Kansas State University

Chemistry professor receives financial award for photosynthesis research

Submitted by Ryszard Jankowiak

Ryszard Jankowiak, professor of chemistry, has received a U.S. Department of Energy Basic Energy Science Award for $500,000 over three years for photosynthesis research. Jankowiak's project is titled, "Resonant and nonresonant hole-burning and delta fluorescence line-narrowing study of Bacteriochlorophyllss in excitonically coupled photosynthetic systems."

Protein environment, pigment organization/energies and pigment-protein interactions in photosynthetic complexes are critical for rapid excitation energy transfer and/or electron transfer processes. This project focuses on excitonically coupled bacteriochlorophylls that are ubiquitous in bacterial photosynthetic complexes. Hole-burning and delta fluorescence line-narrowing spectroscopies and modeling studies will be used to provide additional insight into the excitonic structure and vibrational frequencies, electron-phonon and vibronic couplings, ligation of pigments, and excitation energy transfer processes in the following three protein systems. 1) De novo designed proteins with one and two Zn-Bacteriochlorophylls will provide relatively simple functional models of Bacteriochlorophylls in the protein environment as well as serve as a complementary strategy for better understanding the roles of specific elements within the natural systems. 2) The Zn-reaction center (RC) protein from Rb. Sphaeroides (Zn-RC with six Zn-Bacteriochlorophylls) and its mutants (e.g. the Zn-RC, in which the H_A cofactor has the Zn penta-coordinated with the fifth ligand coming from a histidine introduced by site-directed mutagenesis). It is of interest to evaluate how the coordination state of the Zn in the H_A Zn-bacteriochlorophylls affects excitonic structure, hole-burning spectra, and excitation energy transfer/electron transfer rates. Hole-burning spectra in Zn-RC with the initial electron donor oxidized or missing will provide additional insight into the hole-burning process in proteins. 3) The Fenna-Matthews-Olson (FMO) antenna protein (with 7-8 bacteriochlorophylls per monomer) is a model system for photosynthetic antennas found in anoxygenic green sulfur photosynthetic bacteria.

To resolve some controversies regarding the nature of various optical spectra previously reported for FMO, we propose to reexamine the previously studied species (i.e. P. aestuarii and C. tepidum) and study a new variant of C. tepidum carrying Bacteriochlorophylls a esterified by geranylgeraniol and a new FMO protein from P. phaeum for comparison. This study is warranted due to the discovery of the 8^th bacteriochlorophylls molecule in addition to the seven molecules that have been known and studied experimentally and theoretically for many years. The 8^th pigment is sandwiched between two monomers and has Bacteriochlorophylls ligands from two different subunits. This work will be done in collaboration with R. Blankenship who will provide FMO with a controlled amount of the 8^th pigment and FMO mutants. In this respect, we anticipate the calculated site energies and site energies obtained from a fitting algorithm of various optical spectra, in particular the shapes of hole-burning spectra in the Q_y and Q_x regions, will provide more insight into their excitonic structure. The 8^th pigment should affect the previous interpretation of excitonic structure and dynamics. It is anticipated that our methodologies and excitonic calculations will identify the contribution of the 8^th pigment to the optical spectra of the FMO protein, shed more light on its ligation status, and elucidate the degree of its excitonic coupling with the other pigments. It is important to determine the site energies of all pigments to obtain a more complete picture of the excitation energy transfer pathway in these systems. Finally, we anticipate that better understanding of the experimentally observed anti-hole and photoproduct distribution in Zn-bacteriochlorophylls monomers, excitonically coupled Zn-bacteriochlorophylls dimer, and multichromophoric Bacteriochlorophylls systems will provide a better framework for probing the electronic structure, via hole-burning spectroscopy, of complex biological complexes.