State-of-the-art femtosecond to nanosecond time-resolved spectroscopic techniques are being used to study reaction mechanism in complicated biological systems. Current projects are focused on investigating the primary charge separation and energy transfer processes in the photosystem II reaction center of green plants, reaction mechanism in B12 dependent enzymes, and the photochemical ring-opening and cis-trans isomerization reactions of provitamin D3 (7-dehydrocholesterol) and previtamin D3.
Photosynthetic organisms, both plants and bacteria, store photon energy via charge separation across a membrane. Currently, our group is using femtosecond transient absorption spectroscopy, in conjunction with picosecond fluorescence measurements to unravel the mechanism of primary charge separation in the photosystem II reaction center of plants. Our work on PSII, carried out in collaboration with Professor Yocum, is exploring the charge separation process in a variety of reaction center preparations, differing in the number of proteins, the number of chlorophyll present, and in the preparation protocols.
The second project underway in our group uses time-resolved spectroscopy to study reaction mechanism in B12 dependent enzymes. The key catalytic element in B12 is a unique carbon-cobalt bond. Reactivity in B12 enzymes involves homolysis (adenosylcobalamin dependent enzymes) or heterolysis (methylcobalamin dependent enzymes) of the active carbon-cobalt bond. Although the B12 catalysis is not naturally photoinitiated, a photon can be used to trigger bond homolysis in both enzyme bound and free B12 coenzymes. In addition, protection from photolysis has been proposed as a protein function in B12 systems. The aim of our research program is to use time-resolved spectroscopy to elucidate the role of the protein in controlling and directing the reactivity of the carbon-cobalt bond.
Femtosecond time-resolved deep ultraviolet spectroscopy is being used to investigate polyene photochemistry. Vitamin D3 is produced through ultraviolet photolysis of 7-dehydrocholesterol in the skin. The initial step is the ring-opening reaction of 7-dehydrocholesterol to form previtamin D3. In our laboratory we have been investigating the photochemical ring opening reaction of both 1,3-cyclohexadiene and the more complicated 7-dehydrocholesterol chromophore.