21] Bound iron-sulfur centers in photosynthetic membranes (higher plants and bacteria): Physical detection and occurrence

Abstract

This chapter describes physical detection and occurrence of bound iron–sulfur centers in photosynthetic membranes in higher plants and bacteria. Iron–sulfur proteins of the ferredoxin type are constituents of all photosynthetic cells. In addition to these soluble proteins, bound iron–sulfur groups have been identified in photosynthetic membranes. Iron-sulfur centers in soluble proteins have oxidized minus reduced absorption bands in the visible spectral region (350–450 nm) that have been useful in their characterization, but such absorbance changes have been difficult to detect in membrane fragments from photosynthetic organisms. The presence of large amounts of chlorophyll with strong absorption in the 400 nm region makes observation of iron–sulfur bands almost impossible. The physical technique that has been applied most successfully to the study of these centers is low-temperature electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopic analysis of reduced iron–sulfur centers is commonly performed on an X-band spectrometer equipped with a liquid-helium cryogenic system capable of achieving temperatures down to 4°K. To obtain EPR signals from bound iron–sulfur centers in photosynthetic membranes it is necessary to reduce these centers. Three techniques are available for reduction, and basic procedures of each of these techniques are described in this chapter.