Amino acids are heterofunctional compounds that necessarily contain two functional groups: an amino group - NH 2 and a carboxyl group - COOH, associated with a hydrocarbon radical. The general formula of the simplest amino acids can be written as follows:

Since amino acids contain two different functional groups that influence each other, the characteristic reactions are different from the characteristic reactions carboxylic acids and amines.

Properties of amino acids

The amino group - NH 2 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 bases - amines:

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

The ionization of amino acid molecules depends on the acidic or alkaline nature of the environment:

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

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

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 irreplaceable, because they are not synthesized in the human body. Essential acids are valine, leucine, isoleucine, phenylalanine, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food. 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 the amide group -NH-CO-, for example:

The high molecular weight compounds obtained as a result of this reaction contain big number amide fragments and therefore received the name polyamides.

These, in addition to the synthetic nylon fiber 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.

Alpha amino acid polyamides are called peptides. Depending on the number of amino acid residues, they are distinguished dipeptides, tripeptides, polypeptides. In such compounds, the -NH-CO- groups are called peptide groups.

Isomerism and nomenclature of amino acids

Amino acid isomerism is determined by the different structure of the carbon chain and the position of the amino group, for example:

The names of amino acids are also widespread, in which the position of the amino group is indicated by letters Greek alphabet: α, β, y, etc. Thus, 2-aminobutanoic acid can also be called an α-amino acid:

Methods for obtaining amino acids

1. Amino acids exhibit amphoteric properties and acids and amines, as well as specific properties due to the joint presence of these groups. In aqueous solutions, AMK exist in the form of internal salts (bipolar ions). Aqueous solutions monoaminomonocarboxylic acids are neutral to litmus, because their molecules contain an equal number of -NH 2 - and -COOH groups. These groups interact with each other to form internal salts:

Such a molecule has opposite charges in two places: positive NH 3 + and negative on the carboxyl –COO -. In this regard, the internal salt of AMK is called a bipolar ion or Zwitter ion (Zwitter - hybrid).

A bipolar ion in an acidic environment behaves like a cation, since the dissociation of the carboxyl group is suppressed; in an alkaline environment - as an anion. There are pH values ​​specific for each amino acid, in which the number of anionic forms in solution is equal to the number of cationic forms. The pH value at which the total charge of the AMK molecule is 0 is called the isoelectric point of AMK (pI AA).

Aqueous solutions of monoaminodicarboxylic acids have an acidic reaction:

HOOC-CH 2 -CH-COOH « - OOC-CH 2 -CH–COO - + H +

The isoelectric point of monoaminodicarboxylic acids is in an acidic environment and such AMAs are called acidic.

Diaminomonocarboxylic acids have basic properties in aqueous solutions (the participation of water in the dissociation process must be shown):

NH 2 -(CH 2) 4 -CH-COOH + H 2 O « NH 3 + -(CH 2) 4 -CH–COO - + OH -

The isoelectric point of diaminomonocarboxylic acids is at pH>7 and such AMAs are called basic.

Being bipolar ions, amino acids exhibit amphoteric properties: they are able to form salts with both acids and bases:

Interaction hydrochloric acid HCl produces a salt:

R-CH-COOH + HCl ® R-CH-COOH

NH 2 NH 3 + Cl -

Interaction with a base leads to the formation of a salt:

R-CH(NH 2)-COOH + NaOH ® R-CH(NH 2)-COONa + H 2 O

2. Formation of complexes with metals– chelate complex. The structure of the copper salt of glycol (glycine) can be represented by the following formula:

Almost all of the copper available in the human body (100 mg) is bound to proteins (amino acids) in the form of these stable claw-shaped compounds.

3. Similar to other acids amino acids form esters, halogen anhydrides, amides.

4. Decarboxylation reactions occur in the body with the participation of special decarboxylase enzymes: the resulting amines (tryptamine, histamine, serotinine) are called biogenic amines and are regulators of a number of physiological functions of the human body.

5. Interaction with formaldehyde(aldehydes)

R-CH-COOH + H 2 C=O ® R-CH-COOH

Formaldehyde binds the NH 2 group, the -COOH group remains free and can be titrated with alkali. Therefore, this reaction is used for the quantitative determination of amino acids (Sørensen method).

