Linear Dichroism and the Transition Dipole Moment Orientation of the Carotenoid in the LH2 Antenna Complex in Membranes ofRhodopseudomonas acidophilaStrain 10050

Abstract

Linear dichroism (LD) of the carotenoid, rhodopin glucoside, and the 800 and 850 nm absorbing bacteriochlorophylls in the LH2 antenna complex from membranes of Rhodopseudomonas acidophilastrain 10050 squeezed in polyacrylamide gels is reported. A model is presented for computing the ratio of the LD-to-isotropic absorption (LD/A) for the pigments based on the crystal structure of the LH2 complex solved by X-ray diffraction methods. Semiempirical molecular orbital (MO) calculations have refined the structure of the protein-bound rhodopin glucoside and show that the transition moment of the carotenoid is not collinear with the long axis of the molecule. Rather, it is 9.1° off axis from the extended ﷿-electron conjugated chain. This rotation of the transition moment vector away from the structural long axis emerges as a result of the single/double bond alternation naturally present in all polyenes and carotenoids and is not due to environmental perturbations induced by binding of the molecule to the pigment-protein complex. Calculations of the intensity of polarized absorption for the rhodopin glucoside molecule predict a significant effect of the magnitude of the off-axis angle on the value of the observed LD/A. This paper employs semiempirical MO calculations and seeks to correlate the experimentally observed LD, which gives the orientation of the transition moment of the rhodopin glucoside in the membranes, with the X-ray diffraction results, which reveals the molecular structure of the carotenoid to atomic resolution.