PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Temperature dependence of photo-CIDNP in photosynthetic reaction centres. Eugenio Daviso^*,1, S. Prakash¹, A. Alia¹, P. Gast², H de Groot¹, G. Jeschke³, J. Matysik¹, ¹ Leiden Institute of Chemistry, Leiden, The Netherlands² Leiden Institute of Physics, Leiden, The Netherlands³ Max-Planck-Institut für Polymerforschung, Mainz, Germany

ABSTRACT- Photochemically induced dynamic nuclear polarisation (photo-CIDNP) is well known in liquid NMR. The phenomenon is explained in terms of the radical pair mechanism, in which nuclear spin-sorting controls the chemical fate of the reaction path of the radical pair. Singlet radical pairs recombine, whereas triplet radical pairs disintegrate by diffusion and may form different reaction products. As a result, highly polarised nuclei at different chemical shifts for both reaction paths can be observed by liquid NMR. The observation of photo-CIDNP in frozen photochemical reaction centres [1] raised questions about its origin since diffusion is blocked in solids. Understanding of the exact mechanism may provide a clue for the construction of efficient artificial reaction centres as well as allow to extend the photo-CIDNP MAS NMR studies to other systems. Various mechanisms have been proposed in the literature and have been discussed recently [2]. In the three-spin mixing (TSM) mechanism, nuclear polarisation is produced in a coherent process by the anisotropic part of the hyperfine interaction in the radical pair state [3]. The differential decay (DD) mechanism is based on kinetic spin sorting caused by the fast decay of the triplet radical pair to the molecular triplet on the primary electron donor [4]. Since both triplets stay in equilibrium [5], the efficiency of the DD mechanism depends on the temperature. Here we present the first experimental data probing the temperature dependence of the CIDNP effect. [1] Matysik et al. (2000) Proc. Natl. Acad. Sci. USA 97, 9865; (2002) Biochemistry 41, 9708; (2003) J. Molec. Struc. 661-662, 625. [2] Jeschke & Matysik (2003) Chem. Phys. 294, 239. [3] Jeschke (1997) J. Phys. Chem. 106, 10072; (1998) J. Am. Chem. Soc. 120, 4425. [4] Polenova & McDermott (1999) J. Phys. Chem. B 103, 535. [5] Chidsey et al. (1985) Proc. Natl. Acad. Sci. USA 82, 6850.

KEY WORDS: reaction centres, CIDNP, spin polarization