configuration, and -carbon atom of (+)-serine - D-configuration. Moreover, if the Fischer projection of an amino acid is written so that the carboxyl group is located at the top and R at the bottom, then L-amino acids, the amino group will be on the left, and D- amino acids - on the right. Fischer's scheme for determining amino acid configuration applies to all α-amino acids that have a chiral α-carbon atom.

From the figure it is clear that L-amino acid can be dextrorotatory (+) or levorotatory (-) depending on the nature of the radical. The vast majority of amino acids found in nature are L-row. Their enantiomorphs, i.e. D-amino acids are synthesized only by microorganisms and are called "unnatural" amino acids.

According to (R,S) nomenclature, most "natural" or L-amino acids have the S configuration.

L-Isoleucine and L-threonine, each containing two chiral centers per molecule, can be any member of a pair of diastereomers depending on the configuration at the -carbon atom. The correct absolute configurations of these amino acids are given below.

ACID-BASE PROPERTIES OF AMINO ACIDS

Amino acids are amphoteric substances that can exist in the form of cations or anions. This property is explained by the presence of both acidic ( -COUN), and main ( - N.H.2 ) groups in the same molecule. In very acidic solutions N.H.2 The acid group is protonated and the acid becomes a cation. In strongly alkaline solutions, the carboxyl group of the amino acid is deprotonated and the acid is converted into an anion.

In the solid state, amino acids exist in the form zwitterions (bipolar ions, internal salts). In zwitterions, a proton is transferred from the carboxyl group to the amino group:

If you place an amino acid in a conductive medium and lower a pair of electrodes there, then in acidic solutions the amino acid will migrate to the cathode, and in alkaline solutions - to the anode. At a certain pH value characteristic of a given amino acid, it will not move either to the anode or to the cathode, since each molecule is in the form of a zwitterion (carries both a positive and negative charge). This pH value is called isoelectric point(pI) of a given amino acid.

REACTIONS OF AMINO ACIDS

Most of the reactions that amino acids undergo in the laboratory ( in vitro), common to all amines or carboxylic acids.

1. formation of amides at the carboxyl group. When the carbonyl group of an amino acid reacts with the amino group of an amine, a polycondensation reaction of the amino acid occurs in parallel, leading to the formation of amides. To prevent polymerization, the amino group of the acid is blocked so that only the amino group of the amine reacts. For this purpose, carbobenzoxychloride (carbobenzyloxychloride, benzyl chloroformate) is used. rubs-butoxycarboxazid, etc. To react with an amine, the carboxyl group is activated by treating it with ethyl chloroformate. Protecting group then removed by catalytic hydrogenolysis or by exposure to a cold solution of hydrogen bromide in acetic acid.

2. formation of amides at the amino group. When the amino group of an amino acid is acylated, an amide is formed.

The reaction proceeds better in the basic medium, since this ensures a high concentration of free amine.

3. education esters. The carboxyl group of an amino acid is easily esterified by conventional methods. For example, methyl esters are prepared by passing dry hydrogen chloride gas through a solution of the amino acid in methanol:

Amino acids are capable of polycondensation, resulting in the formation of polyamide. Polyamides consisting of -amino acids are called peptides or polypeptides . The amide bond in such polymers is called peptide communication. Polypeptides with a molecular weight of at least 5000 are called proteins . Proteins contain about 25 different amino acids. When a given protein is hydrolyzed, all of these amino acids or some of them can be formed in certain proportions characteristic of an individual protein.

The unique sequence of amino acid residues in the chain inherent in a given protein is called primary protein structure . Features of twisting chains of protein molecules ( relative position fragments in space) are called secondary structure of proteins . Polypeptide chains of proteins can be connected to each other to form amide, disulfide, hydrogen and other bonds due to amino acid side chains. As a result, the spiral twists into a ball. This structural feature is called tertiary structure squirrel . To exhibit biological activity, some proteins must first form a macrocomplex ( oligoprotein), consisting of several complete protein subunits. Quaternary structure determines the degree of association of such monomers in biological active material.