6. Interaction nitrous acid leads to the formation of hydroxy acids and the release of nitrogen. Based on the volume of released nitrogen N2, its quantitative content in the object under study is determined. This reaction is used for the quantitative determination of amino acids (Van-Slyke method):

R-CH-COOH + HNO 2 ® R-CH-COOH + N 2 + H 2 O

This is one of the ways to deaminate AMK outside the body

7. Acylation of amino acids. The amino group of AMK can be acylated with acid chlorides and anhydrides already at room temperature.

The product of the recorded reaction is acetyl-α-aminopropionic acid.

Acyl derivatives of AMK are widely used in studying their sequence in proteins and in the synthesis of peptides (protection of the amino group).

8.Specific properties reactions associated with the presence and mutual influence amino and carboxyl groups - formation of peptides. Common property a-AMK is polycondensation process, leading to the formation of peptides. As a result of this reaction, amide bonds are formed at the site of interaction between the carboxyl group of one AMK and the amino group of another AMK. In other words, peptides are amides formed as a result of the interaction of amino groups and carboxyls of amino acids. The amide bond in such compounds is called a peptide bond (explain the structure of the peptide group and peptide bond: three-center p, p-conjugated system)

Depending on the number of amino acid residues in the molecule, di-, tri-, tetrapeptides, etc. are distinguished. up to polypeptides (up to 100 AMK residues). Oligopeptides contain from 2 to 10 AMK residues, proteins contain more than 100 AMK residues.B general view The polypeptide chain can be represented by the diagram:

H 2 N-CH-CO-NH-CH-CO-NH-CH-CO-... -NH-CH-COOH

Where R 1, R 2, ... R n are amino acid radicals.

Concept of proteins.

The most important biopolymers of amino acids are proteins - proteins. There are about 5 million in the human body. various proteins that make up the skin, muscles, blood and other tissues. Proteins (proteins) get their name from Greek word“protos” - first, most important. Proteins perform a number of important functions in the body: 1. Construction function; 2. Transport function; 3. Protective function; 4. Catalytic function; 5. Hormonal function; 6. Nutritional function.

All natural proteins are formed from amino acid monomers. When proteins are hydrolyzed, a mixture of AMK is formed. There are 20 of these AMKs.

4. Illustrative material: presentation

5. Literature:

Main literature:

1. Bioorganic chemistry: textbook. Tyukavkina N.A., Baukov Yu.I. 2014

  1. Seitembetov T.S. Chemistry: textbook - Almaty: EVERO LLP, 2010. - 284 p.
  2. Bolysbekova S. M. Chemistry of biogenic elements: tutorial- Semey, 2012. - 219 p. : silt
  3. Verentsova L.G. Inorganic, physical and colloid chemistry: textbook - Almaty: Evero, 2009. - 214 p. : ill.
  4. Physical and colloidal chemistry / Edited by A.P. Belyaev. - M.: GEOTAR MEDIA, 2008
  5. Verentseva L.G. Inorganic, physical and colloidal chemistry, (verification tests) 2009

Additional literature:

  1. Ravich-Scherbo M.I., Novikov V.V. Physical and colloidal chemistry. M. 2003.

2. Slesarev V.I. Chemistry. Fundamentals of living chemistry. St. Petersburg: Khimizdat, 2001

3. Ershov Yu.A. general chemistry. Biophysical chemistry. Chemistry of biogenic elements. M.: VSh, 2003.

4. Asanbaeva R.D., Ilyasova M.I. Theoretical basis buildings and reactivity biologically important organic compounds. Almaty, 2003.

  1. Guide to laboratory classes in bioorganic chemistry, ed. ON THE. Tyukavkina. M., Bustard, 2003.
  2. Glinka N.L. General chemistry. M., 2003.
  3. Ponomarev V.D. Analytical chemistry Part 1.2 2003

6. Control questions (Feedback):

1. What determines the structure of the polypeptide chain as a whole?

2. What does protein denaturation lead to?

3. What is the isoelectric point called?

4. What amino acids are called essential?

5. How are proteins formed in our body?

Amino acids.

Amino acids(aminocarboxylic acids) - organic compounds, the molecule of which simultaneously contains carboxyl (-COOH) And amine groups (-NH 2).


The structure of amino acids can be expressed by the general formula below,
(Where R– hydrocarbon radical, which may contain various functional groups).

Amino acids can be considered as derivatives carboxylic acids, in which one or more hydrogen atoms are replaced by amine groups (-NH2).


