The case of medium-dependent dual mechanisms for photoisomerization: one-bond-flip and hula-twist.
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.
Liu
R S
RS
Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA. rliu@gold.chem.hawaii.edu
Hammond
G S
GS
D007536:Isomerism; D010777:Photochemistry; D011487:Protein Conformation