The case of medium-dependent dual mechanisms for photoisomerization: One-bond-flip and Hula-Twist

Abstract

This paper critically reviews examples in the literature of photochemical cis-trans isomerization paying particular attention to the medium effect and accompanied conformational changes. A case is made that the Hula-Twist mechanism, postulated in 1985 as a photochemical reaction pathway for a polyene chromophore imbedded in a protein binding cavity such as those of rhodopsin and bacteriorhodopsin, is also a dominant reaction pathway for a diene, or a longer polyene confined in a rigid (relative to isomerization rate) medium. The conventional one-bond-flip process is the preferred reaction pathway in a fluid medium. While defining experiments are proposed, this dual mechanistic approach successfully accounts for all examples in the literature on photoisomerization reactions whether involving conformational changes or not.

Dynamic motion and various reaction paths of cobaloxime complexes in crystalline-state photoreaction

“…In this mechanism, the 1,2-hydrogen migration is not required and the amount of atomic movement of the bece group as a whole becomes considerably smaller. This motion is very similar to the 'hula-twist motion', a model of the cis-trans isomerization of retinylidene chromophore in rhodopsin, as shown in Figure 80 [91][92][93][94]. Such a motion is believed to be very suitable for isomerization in a highly restricted environment, for example, in enzymatic reactions in a living cell or catalytic reactions.…”

Volume-conserving trans–cis isomerization pathways in photoactive yellow protein visualized by picosecond X-ray crystallography

“…The isomerization pathway within the chromophore binding pocket that constitutes a “protein cage” is quite different from that in the gas phase, where chromophores are free from host constraints and may isomerize via the geometrically simpler one-bond-flip pathway 34 . Surprisingly, both the BP and HT pathways (Figs.…”

Photophysics of the blue fluorescent protein

“…This facilitates efficient relaxation in the crowded interior of the protein [13], providing possibility to circumvent steric clashes. We can infer that the hulatwist [14] (simultaneous torsion of both bond angles) is energetically advantageous and explains the small barrier for the relaxation on the excited state surface of the chromophore in BFP. This barrier amounts 0.62 kcal/mol as obtained from Arrhenius plot of fluorescence enhancement on temperature decrease [2] and 1.2 kcal/mol as calculated from temperature dependence of lifetime of the shortest decay component.…”