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Abstract

In crystals of reaction centers from the purple bacterium Rhodopseudomonas viridis, the four linearly arranged heme groups of the cytochrome-c subunit are oriented in a way that the two outer groups lie preferentially in direction parallel to the crystalline z-axis whereas the inner groups lie mainly perpendicular to z. Illumination of redox poised crystals with polarized light show that both the outer and inner groups have each a high- and a low-potential heme. The absorption maxima appear at 552, 552.5, 556 and 558.5 nm. The inspection of difference spectra taken at various redox potentials and the consideiration of the individual molecular heme planes in the unit cell lead to the following sequence of heme groups (P-960: special bacteriochlorophyll pair, redox potentials in brackets): P-960, c-558.5 (+370 mV), c-552 (+10 mV), c-556 (+300 mV), c-552.5 (−60 mV).