The conformation of the L-alanine zwitterion (ALAZ) in aqueous solution was examined by an ab initio MO method including the solvent effect with the generalized Born (GB) equation. The geometry optimization with the 6-31++G* basis set gave the (, )=(80, 8) conformation as the most stable conformation of ALAZ in aqueous solution, where is the HNCC torsional angle and is the NCCO torsional angle. Stabilization by the solvent is larger in the =90° conformation than in the =0° conformation. However, the differential solvation is small and the conformation with a nearly planar NCCO skeleton is the most stable conformation in aqueous solution. The barrier to rotation of the CO2 group ( rotation) was calculated to be 6·7 (6-31++G*) and 5·9 (MP2/6-31++G*) kcal mol-1, while that to rotation of the NH3 group ( rotation) was very small (<1 kcal mol-1). The relative free energy of solvation and relative stability among the representative conformations of ALAZ were calculated by Monte Carlo (MC) simulation with the statistical perturbation method. The energy variation along the rotation indicated that the value of =5-10° is favorable for ALAZ in aqueous solution. The MC simulation supports the ab initio GB results for the conformational properties of ALAZ in aqueous solution.
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