Time-Domain EPR Studies of Tyrosine Z of Photosystem II

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9513648 Britt The Photosystem II component of the plant photosynthetic apparatus utilizes light energy to oxidize water and reduce membrane diffusable plastoquinone. Photon-induced charge separation occurs at the chlorophyll moiety P680 . The electron transfer components that donate to the photogenerated chlorophyll cation P+680 are unique to Photosystem II. A tyrosine residue designated as YZ serves as an electron transfer intermediate between P+680 and the manganese cluster of the oxygen-evolving complex (OEC) where water oxidation occurs. Fast optical excitation will be combined with pulsed electron paramagnetic resonance (EPR) spectroscopy to examine the role of YZ in electron transfer and water oxidation. With laser pulsewidths of (3 ns, and microwave pulsewidths of (10 ns, we will be able to measure the kinetics of formation and depletion of the YZ( tyrosyl radical at each of the "S-state" transitions of the oxygen evolving cycle. The kinetic isotope effects will be measured by repeating these experiments in 2H2O-enriched buffer. The time dependence of the YZ EPR lineshapes will be examined at each S-state transition to look for evidence of magnetic interactions with the Mn cluster and/or conformational changes of the tyrosyl radical. Electron spin echo envelope modulation (ESEEM) and electron spin echo - electron nuclear double resonance (ESE-ENDOR) experiments will be performed to further test recent models that postulate that YZ( acts directly to abstract protons or hydrogen atoms from water ligands to the Mn cluster. These experiments will greatly advance our knowledge of the structure and dynamics of the Photosystem II reaction center. The experiments represent a state-of-the-art merging of fast laser excitation with pulsed EPR methods and provide excellent training opportunities for a number of graduate students and postdoctoral researchers. %%% The Photosystem II component of the plant photosynthetic apparatus utilizes light energy to strip electrons from water mo lecules, with molecular oxygen released into the atmosphere as a fortunate byproduct. When light is absorbed by a certain chlorophyll molecule designated P680, it spontaneously transfers an electron to an adjacent pigment molecule. This light-induced charge separation is the first step in converting light energy into useful chemical energy. The resulting positively charged chlorophyll P+680 molecule has a strong affinity to regain an electron, and the electron comes initially from a tyrosine amino acid residue of the surrounding protein. This tyrosine residue, designated YZ, in turn pulls an electron from a small cluster of manganese atoms where the water-splitting chemistry is thought to occur. Our experiments will examine the magnetic properties of this YZ tyrosine during the fast electron transfer events. Laser light pulses of only 3 billions of a second duration will be combined with comparably short microwave pulses to measure the changes of magnetic properties on this ultra-short timescale. The measurements will be interpreted to understand the dynamics of this YZ tyrosine during electron transfer. The goal is to test controversial new models that propose that the YZ tyrosine pulls protons as well as electrons off water molecules bound to the metal cluster. These experiments will greatly advance our knowledge of the structure and dynamics of the Photosystem II reaction center. The experiments represent a state-of-the art merging of fast laser excitation with pulsed magnetic resonance methods and provide excellent training opportunities for a number of graduate students and postdoctoral researchers. ***