Development of Two-dimensional Stark Spectroscopy for the Investigation of Photosynthetic Charge Separation

[摘要] Charge transfer reactions are critical for the efficient function ofphotosynthetic enzymes. With growing energy demand, understanding the design principles of natural photosynthetic systems is important to aidefforts in developing sustainable energy sources that do not add to thecarbon-dioxide burden of the atmosphere. Photosystem II is particularlyinteresting because it is an ideal model for artificial photovoltaic devices for energy applications: it is efficient, stabilizes the energized state for useful times, and is resilient to photo-damage. Despite decades of study, the mechanism of primary charge separation in this system is still under debate, primarily because the charge-transfer intermediates involved in these reactions do not have strong spectral signatures and are extremely short-lived.I have developed a novel spectroscopy method called two-dimensional electronic Starkspectroscopy (2DESS) for the study of fast processesinvolving the movement of charge in photosynthetic proteins. It combines thehigh sensitivity of Stark spectroscopy to charge-transfer reactions andthe high temporal and spectral resolution of two-dimensional electronicspectroscopy. In collaboration with Darius Abramavicius at Vilnius University in Lithuania, I simulated acharge-transfer dimer system similar to the ``special-pair;; chlorophylls foundin PSII RC thought to be involved in the primary charge-separation process inthis system. Based on these simulations, I demonstrated that the 2DESS andStark spectra for CT states in the PSII do not follow typical Liptay models.There is also evidence to suspect that the parameters used to model the PSIIis incorrect. I then demonstrated the experimental technique on an organic polymeroften used for photovoltaic applications, observing first-derivativelineshapes consistent with predictions. Following this demonstration, Iobserved spectral signatures consistent with charge-separation of the PSII RC.Work is underway to extend the simulations to a more completemodel system, as well as utilize the experimentally-obtained data to verifyproposed models of charge-separation in the PSII RC. In combination with other spectroscopy techniques, 2DESS will allow us to obtain a complete description of the initial charge-separation kinetics in photosystem II and may suggest ways to mimic its extraordinary efficiency. We expect thistechnique to be applicable to other systems such as organic photovoltaics, inwhich the role of CT states is unclear or is hard to trace.

[发布机构] University of Michigan

[效力级别] photosynthesis

[关键词] nonlinear optical spectroscopy;photosynthesis;two-dimensional electronic spectroscopy;photosystem II;Physics;Science;Biophysics