Proteins are divided into two large groups - fibrillar (the ratio of the length of the molecule to the width is greater than 10) and globular (ratio less than 10). Fibrillar proteins include collagen , the most abundant protein in vertebrates; it accounts for almost 50% of the dry weight of cartilage and about 30% solid bones. In most regulatory systems of plants and animals, catalysis is carried out by globular proteins, which are called enzymes .

Amino acids

Any compound that contains both a carboxyl and an amino group is an amino acid. However, more often this term is used to refer to carboxylic acids whose amino group is in the a-position to the carboxyl group.

Amino acids, as a rule, are part of polymers - proteins. Over 70 amino acids occur in nature, but only 20 play an important role in living organisms. Essential amino acids are those that cannot be synthesized by the body from substances supplied with food in quantities sufficient to satisfy the physiological needs of the body. Essential amino acids are given in table. 1. For patients with phenylketonuria, tyrosine is also an essential amino acid (see Table 1).

Table 1

Essential amino acids R-CHNH 2 COOH

Amino acids are usually named as substitutes for the corresponding carboxylic acids, denoting the position of the amino group with the letters of the Greek alphabet. For the simplest amino acids, trivial names are usually used (glycine, alanine, isoleucine, etc.). Amino acid isomerism is associated with the arrangement of functional groups and the structure of the hydrocarbon skeleton. An amino acid molecule may contain one or more carboxyl groups and, accordingly, amino acids vary in basicity. Also, an amino acid molecule can contain a different number of amino groups.

METHODS OF OBTAINING AMINO ACIDS

1. About 25 amino acids can be obtained by hydrolysis of proteins, but the resulting mixture is difficult to separate. Usually one or two acids are obtained in much larger quantities than the others, and these acids can be isolated quite easily - using ion exchange resins.

2. From halogenated acids. One of the most common methods for the synthesis of a-amino acids is ammonolysis of an a-halogenated acid, which is usually obtained by the Gehl-Volhard-Zelinsky reaction:

This method can be modified to produce a-bromo acid via malonic ester:

An amino group can be introduced into the ester of an a-halogenated acid using potassium phthalimide (Gabriel synthesis):

3. From carbonyl compounds (Strecker synthesis). The Strecker synthesis of a-amino acids consists of the reaction of a carbonyl compound with a mixture of ammonium chloride and sodium cyanide (this improvement of the method was proposed by N.D. Zelinsky and G.L. Stadnikov).

Addition-elimination reactions involving ammonia and a carbonyl compound produce an imine, which reacts with hydrogen cyanide to form a-aminonitrile. As a result of its hydrolysis, an a-amino acid is formed.

Chemical properties of amino acids

All a-amino acids, except glycine, contain a chiral a-carbon atom and can occur as enantiomers:

It has been proven that almost all natural a-amino acids have the same relative configuration at the a-carbon atom. The a-carbon atom of (-)-serine was conventionally assigned the L-configuration, and the a-carbon atom of (+)-serine was assigned the D-configuration. Moreover, if the Fischer projection of an a-amino acid is written so that the carboxyl group is located at the top and R at the bottom, the L-amino acid will have the amino group on the left, and the D-amino acid will have the amino group on the right. Fischer's scheme for determining amino acid configuration applies to all a-amino acids that have a chiral a-carbon atom.

The figure shows that an L-amino acid can be dextrorotatory (+) or levorotatory (-) depending on the nature of the radical. The vast majority of a-amino acids found in nature belong to the L-series. Their enantiomorphs, i.e. D-amino acids are synthesized only by microorganisms and are called “unnatural” amino acids.

According to (R,S) nomenclature, most "natural" or L-amino acids have the S configuration.

L-Isoleucine and L-threonine, each containing two chiral centers per molecule, can be any member of a pair of diastereomers depending on the configuration at the b-carbon atom. The correct absolute configurations of these amino acids are given below.

ACID-BASE PROPERTIES OF AMINO ACIDS

Amino acids are amphoteric substances that can exist in the form of cations or anions. This property is explained by the presence of both acidic (-COOH) and basic (-NH 2) groups in the same molecule. In very acidic solutions, the NH 2 group of the acid is protonated and the acid becomes a cation. In strongly alkaline solutions, the carboxyl group of the amino acid is deprotonated and the acid is converted into an anion.

