Jump rates for anisotropic particles. Model calculations for the transport of water molecules through membrane channels.

A theoretical analysis for site-to-site jump rates for anisotropic molecules is presented. The molecular shape is regarded as a mechanical anisotropy in the form of finite moments of inertia, as well as anisotropy with respect to the interaction potential. The mutual coupling between rotational and translational motion necessarily produces a competitive effect between the equilibrium alignment in the local field and the precession of the figure axis, leading to an increase of the effective activation energy. As a numerical example the jump rate for a water molecule in a gramicidin-like channel has been calculated, and a temperature-independent reduction of some 15% for the rate as compared to the point-like molecule has been found.