Influence of Carotenoids on the Infra-Red Spectrum of Bacteriochlorophyll in Chromatium

Abstract

ALTHOUGH it is well known that the infra-red maxima in the absorption spectrum of Chromatium exhibit considerable variability, the basis of this phenomenon has remained obscure. Wassink et al. ¹ described in detail the variations which they observed in the infra-red spectrum of both the organisms and colloidal extracts. After considering several explanations, these authors took the view that all these infra-red maxima represent one pigment, namely, bacteriochlorophyll bound to different proteins. More recently, Duysens² also has claimed that each of the infra-red peaks represents bacteriochlorophyll; however, he has not tried to explain the existence of more than one peak. Work in this laboratory has led to a hypothesis of the ultra-structure of the bacterial chromatophore³. It was posulated from this model that the transfer of energy from carotenoids to bacteriochlorophyll has spatial requirements which are met only when the chromatophore is in a suitable environment and further that the complexity of the infra-red spectrum is related to the interaction between these two pigment systems. At this time both postulates have received experimental support. Variation in the concentration of inert solute in the suspension medium has pronounced effects upon the efficiency with which quanta absorbed by the carotenoids are used for photophosphorylation by isolated chromatophores⁴. The loss of the ability to transfer energy is also correlated with specific changes which appear in the infra-red spectrum. These changes which appear in the infra-red spectrum of isolated chromatophores are comparable to the differences which are observed in organisms with differing carotenoid content.