publications list
Simple lower bound on the time of photoisomerization of retinal.
E.V. Tsiper
Department of Chemistry, University of Rochester, Rochester, NY
I suggest a simple mechanistic argument that provides a time estimate
for retinal photoisomerization. Retinal molecule plays key role in
photobiology by working as a nature-created photodetector in vision.
Its light-induced photoisomerization is a first step in a long chain
of events that lead from the absorption of a single photon to the
utilization its energy in a biological function [G. Wald, {\em
Science} {\bf 162}, 230 (1968)]. The photoisomerization of retinal is
believed to be complete in only 100---200 fs, making it one of the
fastest known photoreactions [Q. Wang, R.W. Shoenlein, L.A. Peteanu,
R.A. Mathies, and C.V. Shank, {\em Science} {\bf 266}, 422 (1994)].
My estimate is based on the comparison of the kinetic energy of nuclei
necessary to achieve such a fast reaction rate to the energy
$\hbar\omega$ supplied by the photon. Calculation of the reduced
momentum of inertia of a system in a model of classical nuclei give
the minimum kinetic energy about 16 times larger than
$\hbar\omega\approx2.5$ eV. The paradox can be resolved by assuming
that (i) only a short portion of the molecule is affected by the
structural change, however leading to an increase in the elastic
energy for the rest of the molecule, or (ii) that the first stage of
the reaction is, in fact, not isomerization. The last possibility is
in line with recent {\em ab-initio} calculations [M. Garavelli,
P. Celani, F. Bernardi, M.A. Robb, and M.J. Olivucci, {\em
J. Am. Chem. Phys.} {\bf 119}, 6891 (1997)], and experimental findings
[L. Song and M.A. El-Sayed {\em J. Am. Chem. Soc.} {\bf 120}, 8889
(1998)].
Chem. Phys. Letters 342, 652 (2001)
reprint:
PDF