Amino acids

General molecular structure overview

The molecule of any standard amino acid is built around a single central carbon atom. Four different structural elements are always attached to this carbon "backbone". This configuration ensures the stability and unique chemical properties of the compounds.

Basic structural components:

• Amino group ($—NH_2$) — a functional group that acts as the primary bearer of the basic properties of the molecule.
• Carboxyl group ($—COOH$) — the second functional group, playing the opposite role and providing pronounced acidic characteristics.
• Hydrogen atom ($—H$) — an obligatory element bonded to the central carbon.
• Side chain (radical) ($—R$) — the variable part of the molecule. The radical is a unique group of atoms that serves as a kind of "passport". The structure of the radical is strictly individual and allows one amino acid to be unmistakably distinguished from any other in the standard list.

Physicochemical properties and bonds

In terms of physical characteristics, pure free amino acids are colorless crystalline substances. Their solubility in various media varies: some dissolve remarkably well in ordinary water, while absolutely all of them are insoluble in organic solvents.

From a chemical standpoint, a fundamental feature is their amphotericity. This means that the molecule can act as both an acid and a base. This dual chemical nature is directly due to the simultaneous presence of an amino group ($—NH_2$) and a carboxyl group ($—COOH$) in the structure.

Amino acids are designed to create more complex polymeric structures. They are capable of joining together, forming a specialized peptide bond ($—CO—NH—$). By its chemical nature, this is a classic covalent bond characterized by exceptionally high strength and stability, securely holding monomers together in a single chain.

Diversity of radicals: 20 standard amino acids

Twenty standard amino acids function in biological systems, each with its own unique radical. For ease of study, they can be divided according to key structural features.

Special structures and simple radicals:

1. Glycine — has the most primitive structure, where the position of the radical is occupied by a single hydrogen atom ($R = H$).
2. Proline — stands apart as it is actually an imino acid. It has a specific cyclic structure, with the nitrogen atom included directly in the ring itself.

Sulfur-containing and hydroxyl groups:

1. Cysteine — contains a sulfur atom represented as a sulfhydryl group ($—SH$).
2. Methionine — the second amino acid on the list whose structure contains sulfur.
3. Serine — bears a hydroxyl group ($—OH$) within its radical.
4. Threonine — also belongs to compounds whose radicals contain oxygen as part of hydroxyl or other groups.

Acidic properties and their amides:

1. Aspartic acid — exhibits pronounced acidic properties due to the presence of two carboxyl groups ($—COOH$) at once.
2. Glutamic acid — another representative with prominent acidic characteristics.
3. Asparagine — is a chemical derivative (amide) of aspartic acid.
4. Glutamine — acts as the direct amide of glutamic acid.

Basic and aromatic properties:

1. Arginine — a molecule with pronounced basic properties.
2. Lysine — also belongs to the group of amino acids with basic radical characteristics.
3. Histidine — a basic amino acid additionally distinguished by a complex heterocyclic structure.
4. Tyrosine — an aromatic compound whose structure necessarily contains a phenolic ring.
5. Tryptophan — a large aromatic molecule with a heterocyclic radical nature.
6. Phenylalanine — a classic representative of the aromatic amino acid group.

Aliphatic amino acids: The remaining four compounds — Alanine (17), Valine (18), Leucine (19), and Isoleucine (20) — differ only in the structure of their hydrocarbon chains in the radicals, forming the basis for many biological structures.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of the chemical reaction of peptide bond formation
• Features of the cyclic structure of proline as an imino acid
• The role of the phenolic ring in tyrosine chemical reactions
• The effect of two carboxyl groups on the charge of aspartic acid
• Differences in the solubility of specific amino acids in water

Other topics in the section "Chemical composition of the cell"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.