An Nmr Study Of The Catalytic Cycle Of Photosynthetic Water Oxidation.

Abstract

Flash-induced changes in the solvent proton NMR relaxation rate R$\sb 1$ were used to probe photosynthetic water oxidation in Photosystem II particles prepared from spinach. A series of control experiments were done to show that the measured R$\sb 1$ changes reflected S-state related chemistry. A flash dependent R$\sb 1$ transient profile from 1 to 5 flashes was done and possible Mn oxidation chemistry which would cause similar R$\sb 1$ changes were speculated. The positive R$\sb 1$ transient caused by one flash illumination was consistent with the expected change for a Mn(III) $\rightarrow$ Mn(IV) oxidation for the S$\sb 1$ to S$\sb 2$ transition. The absence of a further R$\sb 1$ enhancement on two-flash illumination suggested that the S$\sb 2$ to S$\sb 3$ transition is not accompanied by a Mn oxidation. A positive R$\sb 1$ transient on the third flash suggested that in going from S$\sb 1$ to S$\sb 0$, a strongly relaxing center is created. This is consistent with a Mn(III) $\rightarrow$ Mn(II) reduction for the S$\sb 1 \rightarrow$ S$\sb 0$ transition. Flash profile of the amplitude of the R$\sb 1$ change showed the expected minimum on the fourth flash. A theoretical simulation of the R$\sb 1$ amplitude flash profile showed a best fit when 15% of the centers were assumed to be damaged, i.e. incapable of S-state cycling beyond the S, state. Hydroxylamine titrations of control and 23 kDa and 17 kDa protein-depleted PS II particles were done. The functional Mn in peptide-depleted OECs was found to be more accessible to solvent water molecules resulting in a larger enhancement contribution to the relaxation of the water molecules.