Optical identification of the long-wavelength (700-1700 nm) electronic excitations of the native reaction centre, Mn₄CaO₅ cluster and cytochromes of photosystem II in plants and cyanobacteria

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 Chl_D1 and Chl_D2. 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 d_z² → d_{x2 - 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.