An example is the protozoa: aminoacetic acid, or glycine, and aminopropionic acid or alanine:


Chemical properties of amino acids

Amino acids are amphoteric compounds, i.e. Depending on the conditions, they can exhibit both basic and acidic properties.


Due to the carboxyl group ( -COOH) they form salts with bases.
Due to the amino group ( -NH 2) form salts with acids.


A hydrogen ion that is eliminated upon dissociation from a carboxyl ( -HE) amino acid, can pass to its amino group with the formation of an ammonium group ( NH3+).


Thus, amino acids also exist and react in the form of bipolar ions (internal salts).


This explains that solutions of amino acids containing one carboxyl and one amino group have a neutral reaction.

Alpha amino acids

From molecules amino acids molecules of protein substances are built or proteins, which, upon complete hydrolysis under the influence of mineral acids, alkalis or enzymes, decompose, forming mixtures of amino acids.


The total number of naturally occurring amino acids reaches 300, but some of them are quite rare.


Among the amino acids, there is a group of 20 most important. They are found in all proteins and are called alpha amino acids.


Alpha amino acids– crystalline substances soluble in water. Many of them have a sweet taste. This property is reflected in the name of the first homolog in the series of alpha amino acids - glycine, which was also the first alpha amino acid found in natural material.


Below is a table with a list of alpha amino acids:


Name
Formula
Remaining name
Amino acids with aliphatic radicals
OH group
Ser
Thr
Amino acids with radicals containing a COOH group
Asp
Glu
Amino acids with radicals containing NH2CO-group
Asn
Gln
Amino acids with radicals containing NH 2-group
Lys
Arg
Amino acids with sulfur-containing radicals
Cys
Met
Amino acids with aromatic radicals
Phe
Tyr
Amino acids with heterocyclic radicals
Trp
His
Pro

Essential amino acids

Main source alpha amino acids food proteins serve the animal body.


Many alpha amino acids are synthesized in the body, but some alpha amino acids necessary for protein synthesis are not synthesized in the body and must come from outside, with food. These amino acids are called irreplaceable. Here is their list:


Amino acid name
Name of food products

grains, legumes, meat, mushrooms, dairy products, peanuts

almonds, cashews, chicken, chickpeas, eggs, fish, lentils, liver, meat, rye, most seeds, soy

meat, fish, lentils, nuts, most seeds, chicken, eggs, oats, brown rice

fish, meat, dairy products, wheat, nuts, amaranth

milk, meat, fish, eggs, beans, beans, lentils and soybeans

dairy products, eggs, nuts, beans

legumes, oats, bananas, dried dates, peanuts, sesame seeds, pine nuts, milk, yogurt, cottage cheese, fish, chicken, turkey, meat

legumes, nuts, beef, chicken, fish, eggs, cottage cheese, milk

pumpkin seeds, pork, beef, peanuts, sesame seeds, yogurt, Swiss cheese

tuna, salmon, pork tenderloin, beef fillet, chicken breasts, soybeans, peanuts, lentils

In some, often congenital, diseases, the list of essential acids expands. For example, with phenylketonuria, the human body does not synthesize another alpha amino acid - tyrosine, which in the body of healthy people is obtained by hydroxylation of phenylalanine.

Use of amino acids in medical practice

Alpha amino acids occupy a key position in nitrogen metabolism. Many of them are used in medical practice as medicines, affecting tissue metabolism.


So, glutamic acid used to treat diseases of the central nervous system, methionine And histidine– treatment and prevention of liver diseases, cysteine– eye diseases.

Amino acids are organic amphoteric compounds. They contain two functional groups of opposite nature in the molecule: an amino group with basic properties and a carboxyl group with acidic properties. Amino acids react with both acids and bases:

H 2 N -CH 2 -COOH + HCl → Cl [H 3 N-CH 2 -COOH],

H 2 N -CH 2 -COOH + NaOH → H 2 N-CH 2 -COONa + H 2 O.

When amino acids are dissolved in water, the carboxyl group removes a hydrogen ion, which can attach to the amino group. In this case, an internal salt is formed, the molecule of which is a bipolar ion:

H 2 N-CH 2 -COOH + H 3 N -CH 2 -COO – .

