7.
Four aromatic amino acids (
p-aminobenzoic, 4,6-diaminoisophthalic,
o-aminobenzoic, and methylene-bis-anthranilic) were studied by FTIR spectroscopy. The first two molecules were found to exist in the solid phase exclusively in neutral form and the latter two in coexisting neutral and ionic forms. The shift of the tautomeric equilibrium from neutral molecule to zwitterion is determined by the character of substitution, molecular conformation, and the possibility of noncovalent bonds formed between the functional groups. The separation of charges becomes possible only if the conformers of the molecule include a structure with an OH….N intramolecular hydrogen bond. The proton is completely transferred from the acid group to the amino group when the strong intermolecular hydrogen bonds can stabilize the formed zwitterion. Otherwise, uncharged complexes with different degrees of proton transfer to the amino group are formed.
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