No abstract

The objective of this work is two-fold. First, the effects of static diagonal disorder on the linear and nonlinear absorption spectra of excitons in circular molecular aggregates are studied by computer modeling. Second, it is demonstrated that this simplified model successfully reproduces the main features of both the ground-state absorption and initial pump−probe absorption difference spectra of LH2 antenna proteins from photosynthetic bacteria measured upon spectrally selective population of excitons at low temperature. Of the usual first-order approximations in the Frenkel exciton theory, our model exploits only two:  the two-state and the zero electron−vibrational coupling approximations. In our model, the molecules of the aggregate are allowed to have different site energies. The coupling between all aggregate molecules is taken into account. An important difference between our study and previous work is that the exciton state selective spectra are calculated corresponding to the recently performed spectrally selective ultrashort pulse excitation experiment. We investigate the behavior of excitons as a function of disorder separately in the B850 and B800 ring aggregates of LH2. Usually, excitations in the B800 ring have been considered completely localized. The present study reinforces the importance of static diagonal disorder in describing the spectral properties of excitons in the LH2 antenna complex at low temperatures. Moreover, it has been demonstrated that two types of spectral disorder govern the inhomogeneously broadened exciton spectra of antenna complexes embedded into the photosynthetic membrane rather than a single source of disorder. From the comparison of simulated and experimental linear absorption spectra, we suggest that the peculiar asymmetry of the B800 band as well as some of the high-energy sideband structures are due to weak coupling of excitons in the B850 and B800 ring aggregates with intramolecular vibrations of bacteriochlorophyll a molecules.

The effects of high hydrostatic pressure (up to 8 kbar) on bacteriochlorophyll a Q y electronic absorption bands of LH2 photosynthetic antenna complexes have been studied at ambient temperature. A variety of samples were studied, including intact membranes and isolated complexes from wild type and mutant photosynthetic bacteria Rhodobacter sphaeroides, Rhodopseudomonas acidophila, and Rhodospirillum molischianum. The spectra of the complexes universally red shift and broaden under elastic compression, while the variations of the integrated intensity remain within the experimental uncertainty. A qualitatively different slope and variation of the slope of the pressure-induced shift is observed for the B800 and B850 absorption bands of LH2 complexes belonging to quasi-monomer and aggregated pigments, respectively. For the complexes from Rhodobacter sphaeroides, e.g., the corresponding slopes are −28 ± 2 and −65 ± 2 cm-1/kbar. The shift rate of the B800 band declines with pressure, while the opposite is observed for the B850 band. The shifts show little if any correlation with hydrogen bonds. Using simple phenomenological arguments and numerical simulations of molecular exciton spectra, it is shown that the shift of the B800 band is governed by pigment−protein interactions, while in addition to that, interpigment couplings (including long-range dipolar and short-range orbital overlap interactions) are instrumental for the B850 band shift. The compressibility of the B800 bacteriochlorophyll binding sites deduced from the B800 band shift at ambient pressure is ∼0.02 kbar-1, and it decreases nonlinearly with pressure. Inter-pigment couplings are responsible for approximately one-third of both the total ambient-pressure solvent shift of the B850 absorption band and its pressure-induced growth. A slight increase with pressure of the B850 band shift due to orbital overlap couplings is predicted.