Exciton Level Structure and Energy Disorder of the B850 Ring of the LH2 Antenna Complex

Abstract

Experimental and theoretical results are presented on the exciton level structure of the B850 ring of bacteriochlorophyll a molecules for the light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila (strain 10 050) and the effects of energy disorder (due to structural heterogeneity) on the level structure. The work is an outgrowth of the accompanying paper (Wu et al. J. Phys. Chem. B 1997, 101, 7641), which reports on the temperature and pressure dependencies of the LH2 absorption spectrum and the zero-phonon hole action spectrum of the lowest energy exciton level of the complex, B870, as well as the structural (nondenaturing) change of the complex near 150 K. The effects of energy disorder are analyzed using the theory of Wu and Small (Chem. Phys. 1997, 218, 225), which employs symmetry-adapted energy defect patterns. The analysis leads to a room temperature value of ∼100 cm-1 for the splitting between B870 and the adjacent, strongly allowed E1 level in the absence of disorder. Using the temperature-dependent data of Wu et al., we arrive at a theoretical estimate for this splitting at temperatures below ∼150 K of ∼150 cm-1, which is 50 cm-1 smaller than the “apparent” value of 200 cm-1 based on the 4.2 K B870 action spectrum. The 50 cm-1 difference is explained in terms of a distribution of values for the energy disorder parameter(s), which leads to a distribution of values for the oscillator strength of B870. Hole-burning data on the temperature dependence of B870's optical dynamics are presented and analyzed. Below ∼15 K the dynamics are dominated by two-level systems of the protein with an effective dephasing frequency that carries a T α dependence with α ≈ 1.3. At temperatures above ∼20 K the dephasing is strongly exponentially driven with an activation energy of ∼100−140 cm-1. A mechanism suggested for this dephasing is that it is due to upward scattering of the B870 level to the adjacent E1 level by one-phonon absorption. New satellite hole spectra for the LH2 complex (isolated and chromatophores) are presented that lead to the assignment of the weak high-energy tail absorption of the B800 and B850 absorption bands to B850 exciton levels of the B850 ring, which are either symmetry forbidden or predicted to be very weakly absorbing in the absence of energy disorder.

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…The oscillator strength for optically allowed transitions accumulates mainly in the lowest exciton states, as indicated by the reddish ellipse. [120,121] c) Low-temperature (1.4 K) absorption spectra from ensembles of peripheral light-harvesting complexes (LH2) from Rps. acidophila (top trace), [15] Rps.…”

Parameters of the Protein Energy Landscapes of Several Light-Harvesting Complexes Probed via Spectral Hole Growth Kinetics Measurements

“…SHB has been applied to explore electronic level structure, electron-phonon coupling and energy and charge transfer processes in a broad variety of complexes [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] , as summarized in a recent review 21 . In terms of low-temperature protein dynamics, the presence of low-energy excitations responsible for the ~T 1.3 dependence of the homogeneous line width, which is characteristic for amorphous solids, was demonstrated for many PS complexes [3][4][5][6][7][8][9][10]12 .…”

The Optical Dynamics of Excitons in Cylindrical J-Aggregates

“…1 Much theoretical and experimental work has been done on these circular structures. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Especially, the efficiency of the excitonic energy transport in these antenna systems has attracted much attention. [16][17][18][19][20][21][22][23][24] Studies on the optical dynamics of excitons in these natural aggregates provide inspiration for the search for artificial antenna systems.…”