Internal neutralization and physical properties
The chemical nature of amino acids is determined by the presence of acidic ($-COOH$) and basic ($-NH_2$) groups. A proton from the carboxyl group is easily transferred to the amino group within the same molecule. A bipolar ion (zwitterion) is formed:
$$H_2NCH_2COOH \rightleftarrows NH_3^+CH_2COO^-$$
In the solid state, a strong electrostatic attraction arises between the oppositely charged poles of these ions. The crystal structure resembles an ionic one, which leads to abnormally high melting points.
The simplest representative is glycine (aminoacetic acid or 2-aminoethanoic acid). It is a white crystalline substance with a melting point of 290 °C, yielding a neutral aqueous solution. Glycine differs fundamentally from monofunctional analogues with the same number of carbon atoms: ethylamine is an alkaline gas, and acetic acid is an infinitely soluble acidic liquid. Glycine is obtained by the action of ammonia on chloroacetic acid:
$ClCH_2COOH + 2NH_3 \rightarrow NH_2CH_2COOH + NH_4Cl$.
Features of $\alpha$-amino acids and stereochemistry
Only $\alpha$-amino acids are used to build proteins. In them, the amino group is attached to the second ($\alpha$-) carbon atom.
Since this carbon atom is bonded to four different substituents, it is an asymmetric center (chiral center). This accounts for the existence of optical isomers.
- The only exception is glycine (Gly). Instead of a side chain, it contains a second hydrogen atom, therefore lacking an asymmetric center and remaining optically inactive.
- Proteins are built solely from isomers of the L-series. In the Fischer projection, the structure of an L-amino acid appears as follows: the carboxyl group is at the top, the side chain ($R$) is at the bottom, and the amino group is on the left.
Classification of protein amino acids
Almost all proteins are built from 20 standard amino acids. Of these, only a small fraction are simple. The majority (14 out of 20) contain an additional functional group in their side chain ($R$), which contributes enormously to the properties of the substance.
| Name | Parameter | Side chain ($R$) characteristics | $t_{m}$, °C |
|---|---|---|---|
| Glycine | Gly | $-H$ | 290 |
| Alanine | Ala | $-CH_3$ | 297 |
| Valine | Val | Isopropyl | 284 |
| Glutamic acid | Glu | $-CH_2CH_2COOH$ (acidic) | 160 |
| Lysine | Lys | Contains $-NH_2$ (basic) | 224 |
Amino acids can be grouped according to the chemical nature of their side chains:
- With a hydrocarbon side chain: Alanine, Valine, Leucine (isobutyl), Isoleucine (sec-butyl), Phenylalanine (benzyl).
- With acidic side chains: Aspartic and Glutamic acids. Tyrosine is also singled out separately as it contains a phenolic hydroxyl group.
- Containing nitrogen in the side chain: Lysine (amino group), Arginine (guanidine group), Asparagine and Glutamine (amide groups).
- With oxygen and sulfur: Serine and Threonine contain alcoholic hydroxyls, Cysteine contains a thiol (sulfhydryl) group, and Methionine contains a thioether group.
- Heterocyclic: Histidine (with an imidazole ring) and Tryptophan (with an indole ring). Proline (Pro) stands apart — it is an imino acid whose nitrogen atom is rigidly incorporated into a pyrrolidine ring.
Briefly: answers to topical questions
What are the structural features and chemical nature of proline?
Proline (Pro) is a heterocyclic imino acid, the main feature of which is the inclusion of the nitrogen atom in a pyrrolidine ring.
Structural features and chemical nature:
- Pyrrolidine ring — includes a nitrogen atom, while the ring itself is incorporated into the peptide backbone.
- Peptide bond — interaction with proline forms a –CO–N– group, since it is an imino acid.
- Physicochemical constants — the melting point is 221 °C, and the molecular mass is 115.1.
Class and exam questions
Why do amino acids have such high melting points?
Due to the process of internal neutralization. Molecules exist in the form of bipolar ions (zwitterions) that strongly attract each other electrostatically. The crystal lattice becomes similar to an ionic one, requiring a lot of energy to break it down.
Do amino acids lose their reactivity due to internal salt formation?
No. Internal neutralization does not eliminate the chemical activity of acidic and basic groups. Amino acids remain amphoteric and react with both acids and bases.
Which $\alpha$-amino acid has no optical isomers?
In glycine (aminoacetic acid). Its $\alpha$-carbon atom has two identical substituents — hydrogen atoms, therefore an asymmetric center is absent.
What else to study on this topic
- Reaction mechanisms of amino acids with acids and bases
- Specific color reactions for amino acids
- Physicochemical properties of the thiol group in cysteine
- The role of heterocyclic amino acids (histidine and tryptophan)
- Differences of proline as an imino acid from classical amino acids
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Other topics in "Chapter 18. Major Classes of Bioorganic Compounds"
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