Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

Abstract

Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe2+-depleted transfer step from intermediate radical pair, P 870 + Q a − which is produced in an electron transfer step from intermediate radical pair, P 870 + I−. (P 870 + is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I− is the reduced bacteriopheophytin acceptor, and Q a − is the reduced primary quinone acceptor.) The lifetime of P 870 + I− can be shortened relative to the lifetime of P 870 + I− in Fe2+-depleted RCs by substitution of Zn2+. We report the first observation of X-band and Q-band ESP EPR signals due to P 870 + Q− from bacterial reaction centers that contain Zn2+. Comparison of these signals to those observed from Fe2+-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model.