Light-induced red shift of the Qx absorption band of light-harvesting bacteriochlorophyll in Rhodopseudomonas capsulata and Rhodopseudomonas sphaeroides

In chromatophores from Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata, the Q_x band(s) of the light-harvesting bacteriochlorophyll (BChl) (λ_max ~590 nm) shifts to the red in response to a light-induced membrane potential, as indicated by the characteristics of the light-minus-dark difference spectrum. In green strains, containing light-harvesting complexes I and II, and one or more of neurosporene, methoxyneurosporene, and hydroxyneurosporene as carotenoids, the absorption changes due to the BChl and carotenoid responses to membrane potential in the spectral region 540–610 nm are comparable in magnitude and overlap with cytochrome and reaction center absorption changes in coupled chromatophores. In strains lacking carotenoid and light-harvesting complex II, the BChl shift absorption change is relatively smaller, due in part to the lower BChl/reaction center ratio.In the carotenoid-containing strains, the peak-to-trough absorption change in the BChl difference spectrum is 5–8% of the peak-to-trough change due to the shift of the longest-wavelength carotenoid band, although the absorption of the BChl band is 25–40% of that of the carotenoid band. The responding BChl band(s) does not appear to be significantly red-shifted in the dark in comparison to the total BChl Q_x band absorption.