Objective
Photosynthetic water oxidation that occurs in photosystem II (PSII) is one of the key processes in photosynthesis. Understanding the reactions in PSII on molecular and electronic level is of fundamental interest for photosynthesis research and also important for the development of artificial photosystems for energy production. A redox active tyrosine (YZ) is known to be essential for the electron transfer reactions that ultimately lead to water oxidation in PSII and it has also been suggested that this amino acid is directly involved in catalysing water oxidation. The mechanism for YZ redox reactions, and specifically the mechanism for proton coupled redox reactions, is however unresolved. The mechanism for YZ oxidation is part of this proposal.
The first crystallographic data on an atomic level that was recently published suggests that a calcium (Ca) ion is the metal closest to YZ and that this Ca ion is an integral part of the catalytic water oxidation site. Clearly the role of Ca is at the heart of the mechanism for YZ redox chemistry as well as for the catalytic water-splitting activity in PSII. To find the impact of Ca on YZ redox kinetics as well as on the water oxidation activity in PSII I will use Ca depleted PSII samples. On this material I will perform a complete study of the effect of pH and temperature on YZ redox kinetics to determine the mechanism for YZ oxidation using time resolved absorption measurements.
I have previously shown that the pH-dependence of tyrosine oxidation can be used for mechanistic discrimination for proton coupled electron transfer reactions in synthetic tyrosine model complexes. In parallel I will examine the redox events occurring in the water oxidising complex using advanced EPR to determine the functional role of Ca for water oxidation activity in PSII. In addition PS II samples where Ca has been replaced with the larger strontium ion will be examined using the same techniques to determine the structural role of Ca.
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Call for proposal
FP6-2002-MOBILITY-5
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