spectroscopic properties of photosynthetic complexes and help unravel their structure-function relationship including their exciton transfer properties [1-4]. [1] M. Aghtar, J. Stru¨mpfer, C. Olbrich, K. Schulten, U. Kleinekatho¨fer, J. Phys. Chem. B 117, 7157 (2013). [2] C. Olbrich, ..., U. Kleinekatho¨fer, J. Phys. Chem. B 115, 8609 (2011). [3] C. Olbrich, J. Stru¨mpfer, K. Schulten, U. Kleinekatho¨fer, J. Phys. Chem. Lett. 2, 1771 (2011). [4] C. Olbrich, J. Stru¨mpfer, K. Schulten, U. Kleinekatho¨fer, J. Phys. Chem. B 115, 758 (2011). 920-Pos Board B675 The Fate of the Triplet Excitations in the Fenna-Matthews-Olson Complex and Stability of the Complex Shigeharu Kihara 1 , Daniel Hartzler 1 , Gregory S. Orf 2 , Robert E. Blankenship 2 , Sergei Savikhin 1 . 1 Physics, Purdue University, West Lafayette, IN, USA, 2 Chemistry, Washington University in St. Louis, Saint Louis, MO, USA. The Fenna-Matthews-Olson (FMO) protein complex contains strongly coupled bacteriochlorophyll a (BChl a) pigments and is part of a light harvesting antenna of green sulfur bacteria. This complex is very stable, in spite of the fact that it contains no carotenoids that are universally found to protect pigment-protein complexes against highly reactive singlet oxygen, which can form due to the transfer of energy from the triplet excited states of BChl mol- ecules. In this work the dynamics of triplet excited states in FMO are investi- gated by means of nanosecond time resolved pump-probe spectroscopy. It is inferred, that triplet excited state is formed concurrently on three pigments within the FMO subunit, and its energy is then rapidly transferred to the lowest triplet energy pigment. Mixed singlet-triplet excitonic quantum-mechanical simulations reveal the structural positions of these pigments. It is concluded, that pigment #3 in the structure has the lowest triplet state energy, and that this energy is below that of the singlet oxygen, blocking energy transfer and preventing the formation of singlet oxygen. The latter is confirmed by direct measurements of BChl a triplet state energy via phosphorescence and indepen- dence of triplet state lifetime (55 ms) in FMO on the oxygen concentration. It is thus feasible to have natural antenna systems that are inherently stable without the need for carotenoids in the structure. The authors acknowledge the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE- FG02-09ER16084 for funding these studies. 921-Pos Board B676 Structural Basis for the Non-Photochemical Quenching Switch of the Green Alga Chlamydomonas Reinhardtii nicoletta liguori, Laura M. Roy, Milena Opacic, Roberta Croce. Department of Physics and Astronomy and Institute for Lasers, Life and Biophotonics, VU University of Amsterdam, Amsterdam, Netherlands. Feedback mechanisms that dissipate excess photo-excitations in Light- Harvesting Complexes (LHCs) are necessary to avoid detrimental oxidative stress in most photosynthetic eukaryotes. Here we demonstrate the unique ability of LHCSR, a stress related LHC from the model organism Chlamydomonas rein- hardtii, to sense pH variations, tuning its conformation from a light-harvesting state to a dissipative one. This conformational change is induced exclusively by the acidification of the environment. We show that this ability to respond to different environments is missing in the related major Light Harvesting Complex II (LHCII) despite the high structural ho- mology. Via mutagenesis and spectros- copy characterization we show that LHCSR uniqueness relies on its peculiar C-terminus subdomain, which acts as a sensor of the lumenal pH, able to tune the quenching level of the complex. 922-Pos Board B677 Single-Molecule Exploration of the Photodynamics of LHCII Complexes in Solution Gabriela S. Schlau-Cohen 1 , Hsiang-Yu Yang 1 , Michal Gwizdala 2 , Tjaart Kru¨ger 3 , Pengqi Xu 2 , Roberta Croce 2 , Rienk van Grondelle 2 , W.E. Moerner 1 . 