Abstract
The identities of the axial ligands to the two hemes of the flavocytochrome c-552 isolated from the photosynthetic purple sulfur bacterium Chromatium vinosum have been investigated by visible / near-infrared absorption and magnetic circular dichroism (MCD) spectroscopies, with parallel electron paramagnetic resonance (EPR) studies. One of the hemes has histidine and methionine as axial ligands and has a local environment that is relatively insensitive to the composition of the bulk medium. The second heme, the local environment of which is sensitive to changes in the composition of the bulk medium, exists as a mixture of two forms, only one of which has histidine / methionine axial ligation. One the basis of its EPR characteristics, the other form most likely has histidine / lysine axial ligation. In aqueous solution near neutral pH, more than half of the second heme is present as the histidine / lysine form, while in 50:50 water / ethylene glycol the histidine / methionine form is the dominant one.
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