Intramolecular Hydrogen Bonds in Amino Acids.

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(structure1 of H-bonded GABA) (structure2 of H-bonded GABA) Gamma-aminobutyric acid is physiologically important as an inhibitory neurotransmitter substance, and thus has been given a commonly used abbreviation: GABA. The neutral form of GABA forms a total of four conformers with O-H···N hydrogen bonds, which form two pairs of enantiomers. One of these enantiomer pairs exhibits an envelope structure with the COOH group, the nitrogen atom, and the carbon atoms alpha and gamma in one (slightly distorted) plane, whereas the other has carbon atoms beta and gamma out of the plane of the COOH group, carbon atom alpha, and the nitrogen atom. The hydrogen bonds are of different strength in these two pairs: stronger in the former, and weaker in the latter. The overlap of the fused spheres in the displays show this different strength clearly by the different amount of overlap.

(global minimum) As in glycine and beta-alanine, the H-bonded conformers are not the global minima of the ab initio potential energy surface. The conformation of the global minima of GABA is a straightforward extension of those of beta-alanine: the COOH group and the carbon atoms alpha and beta are located in one plane, carbon atom gamma sticks out of that plane, and the amino group is oriented towards the COOH group. Again, the interaction between the carbonyl oxygen atom and the amino group hydrogen atom does not qualify as a hydrogen bond.

(only conformer with N-H...O=C bond) In fact, among the 62 symmetry-unique local minima in the potential energy surface of GABA there is only one, in which a N-H···O=C hydrogen bond is formed; the geometry of this high-energetical conformer, which has the COOH-group in the unfavourable trans-orientation, is shown next to this paragraph. It should be noted, that this conformer and its enantiomer, together with the conformers with the strong N···H-O bond, are the only hydrogen bonded conformers in the GABA potential energy surface.  

As in beta-alanine, there are reaction paths that preserve the N···H-O hydrogen bond. In the case of GABA, there are two energetically different paths that connect the different H-bonded conformers in a closed cycle A-B-Am-Bm-A ("m" denoting mirror images). Each H-bonded conformer therefore has the "choice" between two different paths that do not break the hydrogen bond. In contrast to 3-aminoproanol, where there is a similar cycle of hydrogen bond conserving reactions, the reactions of the GABA cycle are the ones with the lowest potential barriers for both H-bonded conformers.

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