Femtosecond Pump−Probe Spectroscopy of the B850 Antenna Complex of Rhodobacter sphaeroides at Room Temperature

Abstract

The photosynthetic bacterium Rhodobacter sphaeroides contains a light-harvesting antenna complex (LH2) with a ring of interacting bacteriochlorophyll molecules (B850). Excitation of membrane-bound LH2 complexes with low-intensity, femtosecond pulses causes changes in absorption and stimulated emission that initially depend on the excitation wavelength but relax to a quasiequilibrium with a time constant of 100 ± 20 fs. Excitation on the blue side of the B850 absorption band is followed by a shift of the signals to longer wavelengths and a decrease in amplitude, whereas the relaxations following excitation on the red side consist mainly of a decrease in amplitude. The signals have an apparent initial anisotropy of approximately 0.5 when the complex is excited with broadband pulses, and 0.35−0.4 with narrower pulses. The anisotropy decays to 0.1 with a time constant of about 30 fs. The anisotropies are similar at wavelengths on either side of the absorption band and are relatively insensitive to the excitation wavelength. Contributions of coherent pump−probe coupling and perturbed free induction decay to the measured anisotropies are considered. Pump−probe coupling could increase the initial anisotropy but cannot account for the decay kinetics. Using a density-matrix formalism, we show that the initial light-induced signals are consistent with coherent excitation of multiple exciton levels in an inhomogeneous ensemble of LH2 complexes and that the main features of the spectral relaxations and the anisotropy can be explained by electronic dephasing and thermal equilibration within the manifold of exciton levels.

Femtosecond dynamics of the forbidden carotenoid S 1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

“…Recently, high-resolution crystal structures of some peripheral light-harvesting complexes of photosynthetic purple bacteria (1-3) have become available, enabling us to study the basic principles of the highly efficient collection of solar energy (4)(5)(6)(7)(8).…”

Tansley Review No. 109.

“…0.1 (De Caro et al, 1994 ;Joo et al, 1996 ;Malkin, 1996 ;Kennis et al, 1997 ;Nagarajan et al, 1999). The rate constant of this decay is less than 100 fs (Joo et al, 1996 ;Nagarajan et al, 1999). If we take the view that the B850* excited state is delocalized over a few of the B850 Bchla molecules, this rapid loss of polarization probably represents ultrafast migration of this excited state around the B850 ' ring '.…”

“…Similarly, as with the B800 excited state population, the energy transfer between the B850 Bchla molecules can be explored by recording the decay of the anisotropy of their excited state population (Jimenez et al, 1996 ;Pullerits & Sundstro$ m, 1996 ;Chachisvilis et al, 1997 ;Ma et al, 1997 ;Nagarajan et al, 1999 ;Sundstro$ m et al, 1999). However, the interpretation of this is complex.…”

Computer Simulation of the Anisotropy of Fluorescence in Ring Molecular Systems: Tangential vs. Radial Dipole Arrangement

“…The high initial anisotropy was ascribed to a coherent excitation of a degenerate pair of states with allowed optical transitions and then relaxation to states at lower energies which have forbidden transitions. Nagarjan et al [6] concluded, that the main features of the spectral relaxation and the decay of anisotropy are reproduced well by a model considering decay processes of electronic coherences within the manifold of the excitonic states and thermal equilibration among the excitonic states. In that contribution the exciton dynamics was not calculated explicitly.…”

“…Time-dependent experiments [3,4] led for the B850 ring in LH2 complex to conclusion that the elementary dynamics occurs on a time scale of about 100 fs [5,6,7]. For example, depolarization of fluorescence was studied already quite some time ago for a model of electronically coupled molecules [8,9].…”