Low temperature optical - MCD and EPR studies on a fully active plant PS II complex containing 32 Chl per reaction centre.

Abstract

A new highly active PS II core complex preparation containing only 32 chlorophyll (chl) a per reaction centre has been developed. Low temperature Optical absorption spectroscopy (<4K) results from this material exhibit a higher degree of spectral homogeneity and pigment component resolution than any other hitherto reported (including RC complexes). In the chl Qy region, the P680 dimer feature is fully reslved with a width of only ~50cm-1. Both reaction centre Pheophytins (Pheo) are resolved in the Qx region, around 540nm, with substantially different line widths. This material generates the Mn derived multiline (ML) signal of the formal S2 state by appropriate low temperature illumination, as well as the near infra red induced transition between this signal and the g4.1 signal. On transition from the dark, S1, state to the ML or g4.1 forms, pronounced Stark effects are observed on the P680 and Pheoa active branch spectral features due to a single electron displacement from the Mn centre to Qa -. These effects are quantitavely very different for the ML and the g4.1 cases (more intense in the latter). The electrochromic responses are calibrated from the known separation between Qa and Pheoa and the most conservative estimates for the protein dielectric behaviour. This then suggests that the positive hole developed in the Mn centre moves around 10Å towards Pheoa in the interconversion from the ML to g4.1 signal forms of the S2 state, and at least 10Å closer to P680. Implications of these results for possible structures of the Mn catalytic site will be discussed.