In the solid state, amino acids exist in the form of zwitterions (bipolar ions, internal salts). In zwitterions, a proton is transferred from the carboxyl group to the amino group:

If you place an amino acid in a conductive medium and lower a pair of electrodes there, then in acidic solutions the amino acid will migrate to the cathode, and in alkaline solutions - to the anode. At a certain pH value characteristic of a given amino acid, it will not move either to the anode or to the cathode, since each molecule is in the form of a zwitterion (carries both a positive and negative charge). This pH value is called the isoelectric point (pI) of a given amino acid.

REACTIONS OF AMINO ACIDS

Most of the reactions that amino acids undergo in laboratory conditions (in vitro) are characteristic of all amines or carboxylic acids.

1. formation of amides at the carboxyl group. When the carbonyl group of an amino acid reacts with the amino group of an amine, a polycondensation reaction of the amino acid occurs in parallel, leading to the formation of amides. To prevent polymerization, the amino group of the acid is blocked so that only the amino group of the amine reacts. For this purpose, carbobenzoxychloride (carbobenzyloxychloride, benzyl chloroformate), tert-butoxycarboxazid, etc. is used. To react with an amine, the carboxyl group is activated by exposing it to ethyl chloroformate. The protecting group is then removed by catalytic hydrogenolysis or by the action of a cold solution of hydrogen bromide in acetic acid.

2. formation of amides at the amino group. When the amino group of an a-amino acid is acylated, an amide is formed.

The reaction proceeds better in the basic medium, since this ensures a high concentration of free amine.

3. formation of esters. The carboxyl group of an amino acid is easily esterified by conventional methods. For example, methyl esters are prepared by passing dry hydrogen chloride gas through a solution of the amino acid in methanol:

Amino acids are capable of polycondensation, resulting in the formation of polyamide. Polyamides consisting of a-amino acids are called peptides or polypeptides. The amide bond in such polymers is called peptide bond. Polypeptides with a molecular weight of at least 5000 are called proteins. Proteins contain about 25 different amino acids. When a given protein is hydrolyzed, all of these amino acids or some of them can be formed in certain proportions characteristic of an individual protein.

The unique sequence of amino acid residues in the chain inherent in a given protein is called the primary structure of the protein. The peculiarities of twisting the chains of protein molecules (the relative arrangement of fragments in space) are called the secondary structure of proteins. Polypeptide chains of proteins can be connected to each other to form amide, disulfide, hydrogen and other bonds due to amino acid side chains. As a result, the spiral twists into a ball. This structural feature is called the tertiary structure of the protein. To exhibit biological activity, some proteins must first form a macrocomplex (oligoprotein) consisting of several complete protein subunits. The quaternary structure determines the degree of association of such monomers in the biologically active material.

Proteins are divided into two large groups - fibrillar (the ratio of molecular length to width is greater than 10) and globular (the ratio is less than 10). Fibrillar proteins include collagen, the most abundant protein in vertebrates; it accounts for almost 50% of the dry weight of cartilage and about 30% of the solid matter of bone. In most regulatory systems of plants and animals, catalysis is carried out by globular proteins, which are called enzymes.

A persistent substance containing a lot of sulfur. Proteins are used to make plastics and glue. Below we provide a table with some information about amino acids and proteins (on the next page). Aminoacyl transfer RNA tRNA with an aminoacyl group attached to the 2" or 3" hydroxyl group of the terminal adenosine residue. The aminoacyl group migrates quickly between 2-...

They can. Such combined food products, which contain complementary proteins, are part of the traditional cuisine of all peoples of the world. CHAPTER 3. ECOLOGICAL FEATURES OF STUDYING THE TOPIC “AMINO ACIDS” The human body cannot store proteins, therefore a person needs a balanced protein diet every day. An adult weighing 82 kg requires 79 g...

Types of animals. Regional differences in methionine concentrations are small. The effect of diet on brain methionine concentrations is also insignificant due to competitive relations with neutral amino acids for transport systems. Methionine in the pool of free amino acids is utilized by 80% for protein synthesis. The metabolism of free methionine to cysteine ​​begins with the formation of S-adenosylmethionine, ...

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 carried out 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 (amino acid esters or hydroxy acid amides can be interclass isomers of amino 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 esters 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. pH value 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.