Disordered Exciton Analysis of Linear and Nonlinear Absorption Spectra of Antenna Bacteriochlorophyll Aggregates: LH₂-Only Mutant Chromatophores of Rhodobacter sphaeroides at 8 K under Spectrally Selective Excitation

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 B 800 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.