A High-Pressure Spectral Hole Burning Study of Correlation between Energy Disorder and Excitonic Couplings in the LH 2 Complex from Rhodopseudomonas Acidophila

Abstract

Low-temperature (5 K) nonphotochemical hole burning and absorption spectroscopies were used to study the effects of high pressure (up to 1015 MPa) on excitonic coupling energies and energy disorder in the light-harvesting complex 2 (LH 2) from Rhodopseudomonas Acidophila (strain 10050). The pressure dependences of the widths and positions of the B850 and B870 (lowest exciton level of the B850 ring) bands were determined. The results show, for the first time, that these parameters are linearly correlated (common scaling law), consistent with recent theoretical predictions (Jang; et al. J. Phys. Chem. B 2001, 105, 6655). Excitonic calculations for the B850 ring of bacteriochlorophyll a molecules were performed in the nearest dimer−dimer coupling approximation (Wu; et al. J. Phys. Chem. B 1997, 101, 7654) with E1+ diagonal energy disorder. Such a symmetry is known to dictate the response of the B870 level (absorption bandwidth, displacement (ΔE) from the B850 band maximum) to either diagonal or off-diagonal energy disorder. The results reveal that the pressure dependences of nearest neighbor coupling energies and the width of the distribution function for energy disorder are positively correlated, both increasing with pressure. Importantly, an effective pressure dependent excitonic Hamiltonian for the B850 ring was obtained. This Hamiltonian was used to show that mixing of the upper excitonic levels of the B850 ring in close proximity to the B800 levels is too weak to account for the additional (other than B800 → B850 energy transfer) decay channel of the B800 band. A simple combinatorial model for intra-B800 band downward energy transfer is presented that qualitatively explains all available low-temperature data on the additional decay channel.