Linear dichroism and the transition dipole moment orientation of the carotenoid in the LH2 antenna complex in membranes of Rhodopseudomonas acidophila strain 10050

Linear dichroism (LD) of the carotenoid, rhodopin glucoside, and the 800 an d 850 nm absorbing bacteriochlorophylls in the LH2 antenna complex from mem branes of Rhodopseudomonas acidophila strain 10050 squeezed in polyacrylami de 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 struc ture of the LH2 complex solved by X-ray diffraction methods. Semiempirical molecular orbital (NIO) calculations have refined the structure of the prot ein-bound rhodopin glucoside and show that the transition moment of the car otenoid is not collinear with the long axis of the molecule. Rather, it is 9.1 degrees off axis from the extended pi -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 presen t in all polyenes and carotenoids and is not due to environmental perturbat ions induced by binding of the molecule to the pigment-protein complex. Cal culations of the intensity of polarized absorption for the rhodopin glucosi de molecule predict a significant effect of the magnitude of the off-axis a ngle on the value of the observed LD/A. This paper employs seiniernpirical MO calculations and seeks to correlate the experimentally observed LD, whic h gives the orientation of the transition moment of the rhodopin glucoside in the membranes, with the X-ray diffraction results, which reveals the mol ecular structure of the carotenoid to atomic resolution.