Volume 198,number 5 CHEMICAL PHYSICSLETTERS 16 October 1992 A search for subpicosecond absorption components in photosystem II reaction centers S.W. McCauley ‘, A.P. Baronavski, Jane K. Rice zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Chemistry Division, Code 6110, Naval Research Laboratory Washington, DC 20375, USA M.L. Ghirardi * and AK Mattoo Plant Molecular Biology Laboratory, USDA-AR8 BeltsviNeAgricultural Research Center- West, Beltsville, MD 20705, USA Received 27 March 1992; in final form 27 July 1992 The transient absorption kinetics of spinach photosystem II reaction centers were measured at 672 nm (detection bandwidth z 11 nm) followingexcitation at 3 10 nm. A temporal resolution of z 50 fs was used which is three times higher resolution than the current literature value. We observed a very fast absorption decrease with a rise time of I SO+ 15 fs followed by a 13f 4 ps recovery. The kinetics of the recovery step did not reveal a 3 ps component, however, a slight break in the data suggestsa more complicated fit may explain the data as well or better. Based on a comparison of the rise time reported here and those reported by Durrant et al., the relaxation from S,,to S, occurs very rapidly, within the 150 fs initial absorption decrease. 1. Introduction A fundamental reaction in plants is the primary charge separation in photosynthesis. This reaction occurs after light energy absorbed by the chlorophyll a (Chl a) antenna forms an excitation which mi- grates to the photosynthetic reaction center (RC). Nanba and Satoh [ 1] have recently demonstrated that it is possible to isolate active RC pigment-pro- tein compIexes from photosystem II (PSII) of higher plants. Reaction centers believed to be analogous to PSI1 have previously been isolated from the purple bacteria Rhodopseudomonas viridis and khodobacter sphaeroides by Reed and Clayton [ 21. More exper- iments have been performed on the bacterial RCs since they are more stable than the corresponding preparations from higher plants. X-ray diffraction studies of RC crystals from Rps. viridis and Rb. sphaeroides by Deisenhofer et al. [ 3 ] and Allen et al. Correspondence to: J.K. Rice, Chemistry Division, Code 6110, Naval Research Laboratory, Washington, DC 20375,USA. I Permanent address: Physics Department, California State Po- lytechnic University, Pomona, CA 91768, USA. ’ Present address: National Renewable Energy Laboratory, Photoconversion Research Branch, Golden, CO 80401,USA. [41 have revealed the threedimensional structure of the RC core. In the RC core, a bacteriochlorophyll (BChl) dimer is associated with two BChl mono- mers and two bacteriopheophytins (BPheo). These chromophores are arranged with an approximate CZ symmetry. Each branch of the RC core starts at the BChl dimer, followed by a BChl monomer and then a BPheo. The distance from the dimer to the center of a BPheo is about 17 A. Time-resolved studies by Woodbury et al. [ 5 1, Martin et al. [ 61, Breton et al. [ 7 1, Fleming et al. [ 8 1, Dressler et al. [ 9 1, and Lau- terwasser et al. [ lo] have indicated that after the di- mer is excited, an electron is transferred in about 3 ps (values vary between 1 and 4 ps) to one of the Bpheos. The dimer becomes oxidized during this same time. Curiously, despite the symmetric struc- ture previously described, only one branch of the RC core is photochemically active. The PSI1 RC of higher plants has not been as well characterized, but the structure is believed to be sim- ilar to that of the bacterial centers. There are a few direct studies of the absorption transients of pig- ments in stable PSI1 RCs. Wasielewskiet al. [ 111, exciting at 610 nm and probing at 674 nm in 277 K samples, have measured a rate of (3.0 f 0.6 ps)-’ 0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved. 437