Acid-base transformations of amino acids in various environments can be represented by the following general diagram:

Aqueous solutions of amino acids have a neutral, alkaline or acidic environment depending on the number of functional groups. Thus, glutamic acid forms an acidic solution (two -COOH groups, one -NH 2), lysine forms an alkaline solution (one -COOH group, two -NH 2).

Like primary amines, amino acids react with nitrous acid, and the amino group is converted into a hydroxo group, and the amino acid into a hydroxy acid:

H 2 N-CH(R)-COOH + HNO 2 → HO-CH(R)-COOH + N 2 + H 2 O

Measuring the volume of nitrogen released allows us to determine the amount of amino acid ( Van Slyke method).

Amino acids can react with alcohols in the presence of hydrogen chloride gas, turning into ester(more precisely, into the hydrochloride salt of the ether):

H 2 N-CH(R)-COOH + R’OH H 2 N-CH(R)-COOR’ + H 2 O.

Amino acid esters do not have a bipolar structure and are volatile compounds.

The most important property of amino acids is their ability to condense to form peptides.

Qualitative reactions .

1) All amino acids are oxidized by ninhydrin

with the formation of products colored blue-violet. The imino acid proline gives a yellow color with ninhydrin. This reaction can be used to quantify amino acids by spectrophotometry.

2) When aromatic amino acids are heated with concentrated nitric acid, nitration of the benzene ring occurs and compounds colored in yellow. This reaction is called xanthoprotein(from the Greek xanthos - yellow).

All natural amino acids can be divided into the following main groups:

1) aliphatic saturated amino acids(glycine, alanine);

2) sulfur-containing amino acids(cysteine);

3) amino acids with an aliphatic hydroxyl group(serine);

4) aromatic amino acids(phenylalanine, tyrosine);

5) amino acids with acid radical(glutamic acid);

6) amino acids with a basic radical(lysine).

Isomerism. In all a-amino acids, except glycine, the a-carbon atom is connected to four different substituents, so all these amino acids can exist in the form of two isomers, which are mirror images of each other.

Receipt. 1. Hydrolysis of proteins usually produces complex mixtures of amino acids. However, a number of methods have been developed that make it possible to obtain individual pure amino acids from complex mixtures.

2. Replacement of halogen with amino group in the corresponding halogen acids. This method of obtaining amino acids is completely analogous to obtaining amines from halogen derivatives of alkanes and ammonia:

Physical properties. Amino acids are solid crystalline substances, highly soluble in water and slightly soluble in organic solvents. Many amino acids have a sweet taste. They melt at high temperatures and usually decompose in the process. They cannot go into a vapor state.

Chemical properties. Amino acids are organic amphoteric compounds. They contain two functional groups of opposite nature in the molecule: an amino group with basic properties and a carboxyl group with acidic properties. Amino acids react with both acids and bases:

When amino acids are dissolved in water, the carboxyl group removes a hydrogen ion, which can attach to the amino group. This creates internal salt, the molecule of which is a bipolar ion:

Acid-base transformations of amino acids in various environments can be represented by the following diagram:

Aqueous solutions of amino acids have a neutral, alkaline or acidic environment depending on the amount functional groups. Thus, glutamic acid forms an acidic solution (two -COOH groups, one -NH 2), lysine forms an alkaline solution (one -COOH group, two -NH 2).

Amino acids can react with alcohols in the presence of hydrogen chloride gas to form an ester:

The most important property of amino acids is their ability to condense to form peptides.

Peptides. Peptides. are the condensation products of two or more amino acid molecules. Two amino acid molecules can react with each other to eliminate a water molecule and form a product in which the fragments are linked peptide bond-CO-NH-.

The resulting compound is called a dipeptide. A dipeptide molecule, like amino acids, contains an amino group and a carboxyl group and can react with one more amino acid molecule:

The reaction product is called a tripeptide. The process of increasing the peptide chain can continue in principle indefinitely (polycondensation) and lead to substances with very high molecular weight (proteins).

The main property of peptides is the ability to hydrolyze. During hydrolysis, complete or partial cleavage of the peptide chain occurs and shorter peptides with lower molecular weight or α-amino acids that make up the chain are formed. Analysis of the products of complete hydrolysis allows us to determine the amino acid composition of the peptide. Complete hydrolysis occurs when the peptide is heated for a long time with concentrated hydrochloric acid.

