The effect of dipole moment on diffusion controlled bimolecular reaction rates

Abstract

Calculation of diffusion controlled bimolecular reaction rates for complicated macromolecular systems is made possible by a computer simulation approach based on the Brownian dynamics (BD) trajectory method. This is applied to the calculation of the diffusion controlled reaction rate between two charged spherical molecules, one of which represents a protein which possesses a strong dipole moment and asymmetric chemical reactivity. The role of the position of the dipole moment relative to reactive surfaces having various extents in media of varying ionic strength is assessed. No dipolar electrostatic effect on the diffusional rate constant of an isotropically reactive dipolar protein is observed. When the dipolar protein has a small axially symmetric reactive surface (e.g., of 10° extent), the dipole exerts a fivefold enhancement on the rate when the dipole vector is along the reactive patch axis, and retards the rate by a factor of 10 when oppositely disposed. A significant electrostatic effect persists through the physiological ionic strength regime. The positioning of dipolar charges relative to the protein surface at fixed dipole strength has an important effect on the electrostatic influence of the dipole. The variation of the protein dipole relative to its reactive surface can in fact be achieved in practice by chemical modification or site-directed mutagenesis.