Linear-dichroism measurements on the LH2 antenna complex of Rhodopseudomonas acidophila strain 10050 show that the transition dipole moment of the carotenoid rhodopin glucoside is not collinear with the long molecular axis

We have applied linear-dichroism experiments to determine the orientation of the transition dipole moment, corresponding to the main absorption band of the carotenoid, rhodopin glucoside, in the light-harvesting complex LH2 from Rhodopseudomonas acidophila strain 10050. The crystal structure of this LH2 complex is known and MO calculations have been performed that indicate that the transition dipole moment is 9.1degrees off axis from the extended pi-electron conjugated chain (Dolan, P. M.; Miller, D.; Cogdell, R. J.; Birge, R. R. Frank, H. A. J. Phys. Chem. B 2001, 105, 12134-12142). Our experimental results confirm that the transition dipole moment is not oriented along the long axis of the carotenoid molecule, and they are in excellent agreement with the MO calculations. However, they disagree with the linear-dichroism results of the same authors. Possible reasons for the discrepancy are discussed.