Charge transfer effects in the GroEL-GroES chaperonin tetramer in solution

In this work, we present the results of a large-scale, semiempirical LocalSCF quantum mechanical study of GroEL-GroES chaperonin in solution containing 2,481,723 atoms. We find that large biological systems exhibit strong quantum mechanical character, the extent of which was not previously known. Our data show that protein transfers -743 electron units of charge to solvent, which is not described by classical force fields. Contrary to the commonly held belief, which is based on classical mechanics, our computational data suggest that the quantum mechanical effects of charge transfer increase with the size of biological systems. We show that the neglect of charge transfer in classical force fields leads to significant error in the electrostatic potential of the macromolecule. These findings illustrate that a quantum mechanical framework is necessary for a realistic description of electrostatic interactions in large biological systems.