Following the trail of light: Energy and electron transfer processes and photoprotection mechanisms in photosynthetic complexes
Abstract
Energy and electron transfer processes and photoprotection mechanisms in photosynthetic complexes have been investigated in this thesis. In particular, ultrafast time-resolved spectroscopy and structure based theoretical simulations have been used to determine the effective dielectric constant deep within a protein. Electrostatic screening that is defined by the effective dielectric constant plays critical role in the conformation and function of proteins. To the best of our knowledge, this study is one of only two studies that directly measured this fundamental constant deep within a protein, as such measurements present experimental challenge. Optical femtosecond time-resolved pump-probe techniques have been applied to study energy and electron transfer kinetics in two complimentary point mutants in both branches of the photosystem I reaction center cofactors to resolve the highly controversial issue of whether one or both branches are active in electron transfer under physiologically relevant conditions. Experimental studies to resolve this issue prior to this work had been focused primarily on measurements of changes in the electron transfer kinetics, which reflect the relatively slow electron transfer step where the electron acceptor is common to both branches, consequently leaving some ambiguity in the interpretation. On the contrary, the very first steps of electron transfer were probed in this study, revealing unequivocal corroboration for the asymmetrical electron transfer for the first time. Optical femtosecond and nanosecond time-resolved pump-probe spectroscopy have been also used to study singlet and triplet excited state dynamics of the unique chlorophyll a and carotenoid molecules in the cytochrome b6f complex. Based on the experimental results, two unconventional protection mechanisms against light-induced photodegradation had been determined. These kinds of photoprotection mechanisms have not yet been reported. Furthermore, alternative quenching mechanism for the singlet excited state of chlorophyll a has been unconcealed in chlorophyll-containing pigment-protein complex for the first time. Explicit molecular dynamics simulation has been employed to investigate intraprotein oxygen channel within the integral membrane pigment-protein cytochrome b6f complex that is proposed to facilitate an anomalous long-range triplet energy transfer. Femtosecond time-resolved transient absorption techniques have been utilized to study the excited state kinetics of the unique chlorophyll a molecule of the cytochrome b6 f complex in two different crystals as well as in re-dissolved crystals. The optical properties of the latter depend on the surrounding local protein structure. In particular, the singlet excited state of the chlorophyll a molecule is quenched by the surrounding protein matrix, and thus the chlorophyll a molecule in the b 6f complex may serve as a probe of the protein structure. By comparing the kinetics in single crystals and in solution, the crystal packing influence on the multi-subunit protein structure has been assessed. To the best of our knowledge, this is one of the pioneering studies in the field. Steady state spectroscopy has been used to explore weakening of the P700 excitonic coupling induced by structural rearrangements propagated through quaternary structure caused by deletion peripheral subunit(s) in photosystem I.
Degree
Ph.D.
Advisors
Savikhin, Purdue University.
Subject Area
Biophysics
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