Dynamics of the emission spectrum of a single LH2 complex: interplay of slow and fast nuclear motions

¹ Moscow State University
² Vrije Universiteit, Amsterdam
³ Vrije Universiteit

^* To whom correspondence should be addressed. E-mail: danielis{at}nat.vu.nl.

Submitted on August 15, 2005
Revised on October 3, 2005
Accepted on 3 January 2006

	Abstract

We have studied the relationship between the realizations of static disorder and the emission spectra observed for a single LH2 complex (Rutkauskas, D., V. Novoderezkhin, R. J. Cogdell, and R. van Grondelle. 2005. Biophys. J. 88:422-435). We show that the experimentally observed spectral fluctuations reflect realizations of the disorder in the B850 ring associated with different degrees of exciton delocalization and different effective coupling of the excitons to phonon modes. The main spectral features cannot be explained using models with correlated disorder associated with elliptical deformations of the ring. A quantitative explanation of the measured single-molecule spectra is obtained using the modified Redfield theory and a model of the B850 ring with uncorrelated disorder of the site energies. The positions and spectral shapes of the main exciton components in this model are determined by the disorder-induced shift of exciton eigenvalues in combination with phonon-induced effects (i.e. reorganization shift and broadening, that increase in proportion to the inverse delocalization length of the exciton state). Being dependent on the realization of the disorder, these factors produce different forms of the emission profile. In addition, the different degree of delocalization and effective couplings to phonons determines a different type of excitation dynamics for each of these realizations. We demonstrate that experimentally observed quasi-stable conformational states are characterized by excitation energy transfer regimes varying from a coherent wavelike motion of a delocalized exciton (with a 100 fs pass over half of the ring) to a hopping-type motion of the wavepacket (with a 350 fs jumps between separated groups of 3-4 molecules) and self-trapped excitations that do not move from their localization site.

Key Words: Redfield theory, conformational dynamics, delocalization, exciton-phonon coupling, photosynthetic light-harvesting, single molecule

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