Amino acids are characterized by properties. Amino acids, their composition and chemical properties: interaction with hydrochloric acid, alkalis, with each other

The acid-base properties of amino acids are associated with the presence in their structure of two ionizable groups - carboxyl and amino groups, therefore, amino acids can exhibit the properties of both acids and bases, i.e. they are amphoteric compounds. In the crystalline state and in aqueous solutions, α-amino acids exist in the form of bipolar ions, also called zwitterions. The ionic structure determines some of the properties of α-amino acids: high melting point (200-300°C), non-volatility, solubility in water and insolubility in non-polar organic solvents. The solubility of amino acids in water is related to their absorption and transport in the body. The ionization of amino acid molecules depends on the pH of the solution. For monoaminomonocarboxylic acids, the dissociation process has the following form:

In strongly acidic solutions, amino acids are present as positive ions, and in alkaline solutions they are present as negative ions.

The acid-base properties of amino acids can be explained based on the Brønsted-Lowry theory of acids and bases. A fully protonated -amino acid (cationic form), from the perspective of the Brønsted theory, is a dibasic acid containing two acid groups: an undissociated carboxyl group (– COOH) and a protonated amino group (NH 3), which are characterized by the corresponding values ​​of pK 1 and pK 2.

pK values ​​for amino acids are determined from titration curves. Consider the alanine titration curve (Fig. 1).

Rice. 1 – curves obtained by titrating a 0.1 M alanine solution with a 0.1 M HCl solution (a) and a 0.1 M NaOH solution (b).

From the titration curve of alanine it follows that the carboxyl group has pK  1 = 2.34, and the protonated amino group has pK  2 = 9.69. At pH = 6.02, alanine exists as a bipolar ion when the total electrical charge of the particle is 0. At this pH value, the alanine molecule is electrically neutral. This pH value is called the isoelectric point and is denoted pH et or pI. For monoaminomonocarboxylic acids, the isoelectric point is calculated as the arithmetic mean of two pK  values. For example, for alanine it is equal to:

рI= ½(рК 1 + рК 2) = ½(2.34 + 9.69) = 6.02

At a pH value above the isoelectric point, the amino acid is negatively charged, and at a pH value below pI, the amino acid carries a net positive charge. For example, at pH = 1.0, all alanine molecules exist in the form of ions

with a total charge of +1. At pH = 2.34, when there is a mixture of equal amounts of ions

total charge = +0.5. Similarly, you can predict the sign and magnitude of the total charge for any other amino acid at any pH value.

Amino acids with an ionizable group in the radical have more complex titration curves, consisting of 3 sections corresponding to three possible stages of ionization, and, therefore, they have three pK values ​​(pK 1, pK ​​2 and pK R). The ionization of acidic amino acids, such as aspartic acid, consists of the following sequential stages:

The isoelectric points of such amino acids are also determined by the presence of the ionizable group of the radical, along with -amino and -carboxyl groups. For monoaminodicarboxylic acids, the isoelectric points are shifted to the acidic pH region and are defined as the arithmetic mean between the pK values ​​for two carboxyl groups (pIaspartic acid = 2.97). For basic amino acids, pI is shifted to the alkaline region and is calculated as the arithmetic mean between pK values ​​for two protonated amino groups (pIlysine = 9.74).

The acid-base properties of amino acids are used for the separation and subsequent identification of amino acids by electrophoresis and ion exchange chromatography. Both of these methods are based on differences in the sign and magnitude of the total electrical charge at a given pH value.

Amino acids, proteins and peptides are examples of the compounds described below. Many biologically active molecules contain several chemically different functional groups that 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:

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 of 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 with hydrochloric acid HCl leads to the formation of 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 with 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 of amino and carboxyl groups - the formation of peptides. The general property of 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. In general, a 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 the 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: textbook - Semey, 2012. - 219 p. : silt
3. Verentsova L.G. Inorganic, physical and colloidal 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 foundations of the structure and reactivity of 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. Test 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?

Related information.

Amino acids exhibit properties of both acids and amines. So, they form salts (due to the acidic properties of the carboxyl group):

NH 2 CH 2 COOH + NaOH (NH 2 CH 2 COO)Na + H 2 O

glycine sodium glycinate

and esters (like other organic acids):

NH 2 CH 2 COOH + C 2 H 5 OHNH 2 CH 2 C(O)OC 2 H 5 + H 2 O

glycine ethylglycinate

With stronger acids, amino acids exhibit the properties of bases and form salts due to the basic properties of the amino group:

glycine wisteria chloride

The simplest protein is a polypeptide containing at least 70 amino acid residues in its structure and having a molecular weight of over 10,000 Da (dalton). Dalton - a unit of measurement for the mass of proteins, 1 dalton is equal to 1.66054·10 -27 kg (carbon mass unit). Similar compounds consisting of fewer amino acid residues are classified as peptides. Peptides by nature are some hormones - insulin, oxytocin, vasopressin. Some peptides are regulators of immunity. Some antibiotics (cyclosporin A, gramicidins A, B, C and S), alkaloids, toxins of bees and wasps, snakes, poisonous mushrooms (phalloidin and amanitin of the toadstool), cholera and botulinum toxins, etc. have a peptide nature.

