Proton NMR relaxation enhancements due to manganese in the photosynthetic oxygen-evolving complex.

Abstract

A very small NMR-PRE-paramagnetic relaxation enhancement (NMR-PRE) has been observed in the S$\sb0$ state of the oxygen evolving center (OEC) of Photosystem II (PSII). This NMR-PRE signal was observed in the S$\sb0$ state of samples containing PSII-enriched thylakoid membrane fragments but not in samples of the octylglucopyranoside-solubilized PSII complex. The appearance of an NMR-PRE signal in the S$\sb0$ state demonstrates that substrate water molecules are in contact with the manganese center of the OEC at this stage of the S-state cycle. The S$\sb0$ NMR-PRE signal has no detectable magnetic field-dependence between 0.48 T and 0.93 T, and very mild if any, temperature-dependence. The absence of detectable temperature-dependence in the S$\sb0$ NMR-PRE signal demonstrates that the relaxation mechanism is not in the chemical exchange-limited regime. This finding excludes the possibility that a water-bound Mn(II) monomer is the origin of the S$\sb0$ NMR-PRE response. The observed proton relaxation efficiency in the S$\sb0$ state is consistent with the expected properties of a Mn(II) ion which is part of a polynuclear exchange coupled center containing one or more Mn(III) ions. An analysis of the aqueous Mn(III) measurements has yielded the chemical exchange lifetime, $\tau\sb{\rm m},$ of water protons (40 $\pm$ 10 $\mu$s), and the inner sphere relaxation time, T$\sb{\rm 1m}$, for deuterons and for protons in the solvation sphere of Mn(III) at 0$\sp\circ$C (920 $\pm$ 230 $\mu$s and 22 $\pm$ 5 $\mu$s respectively), as well as the electron spin relaxation time, $\tau\sb{\rm s},$ of the aqueous Mn(III) ion (20 $\pm$ 10 ps). The observed proton relaxation efficiency of the S$\sb0$ state is approximately the same as that of the aqueous Mn(III) ion, and thus Mn(III) cannot be excluded as the origin of the S$\sb0$ NMR-PRE signal. Alternatively, the proton relaxation efficiency of the S$\sb0$ is consistent with a water-bound Mn(II) ion in a strongly exchange-coupled polynuclear manganese center. Proton relaxation data have been obtained with unprecedented precision, $ca. \pm0.1\%$ for single relaxation measurements and $\pm0.025\%$ with signal averaging. The NMR probe and sample holder were designed to permit highly efficient flash illumination of photosynthetic samples within the NMR probe. (Abstract shortened by UMI.).