Identification of the Major Ultraviolet Absorbance Photochanges in Photosynthetic Systems*

Abstract— –Measurement of the changes in the absorbance spectrum of bacteriochlorophyll upon reversible one electron oxidation in methanol have been extended into the u.v. regions of the spectrum. A striking resemblance between the oxidized minus reduced spectrum of bacteriochlorophyll and the light minus dark spectrum of chromatophore preparations from Rhodospirillum rubrum and Rhodospeudomonas spheroides is demonstrated. After subtracting from the chromatophore light minus dark spectrum changes which are the result of the photo‐oxidation of the primary electron donor molecule, no major changes remain in the u.v. which would be ascribable to quinone or pteridine photoreduction. Major photochanges which are not completely accounted for by oxidation of the primary electron donor molecule are in the spectral region between 320 and 450 nm. Possible explanations of these remaining changes are explored. Reversible oxidation of bacteriochlorophyll was also carried out in dichloromethane in order to determine the dependency of the oxidized minus reduced difference spectrum on the polarity and hydrogen bonding properties of the solvent. Although the same general features were observed as with the experiment in methanol, there were significant shifts of some of the band locations toward the u.v.