Explaining the visible and near-infrared circular dichroism spectra of light-harvesting 1 complexes from purple bacteria: a modeling study.

In this work, we investigate the origin and characteristics of the circular dichroism (CD) spectrum of various light-harvesting 1 (LH1) complexes. The near-infrared (NIR) CD signal of these core antennae is strongly nonconservative, and the nature of this nonconservativity is under examination in this paper. So far, on the basis of the high-resolution structures of LH2, we have been able to model the absorption and CD spectra in the bacteriochlorophyll (BChl) Q(Y) and Q(X) regions of LH2 (Georgakopoulou et al., Biophys. J. 2002, 82, 2184-2197), as well as in the carotenoid region (Georgakopoulou et al., Biophys. J. 2004, 87, 3010-3022). We proceed by applying the same modeling method in order to reproduce the LH1 spectra. We assume a ring of dimers in a perfect circular arrangement with 16-fold symmetry, and account for all excitonic interactions within the ring. Because LH1 complexes exhibit Q(Y) and Q(X) CD signals of very low intensity, higher transitions can easily affect these regions. Therefore, we expand the model and take into account also the Soret and carotenoid transitions. We can now understand the shape of the absorption and CD spectra and contemplate the structure of the LH1 complex. The latter is similar to LH2 in that it is a very symmetric ring dominated by excitonic interactions. The larger number of symmetry and the bigger diameter of LH1, combined with small rotations of the BChl transition dipole moments, are responsible for the display of CD signals that are very low in intensity. The interaction of the Q(Y) with the carotenoid transitions results in complete loss of the conservativity. Interaction energies between all the pigments in the ring are calculated, and their values are in good accordance with what is reported in the literature.