ELSEVIER 28 April 1995 Chemical Physics Letters 236 (1995) 595-602 CHEWCAL PHYSICS LETTERS ESEEM study of spin-spin interactions in spin-polarised P’Q, pairs in the photosynthetic purple bacterium Rhodobacter sphaeroides R26 Sergei A. Dzuba ‘, Peter Gast, Arnold J. Hoff * Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504,230O RA Leiden, The Netherlands Received 5 January 1995; in final form 27 February 1995 Abstract Electron spin echo envelope modulation (ESEEM) of the transient spin-polarised PfQA pairs has been studied at 20 and 220 K. The observed strong out-of-phase modulation is interpreted as resulting from electron-electron dipolar and exchange interactions. Fourier-transformed echo envelopes are consistent with the theory developed recently by Tang, Thumauer and Norris and allow us to obtain readily the values of dipolar and exchange couplings. The results are insensitive to 15N enrichment. 1. Introduction Charge separation in photosynthetic reaction cen- ters CRC) leads to formation of radical pairs. The primary light-induced reactions in RCs of photosyn- thetic bacteria are: (i) The electronically excited state of the primary donor P, a bacteriochlorophyll dimer, produces the primary radical pair P’I-, where I denotes a bacteriopheophytin molecule. This reaction occurs within = 3 ps. (ii> Dark electron transfer within 200 ps from I- to QA, the primary acceptor quinone, creates the secondary radical pair P’Q,. Finally, (iii) within 200 bs the electron reaches Qn, the secondary acceptor quinone. * Corresponding author. ’ On leave from Institute of Chemical Kinetics and Combus- tion, Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. The study of spin-spin interactions between radi- cals in the pairs, which include magnetic dipole-di- pole and spin-exchange interactions, could reveal important structural information. The dipole-dipole interaction reflects mutual radical positions in the pair, while the exchange interaction is related to the overlap of electronic wavefunctions, which correlates with the rate of electron transfer. As the radical pairs mentioned above are formed shortly after the light flash, they are spin polarised. The method of electron spin polarisation (ESP) spec- troscopy is effective in the determination of dipolar and exchange couplings [l-6]. For RCs in which Q, is not reduced, the results of these measurements refer to the P+Q- pairs, because the PfI- pairs decay very fast. It has been known for a long time that spin-spin interactions in radical pairs result in so-called elec- tron spin echo envelope modulation (ESEEM) [7]. The theory developed in Ref. [7] was applied to 0009-2614/95/$09.50 8 1995 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(95)00259-6