Disordered Exciton Model for the Core Light-Harvesting Antenna of Rhodopseudomonas viridis

Abstract

In this work we explain the spectral heterogeneity of the absorption band (Monshouwer et al., 1995. Biochim. Biophys. Acta. 1229:373–380), as well as the spectral evolution of pump-probe spectra for membranes of Rhodopseudomonas ( Rps.) viridis. We propose an exciton model for the LH1 antenna of Rps. viridis and assume that LH1 consists of 24–32 strongly coupled BChl b molecules that form a ring-like structure with a 12- or 16-fold symmetry. The orientations and pigment-pigment distances of the BChls were taken to be the same as for the LH2 complexes of BChl a–containing bacteria. The model gave an excellent fit to the experimental results. The amount of energetic disorder necessary to explain the results could be precisely estimated and gave a value of 440–545 cm −1 (full width at half-maximum) at low temperature and 550–620 cm −1 at room temperature. Within the context of the model we calculated the coherence length of the steady-state exciton wavepacket to correspond to a delocalization over 5–10 BChl molecules at low temperature and over 4–6 molecules at room temperature. Possible origins of the fast electronic dephasing and the observed long-lived vibrational coherence are discussed.