Photooxidation of Antenna Bacteriochlorophyll in Chromatophores from Carotenoidless Mutant Rhodopseudomonas Sphaeroides and the Attendant Loss of Dimeric Exciton Interaction

Intense continuous illumination of purified chromatophores from carotenoidless mutant Rhodopseudomonas sphaeroides results in progressive photooxidative loss of the near infrared absorption band near 860 nm assigned to antenna bacteriochlorophyll. The quantum yield of this reaction is low, approximately 1.7 x 10(-5). The loss in near infrared absorption is accompanied by a proportional shift in the absorption maximum to shorter wavelengths. The double circular dichroism feature in the near infrared decreases at a faster rate than does the absorbance. These results are explained by a model in which the antenna bacteriochlorophyll, initially associated as dimers (lambda(max) = 860.2 nm), is progressively converted to the monomeric state (lambda(max) = 851.9 nm). The wavelength shift is attributed to disruption of exciton coupling in the dimer. Acetone/methanol extraction indicates that the maximum molar extinction coefficients of the dimer and monomer do not differ by more than 4%. The occurrence of an absorption maximum at 852 nm for monomeric bacteriochlorophyll in a protein complex demonstrates that it is not necessary to invoke aggregation of the chromophores as the origin of the shift from 770 nm in typical organic solvents.