Levels of structural organization of protein molecules.

The protein molecule has a complex structure. There are several levels of structural organization of a protein molecule - primary, secondary, tertiary and quaternary structures.

Primary structure is defined as a linear sequence of proteinogenic amino acid residues linked by peptide bonds (Fig. 5):

Fig.5. Primary structure of a protein molecule

The primary structure of a protein molecule is genetically determined for each specific protein in the nucleotide sequence of messenger RNA. The primary structure also determines higher levels of organization of protein molecules.

Secondary structure - conformation (i.e. location in space) of individual sections of the protein molecule. The secondary structure in proteins can be represented by an -helix, -structure (folded sheet structure) (Fig. 6).

Fig.6. Protein secondary structure

The secondary structure of the protein is maintained by hydrogen bonds between peptide groups.

Tertiary structure - conformation of the entire protein molecule, i.e. spatial arrangement of the entire polypeptide chain, including the arrangement of side radicals. For a significant number of proteins, the coordinates of all protein atoms were obtained by X-ray diffraction analysis, with the exception of the coordinates of hydrogen atoms. All types of interactions take part in the formation and stabilization of the tertiary structure: hydrophobic, electrostatic (ionic), disulfide covalent bonds, hydrogen bonds. Radicals of amino acid residues participate in these interactions. Among the bonds holding the tertiary structure, it should be noted: a) disulfide bridge (- S - S -); b) ester bridge (between the carboxyl group and the hydroxyl group); c) salt bridge (between the carboxyl group and the amino group); d) hydrogen bonds.

In accordance with the shape of the protein molecule, determined by the tertiary structure, the following groups of proteins are distinguished:

1) Globular proteins , which have the shape of a globule (sphere). Such proteins include, for example, myoglobin, which has 5 α-helical segments and no β-folds, immunoglobulins, which do not have an α-helix; the main elements of the secondary structure are β-folds

2) Fibrillar proteins . These proteins have an elongated thread-like shape; they perform a structural function in the body. In the primary structure, they have repeating regions and form a secondary structure that is fairly uniform for the entire polypeptide chain. Thus, protein α - keratin (the main protein component of nails, hair, skin) is built from extended α - helices. There are less common elements of secondary structure, for example, polypeptide chains of collagen, forming left-handed helices with parameters that differ sharply from the parameters of α-helices. In collagen fibers, three helical polypeptide chains are twisted into a single right-handed superhelix (Fig. 7):

Fig. 7 Tertiary structure of collagen

Quaternary structure of protein. The quaternary structure of proteins refers to the way in which individual polypeptide chains (identical or different) with a tertiary structure are arranged in space, leading to the formation of a structurally and functionally unified macromolecular formation (multimer). Not all proteins have a quaternary structure. An example of a protein with a quaternary structure is hemoglobin, which consists of 4 subunits. This protein is involved in the transport of gases in the body.

When breaking disulfide and weak types of bonds in molecules, all protein structures, except the primary one, are destroyed (completely or partially), and the protein loses its native properties (properties of a protein molecule inherent in it in its natural, natural (native) state). This process is called protein denaturation . Factors that cause protein denaturation include high temperatures, ultraviolet irradiation, concentrated acids and alkalis, salts of heavy metals and others.

Proteins are divided into simple (proteins), consisting only of amino acids, and complex (proteins), containing, in addition to amino acids, other non-protein substances, for example, carbohydrates, lipids, nucleic acids. The non-protein part of a complex protein is called a prosthetic group.

Simple proteins, consisting only of amino acid residues, are widespread in the animal and plant world. Currently, there is no clear classification of these compounds.

Histones

They have a relatively low molecular weight (12-13 thousand), with a predominance of alkaline properties. Localized mainly in cell nuclei, soluble in weak acids, precipitated by ammonia and alcohol. They have only tertiary structure. Under natural conditions, they are tightly bound to DNA and are part of nucleoproteins. The main function is the regulation of the transfer of genetic information from DNA and RNA (transmission can be blocked).

Protamines

These proteins have the lowest molecular weight (up to 12 thousand). Exhibits pronounced basic properties. Well soluble in water and weak acids. Contained in germ cells and make up the bulk of chromatin protein. Like histones, they form a complex with DNA and impart chemical stability to DNA, but unlike histones, they do not perform a regulatory function.

Glutelins

Plant proteins contained in gluten from the seeds of cereals and some other crops, in the green parts of plants. Insoluble in water, salt solutions and ethanol, but highly soluble in weak alkali solutions. They contain all essential amino acids and are complete food products.

