Experimental determination of the molar differential extinction coefficient of P700

Abstract

The molar differential extinction coefficients of P700 at 435 and 703 nm have been determined by using a directly coupled reaction in the dark involving the reduction of photooxidized P700 and the oxidation of reduced cytochromes. From the coupled oxidation of mammalian cytochrome c by Anabaena high-P700 particles, the molar differential extinction coefficients of P700 at 435 and 703 nm are calculated to be 8.6·10 4 and 1.20·10 5 M −1·cm −1, respectively, and the ratio of the red to blue band heights is 1.4. From the coupled oxidation of Euglena cytochrome-552 by Triton-fractionated pigment system 1 subchloroplast particles enriched in P700, the calculated molar differential extinction coefficients of P700 at 435 and 703 nm are very close to the values given above. Using an absorbance decrease at 580 nm as a measure of the photoreduction of dichlorophenolindophenol by Triton-fractionated pigment system 1 subchloroplast particles enriched in P700 tends to yield a low extinction value because of other absorbance changes which occur at this wavelength and the non-reproducibility of the values obtained. Comparisons are made between the extinction values of P700 and the corresponding extinction values of the bacteriochlorophyll reaction centers in photosynthetic bacteria.