Very high frequency (135 GHz) EPR of the oxidized primary donor of the photosynthetic bacteria Rb. sphaeroides R-26 and Rps. viridis and of Y.D, (signal II) of plant photosystem II

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Abstract
Mm EPR spectroscopy reveals significant differences between the g-tensors of the oxidized primary donor in reaction centers of the photosynthetic bacteria Rb. sphaeroides R-26 and Rps. viridis: the principal values are g, = 2.00159, gw = 2.00224 and g, = 2.00402, and g, = 2.00180, gw = 2.00235 and g, = 2.00380, respectively. Similarly, the g-tensors of the Yi tyrosyl radical in the D, subunit of the reaction center of plant photosystem I1 and of oxidized tyrosine in vitro are found to be quite different, with principal values of g, = 2.00212, gV = 2.00426 and g, = 2.00752, and g,, = 2.00181, gw = 2.00381 and g, = 2.00613, respectively. The observed differences probably reflect differences in the molecular structure of the radicals investigated. In this communication we report on the amxlication of 2 mm (135 GHz) EPR spectroscopy on a number of photosynthetic preparations with the aim to determine the principal values of the g-tensor of some co-factors: the primary donor of two purple bacteria and the tyrosyl donor Y; located on the D, subunit of the reaction center of plant photosystem 11. The much enhanced resolution of 2 mm EPR allows such a determination even for protonated, randomly-oriented preparations. It is shown that the two purple bacteria differ considerably in the g-anisotropy of their primary donors, while the g-anisotropy of the YL radical is appreciately different from that of oxidized tyrosyl in vitro. Since these differences most probably reflect differences in the detailed molecular structure of the co-factors, it is hoped that our data will be an incentive for correlating theoretically the observed g-anisotropies and the co-factor structure with the aid of the recently resolved crystal structure of the reaction centers (the purple bacteria5s6i7) or the models based on these resolved structures (plant photosystem IIh9).

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