Picosecond absorbance difference spectroscopy on the primary reactions and the antenna-excited states in Photosystem I particles

Abstract

Absorbance difference spectra at various delay times, and kinetics of absorbance changes induced by a 35 ps excitation pulse at 532 nm, were measured of relatively intact Photosystem I particles from spinach containing about 70 chlorophyll a molecules per photoactive primary electron donor P-700. The excitation pulse produced absorbance changes due to the formation of singlet- and triplet-excited antenna chlorophyll a, and, in the case of active reaction centers, also those due to the oxidation of P-700. The formation of excited chlorophyll a was accompanied by the bleaching of the Q y ground state absorption band and by the appearance of a rather flat absorption increase in the region 550–900 nm. The lifetime of singlet-excited chlorophyll a was found to be 40 ± 5 ps. When the iron-sulfur centers were prereduced (photo)chemically, the formation of a radical pair consisting of P-700 + and a chlorophyllous anion was observed. The absorbance-difference spectrum calculated for the reduction of the acceptor was similar to that measured earlier (Shuvalov, V.A., Klevanik, A.V., Sharkov, A.V., Kryukov, P.G. and Ke, B. (1979) FEBS Lett. 107, 313–316), and indicated that the acceptor is a chlorophyll a species absorbing around 693 nm. The lifetime of the radical pair was at least 25 ns. If, however, the acceptor complex was in the oxidized state before the flash, only the oxidation of P-700 was observed. No direct evidence was obtained for the reduction of the chlorophyllous acceptor, implying that if such an anion is formed, it must be reoxidized within 50 ps.