Lennard-Jones Parameters for B3LYP/CHARMM27 QM/MM Modeling of Nucleic Acid Bases

Combined quantum mechanics/molecular mechanics (QM/MM) methods allow computations on chemical events in large molecular systems. Here, we have tested the suitability of the standard CHARMM27 forcefield Lennard-Jones van der Waals (vdW) parameters for the treatment of nucleic acid bases in QM/MM calculations at the B3LYP/6-311+G(d,p)-CHARMM27 level. Alternative parameters were also tested by comparing the QM/MM hydrogen bond lengths and interaction energies with full QM [B3LYP/6-311+G(d,p)] results. The optimization of vdW parameters for nucleic acid bases is challenging because of the likelihood of multiple hydrogen bonds between the nucleic acid base and a water molecule. Two sets of optimized atomic vdW parameters for polar hydrogen, carbonyl carbon, and aromatic nitrogen atoms for nucleic acid bases are reported: base-dependent and base-independent. The results indicate that, for QM/MM investigations of nucleic acids, the standard forcefield vdW parameters may not be appropriate for atoms treated by QM. QM/MM interaction energies calculated with standard CHARMM27 parameters are found to be too large, by around 3 kcal/mol. This is because of overestimation of electrostatic interactions. Interaction energies closer to the full QM results are found using the optimized vdW parameters developed here. The optimized vdW parameters [developed by reference to B3LYP/6-311+G(d,p) results] were also tested at the B3LYP/6-31G(d) QM/MM level and were found to be transferable to the lower level. The optimized parameters also model the interaction energies of charged nucleic acid bases and deprotonation energies reasonably well.