Optical identification of the long-wavelength (700-1700 nm) electronic excitations of the native reaction centre, Mn 4 CaO 5 cluster and cytochromes of photosystem II in plants and cyanobacteria

Abstract

Visible/UV absorption in PS II core complexes is dominated by the chl-a absorptions, which extend to ~ 700 nm. A broad 700-730 nm PS II core complex absorption in spinach has been assigned [1] to a charge transfer excitation between ChlD1 and ChlD2. Emission from this state, which peaks at 780 nm, has been seen [2] for both plant and cyanobacterial samples. We show that Thermosynechococcus vulcanus PS II core complexes have parallel absorbance in the 700-730 nm region and similar photochemical behaviour to that seen in spinach. This establishes the low energy charge transfer state as intrinsic to the native PS II reaction centre. High-sensitivity MCD measurements made in the 700-1700 nm region reveal additional electronic excitations at ~ 770 nm and ~ 1550 nm. The temperature and field dependence of MCD spectra establish that the system peaking near 1550 nm is a heme-to-Fe(III) charge transfer excitation. These transitions have not previously been observed for cyt b559 or cyt c550. The distinctive characteristics of the MCD signals seen at 770 nm allow us to assign absorption in this region to a dz 2 → dx2 - y2 transition of Mn(III) in the Ca-Mn4O5 cluster of the oxygen evolving centre. Current measurements were performed in the S1 state. Detailed analyses of this spectral region, especially in higher S states, promise to provide a new window on models of water oxidation.