Calculations of Spectroscopic Properties of the LH2 Bacteriochlorophyll−Protein Antenna Complex from Rhodopseudomonas acidophila

Abstract

Absorption and CD spectra of a photosynthetic bacterial antenna complex are calculated on the basis of the crystal structure of the LH2 (B800-850) complex from Rhodopseudomonas acidophila. This complex contains a ring of 18 tightly coupled bacteriochlorophylls (B850) and a ring of 9 more weakly coupled bacteriochlorophylls (B800). Molecular orbitals for bacteriochlorophylls with the three different geometries seen in the crystal structure are obtained by semiempirical quantum mechanical calculations (QCFF/PI). Exciton and charge-transfer interactions are introduced at the level of configuration interactions. Particular attention is paid to the dependence of these interactions on the interatomic distances and on dielectric screening. Absorption band shapes are treated with the aid of vibronic parameters and homogeneous line widths that have been measured by hole burning (Reddy, N. R. S., et al., Photochem. Photobiol. 1993, 57, 35-39). Inhomogeneous broadening due to diagonal disorder in the monomeric and charge-transfer transition energies is included by a Monte Carlo method. The calculations successfully 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 ring of pigments while the excited states of the B800 bacteriochlorophylls are much more localized.

Exciton (De)Localization in the LH2 Antenna of Rhodobacter sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit

“…3 In our simulations of the B850 absorption band we have taken into account the interactions between the Q y transitions of BChls, neglecting their mixing with Q x , B y , B x transitions as well as with charge transfer states. 8 We supposed that the interaction energies between the BChl molecules are M 12 ) 400 cm - 16 For the LH2 complex of Rb. sphaeroides an analysis of the CD spectrum yielded M 12 ) 300 cm -1 and M 23 ) 233 cm -1 .…”

Scaling of Fluorescence Stokes Shift and Superradiance Coherence Size in Disordered Molecular Aggregates

“…14, [18][19][20][21][22][23][24][25][26][27][28][29][30] Various spectroscopic measurements, such as absorption, 4,5 fluorescence, 11,12 pump-probe, 12 and photon echo, 6,7 have been employed for probing exciton motions in molecular aggregates.…”

Photophysical Consequences of Coupling Bacteriochlorophyll a with Serine and its Resulting Solubility in Water¶

“…In this equation, the Lorentz correction factor is used (20). Using this formula, we found that integration over the Voigt-type absorption band previously described, provides a transition dipole moment of (5.2 Ϯ 0.5) D for BChl-ser in PBS, pH ϭ 7.4.…”