  A mechanism for relationship of solvent viscosity with reaction rate constant
at enzyme action is suggested. It is based on fluctuations of electric field in
enzyme active site produced by thermally equilibrium rocking (cranckshaft
motion) of the rigid plane (in which the dipole moment $\approx 3.6 D$ lies) of
a favourably located and oriented peptide group (or may be a few of them). Thus
the rocking of the plane leads to fluctuations of the electric field of the
dipole moment. These fluctuations can interact with the reaction coordinate
because the latter in its turn has transition dipole moment due to separation
of charges at movement of the reacting system along it. The rocking of the
plane of the peptide group is sensitive to the microviscosity of its
environment in protein interior and the latter is a function of the solvent
viscosity. Thus we obtain an additional factor of interrelationship for these
characteristics with the reaction rate constant. We argue that due to the
properties of the cranckshaft motion the frequency spectrum of the electric
field fluctuations has a sharp resonance peak at some frequency and the
corresponding Fourier mode can be approximated as oscillations. We employ a
known result from the theory of thermally activated escape with periodic
driving to obtain the reaction rate constant and argue that it yields reliable
description of the preexponent where the dependence on solvent viscosity
manifests itself. The suggested mechanism is shown to grasp the main feature of
this dependence known from the experiment and satisfactorily yields the upper
limit of the fractional index of a power in it.
