The P700 triplet state in an intact environment detected by ODMR - A well resolved triplet minus singlet spectrum

Optically detected magnetic resonance (ODMR) of P700 triplet state has been performed in large photosystem I (PSI) particles. The 'intactness' of the P700 environment in the PSI particles has been proved by the comparison of the spectra with those obtained in thylakoids and leaves. Effect of degradation of the samples on the spectra has also been tested. When detected in 'native' PSI centers the microwaves induced triplet-minus-singlet absorption spectra (T - S) show very sharp features which allow to estimate the minimum number of bands involved in the change of the electronic state of the primary donor (singlet to triplet), and can be interpreted in terms of interactions among pigments in the reaction center. The analogy with the reaction centers of purple bacteria is discussed. The previously published spectra (H.J. den Blanken, A.J. Hoff, Biochim. Biophys. Acta 724 (1983) 52-61; J. Vrieze, P. Cast, A.J. Hoff, J. Phys. Chem. 100 (1996) 9960-9967) obtained in particles having smaller antenna size, resemble the ones obtained in our degraded samples: it is suggested that the isolation procedure to small particles may produce an increase in the heterogeneity of the environment of the primary donor and/or a change of the mutual orientations and interactions among pigments. The fluorescence detected magnetic resonance (FDMR) in large PSI particles supports an excitation energy transfer model derived from time resolved optical studies (R. Van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 (1994) 1-65) and adapted for helium temperature, characterized by the presence of antenna pools emitting at 690, 720 and 735 nm, respectively. The FDMR signal of P700 changes sign (from positive to negative) when detected at emission wavelength shorter than 700 nm. Comparison of FDMR of P700 triplet state in large PSI-200 particles, in thylakoids and, for the first time, in leaves has been done. (C) 1997 Elsevier Science B.V.