Hydrolysis of peptides can occur in an acidic or alkaline environment, as well as under the action of enzymes. Amino acid salts are formed in acidic and alkaline environments:

Enzymatic hydrolysis is important because it occurs selectively, T . e. allows the cleavage of strictly defined sections of the peptide chain.

Qualitative reactions to amino acids. 1) All amino acids are oxidized ninhydrin with the formation of products colored blue-violet. This reaction can be used to quantify amino acids by spectrophotometry. 2) When aromatic amino acids are heated with concentrated nitric acid, nitration of the benzene ring occurs and yellow-colored compounds are formed. This reaction is called xanthoprotein(from Greek xanthos - yellow).

Squirrels. Proteins are natural polypeptides with high molecular weights (from 10,000 to tens of millions). They are part of all living organisms and perform a variety of biological functions.

Structure. Four levels can be distinguished in the structure of the polypeptide chain. The primary structure of a protein is a specific sequence of amino acids in a polypeptide chain. The peptide chain has a linear structure in only a small number of proteins. In most proteins, the peptide chain is folded in a certain way in space.

Secondary structure is the conformation of the polypeptide chain, i.e. the way the chain is twisted in space due to hydrogen bonds between NH and CO groups. The main method of laying a chain is a spiral.

The tertiary structure of a protein is a three-dimensional configuration of a twisted helix in space. The tertiary structure is formed due to disulfide bridges -S-S- between cysteine ​​residues located in different places of the polypeptide chain. Also involved in the formation of the tertiary structure ionic interactions oppositely charged groups NH 3 + and COO- and hydrophobic interactions, i.e., the tendency of the protein molecule to fold so that the hydrophobic hydrocarbon residues are inside the structure.

Tertiary structure - highest form spatial organization proteins. However, some proteins (for example, hemoglobin) have quaternary structure, which is formed due to the interaction between different polypeptide chains.

Physical properties proteins are very diverse and are determined by their structure. By physical properties Proteins are divided into two classes: globular proteins dissolve in water or form colloidal solutions, fibrillar proteins insoluble in water.

Chemical properties. 1 . The destruction of the secondary and tertiary structure of a protein while maintaining the primary structure is called denaturation . It occurs when heated, changes in the acidity of the environment, or exposure to radiation. An example of denaturation is the coagulation of egg whites when eggs are boiled. Denaturation can be reversible or irreversible. Irreversible denaturation can be caused by the formation insoluble substances upon the action of salts on proteins heavy metals- lead or mercury.

2. Protein hydrolysis is the irreversible destruction of the primary structure in an acidic or alkaline solution with the formation of amino acids. By analyzing hydrolysis products, it is possible to determine the quantitative composition of proteins.

3. For proteins, several are known qualitative reactions. All compounds containing peptide bond, give a violet color when exposed to copper (II) salts in an alkaline solution. This reaction is called biuret. Proteins containing aromatic amino acid residues (phenylalanine, tyrosine) give a yellow color when exposed to concentrated nitric acid (xanthoprotein reaction).

Biological significance proteins:

1. Everything chemical reactions occur in the body in the presence of catalysts - enzymes. All known enzymes are protein molecules. Proteins are very powerful and selective catalysts. They speed up reactions millions of times, and each reaction has its own single enzyme.

2. Some proteins perform transport functions and transport molecules or ions to sites of synthesis or accumulation. For example, the protein contained in the blood hemoglobin carries oxygen to tissues, and protein myoglobin stores oxygen in muscles.

3. Proteins are construction material cells. Supporting, muscle, and integumentary tissues are built from them.

4. Squirrels playing important role V immune system body. There are specific proteins (antibodies), which are able to recognize and bind foreign objects - viruses, bacteria, foreign cells.

5. Receptor proteins perceive and transmit signals coming from neighboring cells or from environment. For example, the effect of light on the retina of the eye is perceived by the photoreceptor rhodopsin. Receptors activated by low molecular weight substances such as acetylcholine transmit nerve impulses at the junctions of nerve cells.

From the above list of functions of proteins it is clear that proteins are vital for any organism and are, therefore, the most important integral part food products. During the digestion process, proteins are hydrolyzed to amino acids, which serve as the starting material for the synthesis of proteins necessary for a given organism. There are amino acids that the body is not able to synthesize itself and acquires them only with food. These amino acids are called irreplaceable.

Lecture: General information about polymers and their classification.