Intramolecular Hydrogen Bonds in Amino Acids.

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-alanine forms one symmetry unique conformer with a O-H···N hydrogen bond. This conformer has a non-symmetric geometry that is of the envelope-type: the atoms of the COOH group, the carbon atom, and the nitrogen atom are located almost exactly in one plane, whereas the carbon atom sticks out of this plane. Due to this non-symmetry, there are in fact two H-bonded conformers in the ab initio potential energy surface, namely the one shown in the display and its mirror image.

Although they do not have any asymmetric carbon atoms, the two H-bonded conformers thus constitute an enantiomer pair, since they are unsymmetric and mirror images of each other. This of special interest, because there exists a H-bond preserving reaction that interconnects these two conformers without breaking the hydrogen bond. The transition state of this reaction path is not the mirror symmetrical conformation, in which the cycle H-O-C-C-C-N forms a planar (but irregular) hexagon. Instead, there are two non-symmetrical transition states, in which the amino group is twisted approximately 20°off the symmetrical orientation (clockwise or counter-clockwise). Hence there are two (energetically equivalent) H-bond preserving reaction paths that connect the two enantiomers, both of which avoid the point of achirality and thus remain chiral (although changing parity) from beginning to end. The corresponding segment of the energy hypersurface in the vicinity of this point of achirality is shown in the graphic next to this paragraph.

As in glycine, the H-bonded conformers are not the global minima of the potential energy surface. The reason for this is that the COOH group is about 35 kJ/mol more stable in the cis orientation, where the groups C=O and O-H point to the same side of the C-O bond, than in the trans orientation that is necessary to form the H-bond. Thus, the energy gain by the hydrogen bond formation (about 32 kJ/mol) is cancelled by the energy loss of the reorientation of the COOH group. The geometry of the global minima is characterized by the COOH group, the , and the carbon atom in one plane, and the nitrogen atom sticking out of that plane. The carbonyl oxygen atom and one amino group hydrogen atom come close to each other in this orientation, but not close enough to form a hydrogen bond.

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