Biophys J, November 2002, p. 2440-2456, Vol. 83, No. 5

Spin-Lattice Relaxation of Coupled Metal-Radical Spin-Dimers in Proteins: Application to Fe²⁺-Cofactor (Q, Q, ) Dimers in Reaction Centers from Photosynthetic Bacteria

Rafael Calvo,^* Roger A. Isaacson, Edward C. Abresch, Melvin Y. Okamura, and George Feher

 ^*Departamento de Física, Facultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral and INTEC (CONICET-UNL), 3000 Santa Fe, Argentina, and  Department of Physics, University of California, San Diego, La Jolla, California 92093-0319 USA

The spin-lattice relaxation times (T₁) for the reduced quinone acceptors Q and Q, and the intermediate pheophytin acceptor , were measured in native photosynthetic reaction centers (RC) containing a high spin Fe²⁺ (S = 2) and in RCs in which Fe²⁺ was replaced by diamagnetic Zn²⁺. From these data, the contribution of the Fe²⁺ to the spin-lattice relaxation of the cofactors was determined. To relate the spin-lattice relaxation rate to the spin-spin interaction between the Fe²⁺ and the cofactors, we developed a spin-dimer model that takes into account the zero field splitting and the rhombicity of the Fe²⁺ ion. The relaxation mechanism of the spin-dimer involves a two-phonon process that couples the fast relaxing Fe²⁺ spin to the cofactor spin. The process is analogous to the one proposed by R. Orbach (Proc. R. Soc. A. (Lond.). 264:458-484) for rare earth ions. The spin-spin interactions are, in general, composed of exchange and dipolar contributions. For the spin dimers studied in this work the exchange interaction, J_o, is predominant. The values of J_o for QFe²⁺, QFe²⁺, and Fe²⁺ were determined to be (in kelvin) 0.58, 0.92, and 1.3 × 10³, respectively. The |J_o| of the various cofactors (obtained in this work and those of others) could be fitted with the relation exp(_Jd), where d is the distance between cofactor spins and _J had a value of (0.66-0.86) Ź. The relation between J_o and the matrix element |V_ij|² involved in electron transfer rates is discussed.

Biophys J, November 2002, p. 2440-2456, Vol. 83, No. 5
© 2002 by the Biophysical Society   0006-3495/02/11/2440/17  $2.00