11 - Electron Spin Polarization in Photosynthetic Reaction Centers

Abstract

This chapter focuses on electron spin polarization (ESP) in photosynthetic reaction centers (RCs). The photosystems are facilitated by both the unique understanding of the sequential electron transfer steps that yield the stabilized charge-separated state and the availability of the crystal structure of its RC protein. In addition, the purple bacterial RC protein can be modified in a number of ways, allowing the rates of electron transfer or the magnitudes of the magnetic interactions in the transient radicals produced by this process to be varied systematically. Three models have been invoked to explain the ESP of the charge-separated state P870+Q– observed in purple bacterial RCs. The expected ESP for all cases from predominantly chemically induced dynamic electron polarization (CIDEP), to a mixture of CIDEP and the correlated radical pair polarization (CRPP), to pure CRPP by varying the lifetime of P870+I–, are simulated. The electron transfer sequence in green sulfur bacteria is believed to be analogous to PSI yet, unlike PSI sample reduction, is required to observe ESP in green sulfur bacteria. The observed ESP behavior in these reduced samples more closely resembles the CIDEP behavior observed with Quinone-replaced iron-containing purple bacterial RCs than the predominant CRPP behavior observed with PSI. Assuming that the ESP arises from the first stabilized charge-separated state, this observation suggests that green sulfur bacteria have some properties similar to those of purple photosynthetic bacteria.