CALCULATIONS OF SPECTROSCOPIC PROPERTIES OF THE LH2 BACTERIOCHLOROPHYLL - PROTEIN ANTENNA COMPLEX FROM RHODOPSEUDOMONAS-ACIDOPHILA

Absorption and CD spectra of a photosynthetic bacterial antenna comple x are calculated on the basis of the crystal structure of the LH2 (B80 0-850) complex from Rhodopseudomonas acidophila. This complex contains a ring of 18 tightly coupled bacteriochlorophylls (B850) and a ring o f 9 more weakly coupled bacteriochlorophylls (B800). Molecular orbital s for bacteriochlorophylls with the three different geometries seen in the crystal structure are obtained by semiempirical quantum mechanica l calculations (QCFF/PI). Exciton and charge-transfer interactions are introduced at the level of configuration interactions. Particular att ention is paid to the dependence of these interactions on the interato mic distances and on dielectric screening. Absorption band shapes are treated with the aid of vibronic parameters and homogeneous line width s that have been measured by hole burning (Reddy, N. R. S., et al., Ph otochem. Photobiol. 1993, 57, 35-39). Inhomogeneous broadening due to diagonal disorder in the monomeric and charge-transfer transition ener gies is included by a Monte Carlo method. The calculations successfull y reproduce the main features of measured absorption and CD spectra of the complex. The results support the view that the excited states of the B850 bacteriochlorophylls are extensively delocalized over the rin g of pigments while the excited states of the B800 bacteriochlorophyll s are much more localized.