Anisotropic Magnetic Field Effects of the Photosynthetic Bacterial Reaction Center of Rhodobacter sphaeroides R-26, Studied by Linear Dichroic Magneto-optical Difference Spectroscopy (LD-MODS) in the Temperature Range 1.2 – 310 K

Abstract

Recently we have introduced magneto-optical difference spectroscopy (MODS) to measure triplet-minus-singlet absorbance difference (T – S) spectra of bacterial photosynthetic reaction centers (RC) over a wide range of temperatures (Hoff et al., 1985; Lous and Hoff, 1986). The MODS technique rests upon the change in yield of the triplet state of the primary donor, 3P, effected by a magnetic field of small amplitude (a few tens of millitesla). The field is modulated at a few hundred hertz and the resulting modulation in absorbance lock-in detected over a wide range of wavelengths. Since the magnetic field BO is a vectorial quantity and the magnetic field effect (MFE) sensitive to the orientation of RC with respect to \( {\vec B_O} \) (see below), one expects that it should be possible to perform a linear dichroic (LD)-MODS experiment, which would result in a LD-(T – S) spectrum. Knowledge of the orientational dependence of the MFE should then allow to extract information on e.g. the magnitude and direction of the dipolar interaction of the primary radical pair (RP) P+I−, where I is the bacteriopheophytin acceptor. The dipolar interaction between P+ and I− plays a significant role in the interpretation of reaction yield detected magnetic resonance (RYDMR) and MFE spectra (Lersch and Michel-Beyerle, 1982; Tang and Norris, 1983; Moehl et al., 1985; Hunter et al., 1987). An independent determination of the dipolar interaction would help in obtaining a reliable value of the isotropic exchange interaction J(P+I−), which is of great interest for understanding photoinduced electron transport (Marcus, 1987; Bixon, 1987).