Absorption properties of the carotenoids after alkaline denaturation of the light-harvesting complex II from Ectothiorhodospira sp.

Alkaline treatment of the Ectothiorhodospira sp. light harvesting system II induces monomerisation of the bacteriochlorophylls and a bleaching of the carotenoid absorption bands in the visible region. Concomitantly, the maxim um of absorption observed around 373 nm shifts towards 354 nm. This shift d oes not result from the Soret band but from a change of the absorption prop erties of the carotenoids. Furthermore, these pigments are not modified che mically but the spectral conversion results from environmental changes. It is assumed that the dissociation of the bacteriochlorophylls in alkaline me dium is accompanied by a structural reorganisation of the complex which rei nforces the interactions between the polypeptides and the carotenoids.