EFFECT OF POLARIZABILITY ON THE POTENTIAL OF MEAN FORCE OF 2 CATIONS - THE GUANIDINIUM-GUANIDINIUM ION-PAIR IN WATER

The potential of mean force of two rigid guanidinium ions constrained to remain parallel is investigated in liquid water by means of free en ergy perturbation (FEP) molecular dynamics simulations, using various intermolecular potentials, The first simulation is carried out employi ng the Amber force field and the transferable intermolecular potential TIP3P water model. The second simulation is performed with the extend ed simple point charge SPC/E water model. In a third simulation, the p olarizability of the water molecule is introduced via The use of the p olarizable simple point charge model PSPC, whereas for the ions, distr ibuted polarizabilities derived from the topological partitioning of e lectrostatic properties (TPEP) are incorporated on heavy atoms. For th e last two simulations, atom-atom Lennard-Jones parameters and charges are derived from ab initio calculations on monomers and guanidinium-w ater pairs, The comparison with a previous simulation using the transf erable intermolecular potential TIP4P, by Boudon et al. (J. Phys. Chem . 1990, 94, 6056-61), reveals that (i) all the models predict a stable contact ion pair (CIP) at a distance of 3.0-3.4 Angstrom, and a solve nt-separated ion pair (SSIP) at about 6.5 Angstrom, (ii) the stabiliza tion energy of the CIP is strongly model-dependent, varying from 10.0 kcal.mol(-1), for the TIP4P model to 4.7 and 2.7 kcal.mol(-1) for the SPC/E and PSPC models respectively, and (iii) in al cases, the SSIP fr ee energy minimum is very shallow and nearly disappears for the simula tion using a polarizable model. Consideration of the distribution and the orientation of the solvent molecules around the ions for the non-p olarizable (SPC/E) and the polarizable (PSPC) cases does not reveal an y significant difference between the two models.