Electronic and vibrational polarizabilities of the twenty naturally occurring amino acids.

The geometries, relative energies, gas-phase static and dynamic dipole polarizabilities of the two most stable neutral forms and of the zwitterionic form of the twenty naturally occurring amino acids have been obtained by Density Functional and conventional ab initio Hatree-Fock theories using correlation consistent basis sets. Mean electronic polarizabilities (<alpha(e)>s) are encompassed in the 40-160 a.u. range and are little dependent on the amino acid framework conformation and structure. The relation between <alpha(e)> and the number of electrons in the molecule makes to classify the amino acids as one of the most polarizable family of compounds. Calculated <alpha(e)> values of the neutral forms linearly relate to the molecular volume and molecular hardness as well as, rather unexpectedly, with the experimental values in water solution, where amino acids are known to be in a zwitterionic form. Vibrational polarizabilities amount to 15-45 a.u.. They come essentially from the low-frequency angular deformation modes of the -OH and -NH(2) groups.