Near-IR Irradiation of the S[sub 2] State of the Water Oxidizing Complex of Photosystem II at Liquid Helium Temperatures Produces the Metalloradical Intermediate Attributed to S[sub 1]Y[sub Z]•.

Near-IR (NIR) excitation at liquid He temperatures of photosystem II (PSII) membranes from the cyanobacterium Synechococcus vulcanus or from spinach poised in the S[sub 2] state results in the production of a g = 2.035 EPR resonance, reminiscent of metalloradical signals. The signal is smaller in the spinach preparations, but it is significantly enhanced by the addition of exogenous quinones. Ethanol (2-3%, v/v) eliminates the ability to trap the signal. The g = 2.035 signal is identical to the one recently obtained by Nugent et al. by visible-light illumination of the S l state, and preferably assigned to S[sub 1]Y[sub Z, sup •] [Nugent, J. H. A., Muhiuddin, I. P., and Evans, M. C. W. (2002) Biochemistry 41, 4117-4126]. The production of the g = 2.035 signal by liquid He temperature NIR excitation of the S[sub 2] state is paralleled by a significant reduction (typically 40-45% in S. vulcanus) of the S[sub 2] state multiline signal. This is in part due to the conversion of the Mn cluster to higher spin states, an effect documented by Boussac et al. [Boussac, A., Un, S., Homer, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001-4007], and in part due to the conversion to the g = 2.035 configuration. Following the decay of the g = 2.035 signal at liquid helium temperatures (decay halftimes in the time range of a few to tens of minutes depending on the preparation), annealing at elevated temperatures (-80 °C) results in only partial restoration of the S2 state multiline signal. The full Size of the signal can be restored by visible-light illumination at -80 °C, implying that during the near-IR excitation and subsequent storage at liquid helium temperatures recombination with Q[sub A, sup -] (and therefore decay of the S[sub 2] state to the S[sub 1] state) occurred in a fraction of centers. In support of this conclusion, the g = 2.035 signal remains stable for several hours (at 11 K) in centers poised in the S[sub 2]···Q[sub A]... [ABSTRACT FROM AUTHOR]