Dipolar ion and its effect on properties
The specificity of amino acids lies in the intramolecular proton transfer from the carboxyl group to the amino group. This results in the formation of a so-called dipolar ion (zwitterion). This state largely determines the physical and chemical properties of the substances.
Clear evidence of the effect of internal ionization is esterification. Treating an amino acid with an alcohol (e.g., ethanol) yields an ester. Unlike the free amino acid, its ester is a volatile compound. This occurs because the labile proton of the carboxyl group in the ester is replaced by an alkyl radical, preventing the formation of a dipolar ion and weakening intermolecular interactions.
Amphoteric properties
As amphoteric electrolytes, amino acids can interact with reagents of opposite character. Let us examine this using glycine as an example.
- With acids, the negatively charged carboxylate group of the dipolar ion reacts (acting as a base by accepting a proton). The reaction product is a salt at the amino group (e.g., glycine hydrochloride).
- With alkalis, the positively charged ammonium group enters into the reaction (acting as an acid by donating a proton). A salt at the carboxyl group is formed (e.g., sodium glycinate).
Due to their amphoteric nature, amino acids resemble inorganic salts such as ammonium acetate. The main difference is that in an amino acid, the acidic and basic centers are not free, but are covalently linked by a carbon skeleton.
Acid-base environment in solutions
The pH of an aqueous amino acid solution depends on the ratio of functional groups in its structure.
- Acidic environment (pH < 7). Typical of monoaminodicarboxylic acids (e.g., glutamic or aspartic acid). One carboxyl group is involved in the formation of the dipolar ion, while the second freely dissociates in water, releasing protons.
- Alkaline environment (pH > 7). Occurs when dissolving diaminomonocarboxylic acids (e.g., lysine). Here, one amino group participates in internal neutralization, while the second binds a proton from a water molecule, leading to the accumulation of hydroxide ions.
Specific reactions of side chains
In addition to reactions involving the main functional groups, amino acids can undergo chemical interactions via their side chains (radicals). An important example is reactions involving the aromatic ring.
Tyrosine, containing a phenolic ring, readily undergoes electrophilic substitution reactions such as halogenation. Treatment with iodine leads to the substitution of hydrogen atoms in the ortho-positions relative to the hydroxyl group, yielding 3,5-diiodotyrosine. The biological significance of this reaction is immense: this is how precursors of vital thyroid hormones are synthesized in the body.
Complexation
Amino acids are excellent ligands. Amino acid anions can bind to heavy metal ions (d-block elements), forming highly stable inner-complex compounds. Such structures are called chelates. For instance, the glycinate ion forms a characteristic complex with the copper ion that does not dissociate into ions in aqueous solution.
Briefly: answers to topical questions
What qualitative (color) reactions are used to detect alpha-amino acids in solution?
For the qualitative detection of alpha-amino acids in solution, the ninhydrin test is used, along with specific methods for terminal residues.
- Ninhydrin test — produces a characteristic purple-blue coloration for α-amino acids.
- Sanger and Edman reactions — used to determine the N-terminal amino acid in the structure.
Additionally, UV spectrophotometry can be used for quantitative determination.
What chemical reactions are characteristic of the carboxyl group of amino acids, aside from esterification?
In addition to esterification, reactions involving the carboxyl group or carboxylate residue, as well as peptide bond formation, are well-documented for amino acids.
- Reaction with acids — the carboxyl group residue ($COO^-$) acts as a base; salts at the amino group are formed, e.g., glycine hydrochloride.
- Reaction with alkalis — salts at the carboxyl group are formed, e.g., sodium glycinate upon reaction of glycine with sodium hydroxide.
- Polycondensation — amino acid molecules join to form a peptide (amide) bond $-CO-NH-$ with the elimination of water.
How does the peptide bond formation reaction proceed between two amino acids?
The peptide bond formation reaction between amino acids proceeds via a polycondensation mechanism.
- Formation of a dipeptide — amino acid molecules are joined by a peptide (amide) bond ($-CO-NH-$) with the elimination of water.
- Isomerism — depending on the order of connection, isomeric dipeptides may form (e.g., Gly-Ala and Ala-Gly).
- Structure of the bond — the resulting $CONH$ group has a planar structure due to the mesomeric effect.
The resulting dipeptide retains free N- and C-termini, allowing the condensation reaction to continue.
Class and exam questions
Why are amino acid esters more volatile than free amino acids?
The ester molecule lacks a labile proton at the carboxyl group; therefore, a dipolar ion does not form, and hydrogen bonds are weaker.
Which amino acid group reacts upon the addition of hydrochloric acid?
During the reaction with acids, the carboxylate anion of the dipolar ion interacts by accepting a proton.
What is the pH environment of an aspartic acid solution?
In an acidic environment (pH < 7), because it has two carboxyl groups, one of which additionally dissociates in water.
Where does substitution occur during the iodination of tyrosine?
Hydrogen is substituted by iodine in the aromatic ring at the ortho-position relative to the phenolic hydroxyl.
What else to study on this topic
- Detailed mechanism of dipolar ion formation
- Specific reactions of other side chains
- Mechanism of chelate formation with copper ions
- Comparison of properties of various diaminomonocarboxylic acids
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