A fluctuating charge interaction potential function for alanine-water was constructed in the spirit of newly developed ABEEMax/MM（atom-bond electronegativity equalization method at the azc level fused into molecular mechanics）. The properties of gaseous neutral alanine-（H20）n（n=l--7） clusters were systematically investigated by quantum mechanics（QM） and the constructed ABEEMax/MM potential, such as conformations, hydrogen bonds （H-bonds）, interaction energies, charge distributions, and so on. The results of ABEEM^rrc/MM model are in fair agreement with those of QM and available experimental data. For isolated alanine, compared with those of experi- mental structure, the average absolute deviations（AAD） of bond length and bond angle are 0.002 nm and 1.4~, re- spectively. For alanine-water clusters, the AAD of interaction energies and H-bond lengths are only 3.77 kJ/mol and 0.012 nm, respectively, compared to the results of MP2/aug-cc-pVDZ//MP2/6-31 I＋G＊＊ method. The ABEEMa charges fluctuate with the changing conformation of the system, and can accurately and reasonably reflect the inter- polarization between water and alanine. The presented alanine-water potential function may provide a basis for fur- ther simulations on related aqueous solutions ofbiomolecules.
Constrained molecular dynamics simulations have been used to investigate the LiCl and NaCl ionic association in water in terms of atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM). The simulations make use of the seven-site fluctuating charge and flexible ABEEM-7P water model, based on which an ion-water interaction potential has been constructed. The mean force and the potential of mean force for LiCl and NaCl in water, the charge distributions, as well as the structural and dynamical properties of contact ion pair dissociation have been investigated. The results are reasonable and informative. For LiCl ion pair in water, the solvent-separated ion pair configurations are more stable than contact ion pair configurations. The calculated PMF for NaCl in water indicates that contact ion pair and solvent-separated ion pair configurations are of comparable stability.
The microscopic mechanisms of ion hydration and ion selectivity in biomolecular systems are long-standing research topics, in which the difficulty is how to reasonably and accurately describe the ion-water and ion-biomolecule interactions. This paper summarizes the development and applications of the atom-bond electronegativity equalization fluctuating charge force field model, ABEEM/MM, in the investigations of ion hydration, metalloproteins and ion-DNA bases systems. Based on high-level quantum chemistry calculations, the parameters were optimized and the molecular potential functions were constructed and applied to studies of structures, activities, energetics, and thermodynamic and kinetic properties of these ion-containing sys- tems. The results show that the performance of ABEEM]MM is generally better than that of the common force fields, and its accuracy can reach or approach that of the hlgh-level ab initio MP2 method. These studies provide a solid basis for further investigations of ion selectivity in biomolecular systems, the structures and properties of metalloproteins and other related ion-containing systems.