Prolamins

Plant proteins. Contained in gluten of cereal plants. They are soluble only in 70% alcohol (this is explained by the high content of proline and non-polar amino acids in these proteins).

Proteinoids.

Proteinoids include proteins of supporting tissues (bone, cartilage, ligaments, tendons, nails, hair); they are characterized by a high sulfur content. These proteins are insoluble or poorly soluble in water, salt and water-alcohol mixtures. Proteinoids include keratin, collagen, fibroin.

Albumin

These are acidic proteins of low molecular weight (15-17 thousand), soluble in water and weak saline solutions. Precipitated by neutral salts at 100% saturation. They participate in maintaining the osmotic pressure of the blood and transport various substances with the blood. Contained in blood serum, milk, egg white.

Globulins

Molecular weight up to 100 thousand. Insoluble in water, but soluble in weak salt solutions and precipitate in less concentrated solutions (already at 50% saturation). Contained in plant seeds, especially legumes and oilseeds; in blood plasma and some other biological fluids. They perform the function of immune defense and ensure the body's resistance to viral infectious diseases.

Amino acids are heterofunctional organic compounds whose molecules include an amino group NH2 and a carboxyl group COOH

Aminoacetic acid

aminopropanoic acid

Physical properties.
Amino acids are colorless crystalline substances soluble in water. Depending on the radical, they can be sour, bitter and tasteless.

Chemical properties

Amino acids are amphoteric organic compounds (due to the amino group, they exhibit basic properties and due to the carboxyl group COOH, they exhibit acidic properties)

Reacts with acids

H 2 N – CH 2 – COOH + NaOH = Cl- aminoacetic acid

Reacts with alkalis

H 2 N – CH 2 – COOH + NaOH = H 2 N – CH 3 – COONa + H 2 O- sodium salt of glycine

React with basic oxides

2H 2 N – CH 2 – COOH + CuO = (H 2 N – OH 2 – COO) 2 + H 2 O- copper glycine salt

Ticket No. 17

The relationship between structure, properties and applications using the example of simple substances.

Most non-metals of simple substances are characterized by a molecular structure, and only a few of them have a non-molecular structure.

Non-molecular structure

C, B, Si

These nonmetals have atomic crystal lattices, so they have great hardness and very high melting points.

The addition of boron to steel and alloys of aluminum, copper, nickel, etc. improves their mechanical properties.

Application:

1. Diamond – for drilling rocks

2. Graphite - for the manufacture of electrodes, neutron moderators in nuclear reactors, as a lubricant in technology.

3. Coal, consisting mainly of carbon, is adsbent - for the production of calcium carbide and black paint.

Molecular structure

F 2, O 2, Cl 2, Br 2, N 2, I 2, S 8

These nonmetals are characterized by molecular crystal lattices in the solid state and, under normal conditions, are gases, liquids or solids with low melting points.

Application:

1. Acceleration of chemical reactions, including in metallurgy

2. Metal cutting and welding

3. In liquid form in rocket engines

4. In aviation and submarines for breathing

5. In medicine

Proteins are like biopolymers. Primary, secondary and tertiary structure of proteins. Properties and biological properties of proteins.

Proteins are biopolymers whose molecules include amino acid residues

Proteins have primary, secondary, tertiary, and quaternary structures.

The primary structure is one consisting of amino acid residues interconnected by pectide bonds.

The secondary structure is a chain coiled into a spiral and, in addition to peptide bonds, there are hydrogen bonds

The tertiary structure is a spiral coiled into a ball and additionally has S-S sulfide bonds

Quaternary structure - double helix coiled into a ball

Physical properties

Proteins are amphoteric electrolytes. At a certain pH value of the environment, the number of positive and negative charges in a protein molecule is equal. Proteins have a varied structure. There are proteins that are insoluble in water, and there are proteins that are easily soluble in water. There are proteins that are chemically inactive and resistant to agents. There are proteins that are extremely unstable. There are proteins that look like threads reaching hundreds of nanometers in length; There are proteins that have the shape of balls with a diameter of only 5–7 nm. They have a large molecular weight (104-107).

Chemical properties
1. The denaturation reaction is the destruction of the primary structure of a protein under the influence of temperature.
2. Color reactions to proteins
a) Interaction of protein with Cu(OH)2
2NaOH + CuSO 4 = Na 2 SO 4 + Cu(OH) 2
b) Interaction of protein with HNO 3
The reagent for sulfur is lead acetate (CH 3 COO) 2 Pb, a black precipitate PbS is formed

Biological role
Proteins are building materials
Proteins are an essential component of all cellular structures
Proteins are enzymes that act as catalysts
Regular proteins: these include hormones
Proteins are a means of protection
Proteins as a source of energy