1 Department of Chemistry, Stanford University, Stanford, CA, USA, 2 Department of Physics and Astronomy, VU University, Amsterdam, Netherlands, 3 Department of Physics, University of Pretoria, Pretoria, South Africa. Plants can safely dissipate excess excitation energy during light harvesting to prevent the formation of triplet chlorophyll, which can generate deleterious singlet oxygen. With this regulation, known as non-photochemical quenching (NPQ), efficient light harvesting and photosynthesis can be balanced under fluctuating sunlight intensity without damage to the photosynthetic machinery. NPQ has been extensively studied and key physiological regulatory factors have been identified. For example, it is known that the change in lumen pH or a pH gradient across thylakoid membrane can trigger an energy-dependent quenching (qE) pathway that activates on a timescale of seconds to minutes. However, understanding the molecular mechanism behind NPQ is still a chal- lenge. One important question is whether, upon activation of qE, the number of quenched complexes increases or the degree of quenching in each complex changes. These two cases cannot be differentiated in ensemble-averaged mea- surements. Therefore, we use the Anti-Brownian ELectrokinetic (ABEL) trap to investigate single copies of light-harvesting complex II (LHCII), the primary antenna in higher plants. Different from other single-molecule techniques that utilize immobilization, perturbations due to surface attachment or encapsula- tion are avoided, and therefore the intrinsic dynamics and heterogeneity of in- dividual complexes are revealed. We perform measurements of fluorescence intensity, excited-state lifetime, and emission spectra of single LHCII com- plexes in the ABEL trap. By analyzing the correlated changes in these proper- ties over time, we observe that individual LHCII complexes are found in distinct forms with different extents of quenching. Comparing the results from different conditions known to correlate with qE activation will give a bet- ter understanding of NPQ at the molecular level. 923-Pos Board B678 Coherent Exciton Dynamics in Photosynthetic Bacteria Petra E. Edlund. Chemistry an molecularbiology, Gothenburg University, Gothenburg, Sweden. Rhodobacter sphaeroides is a photosynthetic bacteria that has the ability to adopt to the environment and live either as chemotrophs or phototrophs, car- rying out photon capture to acquire energy. Photosynthetic bacteria harvest sun light in so called antenna pigments and then the excited state energy is transferred to a membrane bound protein called the reaction center. The reaction center contains chromophores and can also absorb sunlight and utilize the light energy to create a transmembrane electrochemical potential over the membrane, which is the driving force for the synthesis of ATP, the energy storage in the cells. The excitation is used to transport an elec- tron through the protein, a process called charge separation and efficiency of this is extraordinary. We study the role of coherences between the cofactors in the photo response. Time resolved laser spectroscopy has been a tool for characterization of the light harvesting processes in detail and the development of ultrashort laser pulses in the femtosecond timescale, allowed resolution of the fastest light har- vesting processes and made it possible to prepare coherent superposition of excited electronic and vibrational states of the molecules. With the use of two-dimensional electronic spectroscopy and by controlling the polarization of the laser beams used to trigger and probe the energy transfer between the co- factors in the protein we could study the oscillatory dynamics between intermo- lecular chromophores and assign them to electronic coherences. The coherences of the energy transfer lives longer than 1 ps, indicating that, at this time scale, the reaction center coherently retains excitation energy in the chromophores. This provides a suitable mechanism for delocalization of the excitation energy over the chromophores which would facilitate the electron transfer and thereby setting the stage for efficient charge separation. 924-Pos Board B679 Regioselective Reduction Pathway of Geranylgeranyl Moiety in Chlorophyll Biosynthesis Kota Nomura, Tadashi Mizoguchi, Hitoshi Tamiaki. Ritsumeikan University, Kusatsu, Japan. Chlorophylls in photosynthetic organisms are uti- lized in light-harvesting and energy-transfering processes at antenna complexes as well as charge-separating events at reaction centers. Naturally occurring hlorophylls except for most chlorophylls-c possess an isoprenoid ester group in the propionate-type residue at the 17-position. At the final stage of their biosynthetic pathway, a geranylgeranyl group (GG) at the ester chain is regioselectively and stereoselectively reduced to a phytyl group via the intermediates, dihydrogera- nylgeranyl (DHGG) and etrahydrogeranylgeranyl (THGG) groups. In this study, three regioiso- meric 6,7-, 10,11-, and 14,15-DHGG alcohols 182a Sunday, February 16, 2014