Theoretical study for partial molar volume of amino acids in aqueous solution: Implication of ideal fluctuation volume

A Kirkwood-Buff equation for the partial molar volumes of polyatomic molecu les in solutions is derived based on the reference interaction site model ( RISM) theory of molecular liquids. The partial molar volume of the twenty a mino acids in aqueous solution at infinite dilution are calculated using th e equation, and the results are discussed in comparison with the experiment al data. The results indicate that ionizations of the C- and N-terminus gro ups give negative contributions to the volume ranging from -3.2 cm(3)/mol t o -9.7 cm(3)/mol depending on the amino acid. Ionization of the dissociable residues also give negative contribution which ranges from -3.0 cm(3)/mol to -6.0 cm(3)/mol. On the other hand, contribution of the fractional charge s on atoms to the volume is not necessarily negative, but rather slightly p ositive with few exceptions. It is clarified that contribution from an atom group to the volume is largely dependent on the situation where the group is placed. Therefore, it is concluded that the conventional way of determin ing the partial molar volume from group contributions is not reliable. The theoretical results for the partial molar volume exhibit a systematic devia tion from corresponding experimental data, which increases nearly proportio nally with increasing temperature and with the number of atoms in the amino acids. In order to account for the deviation, a concept of the "ideal fluc tuation volume" is proposed, which is the ideal gas contribution to the vol ume, originating from the intramolecular fluctuation of solute. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)51020-4].