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Zwitterionic versus Neutral Form.

The structural behaviour of amino acids is exemplified here for -aminobutyric acid (GABA). The zwitterionic form, as determined via crystal structure analysis, was used as the initial geometry in an ab initio geometry optimization, in which the maximum geometry change was kept deliberately small. Immediately, the trans orientation of the N-C-C-C fragment starts to bend and the -NH group approaches the -COO group. After 33 geometry steps a -NH-H···O-CO- hydrogen bond forms, and after 48 geometry steps the proton has shifted to the oxygen atom. The -NH···HOOC- hydrogen bond, which has formed by then, does not break upon further geometry optimization, rather the main chain torsions adjust to minimize the steric strain. After 88 geometry steps, the final converged and relaxed structure is obtained.

Initial and final geometry are shown here; an animation of the complete sequence (100K, playing time: 30s) is available on a separate page.