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Abstract

It is shown that high-quality light-induced FTIR-difference spectra can be obtained from reaction center crystals of Rhodopsendomonas viridis. Difference spectra between the ground state, PQ, and a light-activated state, P⁺Q⁻, have been recorded. The difference spectra are in good agreement with those reported previously for reaction centers reconstituted into lipid vesicles [(1985) FEBS Lett. 187, 227–232]. This good correspondence indicates that in both sample preparations the same intramolecular processes take place during this transition. In addition to measurements of absorbance changes in the visible spectral region, which indicate reactions of the chromophores and their microenvironments, those in the infrared spectral region also show that the protein side groups and backbone undergo the same light-induced changes in the crystals. It is observed that, besides the porphyrin ring system, the C₉ = O keto and ester groups of, most likely, the primary donor, undergo light-induced changes in charge distribution during oxidation of the primary donor. Large conformational changes of the protein backbone can be excluded for the observed transition.