Optimizing the hydrogen-bond network in Poisson-Boltzmann equation-based pK(a) calculations

pK(a) calculation methods that are based on finite difference solutions to the Poisson-Boltzmann equation (FDPB) require that energy calculations be p erformed for a large number of different protonation states of the protein. Normally, the differences between these protonation states are modeled by changing the charges on a few atoms, sometimes the differences are modeled by adding or removing hydrogens, and in a few cases the positions of these hydrogens are optimized locally. We present an FDPB-based pK(a) calculation method in which the hydrogen-bond network is globally optimized for every single protonation state used. This global optimization gives a significant improvement in the accuracy of calculated pK(a) values, especially for bur ied residues. It is also shown that large errors in calculated pK(a) values are often due to structural artifacts induced by crystal packing. Optimiza tion of the force fields and parameters used in pK(a) calculations should t herefore be performed with X-ray structures that are corrected for crystal artifacts. Proteins 2001;43:403-412. (C) 2001 Wiley-Liss, Inc.