The visual pigment 11-cis-retinal is covalently bonded to Lys-296 of the heptahelical membrane protein rhodopsin through a protonated Schiff base linkage. Photoisomerization of this chromophore induces a structural change in the protein that initiates the vision process. Studies have found this isomerization process occurs inside the protein in about 200 femtoseconds. Considering the relative size of the chromophore, its constrained environment, and the way in which a molecule is traditionally considered to rotate during the isomerization process, this time constant is surprisingly fast. Even more intriguing is that in solution, where the chromophore is free from the constraints of the protein, the time constant for the rate of isomerization increases dramatically to 3 picoseconds. Though it is generally accepted the surrounding protein catalyzes the rapid rate of isomerization, the mechanism by which this occurs is not well understood. Volume-conserving mechanisms of photoisomerization, such as the bicycle pedal (BP) and the Hula-twist (HT) have been proposed to explain this enhanced rate. However, little experimental data is available to support this claim. This project proposes to unveil the mechanisms of isomerization in constrained environments. A sterically hindered derivative of 1,4-diphenyl-1,3-butadiene (DPB), 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) will be employed to understand the mechanism of photoisomerization in constrained environments. Through the use of femtosecond transient absorption spectroscopy and picosecond spectroscopy, we will investigate the excited state dynamics of TPB and compare the results to previously studied DPB. From this, we hope to gain new information about the influence of structural details and solvent environment on the isomerization process.The Battelle/Bertram D. Thomas Scholarship FundNo embarg
