ARGONNE NATIONAL LABORATORY

CHEMICAL SCIENCES AND ENGINEERING

DIVISION SEMINAR

 

SPEAKER:   Serguei Vassiliev

                        Brock University   

                        St. Catharines, Ontario, Canada

 

TITLE:           Molecular Mechanisms of Light Conversion in Photosystem II

 

DATE:            Wednesday Morning

                        November 21, 2007

 

TIME:             11:00 am

 

PLACE:         Building 200, J-193

 

HOST:           David Tiede

 

Abstract:  Conversion of light energy in photosynthesis is extremely fast and efficient, and understanding the nature of this complex photophysical process is challenging as individual cofactors are hard to “see” spectroscopically due to the many pigments involved. Extracting rates for energy transfer and electron transport requires the application of kinetic models to simulate experimental data. I will talk about our approach to kinetic modeling of excited state dynamics and trapping in PSII. We used structure-based kinetic models to analyze chlorophyll fluorescence decay data from isolated PSII and found that dynamic changes in energy trapping during the lifetime of the excited state were required to describe the data. This result stimulated our desire to learn more about the protein dynamics of PSII. We have recently performed molecular dynamics simulation to study photosystem II structure and function. Structural information obtained from simulations was combined with computations of chromophore excited states and electron transfer rates.  We found that the fluctuating energy levels of antenna chromophores had a larger impact on quantum yield than did their relative positions. Variations in electron transport rates had the most significant effect and were sufficient to explain the experimentally observed multicomponent decay of excitation in PSII. As successful as it was, our first simulation did not consider the appearance of charges within PSII associated with photochemistry. Light induced formation of the charge separated state and subsequent electron transport will influence protein structure/dynamics and affect the protonation state of titratable amino acids. Charges arising from protonation changes may, in turn, influence the dynamic structure and subsequent electron transport steps. We are currently calculating protonation states in PSII and are integrating these calculations into our existing computational strategies to form a powerful combination of interacting approaches able to elucidate the dynamic evolution of PSII structure and function during photochemistry.