Hula-Twist, A Supramolecular Photochemical Reaction Mechanism. A New Perspective of Photoisomerization

University Of Hawaii, Honolulu

Investigators

Abstract

With the support of the Organic and Macromolecular Chemistry Program in the Chemistry Division, Professor Robert S. H. Liu, of the Department of Chemistry at the University of Hawaii- Manoa, will study a volume-conserving photochemical cis-trans isomerization reaction mechanism postulated in 1985 to account for the rapid photoisomerization reaction of the retinyl chromophore in the visual pigment rhodopsin. This mechanism is referred to as the Hula-Twist (HT). Its salient stereochemical feature is the simultaneous configurational and conformational transformation of two adjacent double and single bonds. Model compounds designed to provide distinguishable chemistry between Hula-Twist (HT) and the conventional one-bond-flip (OBF) mechanism will be prepared and their photochemical and photophysical properties will be examined. The medium is expected to play an important role to inhibit the OBF process allowing detection of the normally more difficult HT process. However, other conditions that might result in re-directing molecules from reacting in an OBF manner to the HT manner are also present and discussed. Meaningful model compounds used for test experiments are proposed. Among photosensitive bio-molecules containing a Pollyanna chromophore, the isomerization of PYP (photoactive yellow protein that makes bacteria response repulsively toward blue light) and bilirubin (the yellow bile pigment involved in jaundice and its phototherapy) can be rationalized by the involvement of HT in their photochemical reactions. Experiments are now proposed to prove unambiguously of its involvement. For the visual pigment rhodopsin and the energy storage system bacteriorhodopsin, HT is no longer considered likely to be the sole process responsible for the observed photochemistry. Recent X-ray crystal structural work on the primary photoproduct of bR (K) showed that the structure is not that from the once proposed HT -I 4 process. A modified pathway for both bR and rhodopsin is now proposed. HT has been retained in the initial photochemical process. However, before its completion, the process is likely to be re-directed by the rigid protein cavity to a series of volume-conserving bicycle pedal (BP) processes (in the ground state) leading to the more stable 13-cis or all-trans products. New experiments designed to test these ideas are proposed. The proposed experiments will be carried out by Professor Liu's group, concentrating on chemical synthesis and low temperature photochemical-spectroscopic studies in collaboration with a network of international specialists (time-resolved UV-Vis and FT-IR and other spectroscopic and biophysical studies). The common goal is to understand the nature of the very fundamental photochemical cis-trans isomerization that happens in native systems as well as in laboratories and to determine the scope of the novel two-bond twist mechanism. Perhaps this research can finally provide a definitive answer to the question of how the seemingly volume-demanding photoisomerization process of a polyene chromophore in a highly specific protein binding site can take place in less than a picosecond, a time scale much too fast for protein reorganization.