Optically detected magnetic field effects on reaction centers of Rhodobacter sphaeroides 2.4.1 and its Tyr M210 → Trp mutant

Abstract

In this work, the magnetic field effects (MFE) on the triplet yield in reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides 2.4.1 and its Tyr M210 → Trp mutant are compared. The MFE is measured between 5 and 225 K by monitoring the absorbance. Using monochromatic polarised and unpolarised light, linear dichroic (LD-)MFE curves were obtained. Simulations of the (LD-) MFE measured at low temperatures (25 K) are presented, which suggest that the exchange interaction in the mutant and in the wild type have opposite sign. This is explained by considering the free energy differences between the excited primary donor state 1P ∗ and the charge-separated state. The magnetic field dependence of the MFE of Q-depleted mutant RCs at low magnetic fields is similar for temperatures between 25 and 225 K, implying that k T is practically temperature independent. This suggests that the temperature dependence of the triplet yield is due to a change in radical pair singlet recombination rate k S. The exchange interaction between the oxidised primary donor and reduced intermediary bacteriopheophytin acceptor of mutant RCs is twice as large as that of the wild type. This is attributed to an upward change of the energy level of the charge-separated state caused by the mutation.