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The noncovalently bound and structurallyidentical bacteriochlorophyll achromophores in theperipheral light-harvesting complexesLH2 (B800–850) and LH3 (B800–820) from photosyntheticpurple bacteria ensure the variability of the exciton spectra in thenear-infrared (820–850 nm) wavelength region. As a result,the spectroscopic properties of the antenna complexes, such as positionsof the maxima in the exciton absorption spectra, give rise to veryefficient excitation transfer toward the reaction center. In thiswork, we investigated the possible molecular origin of the excitonicallycoupled B820 bacteriochlorophylls in LH3 using femtosecond transientabsorption spectroscopy, deconvolution of steady-state absorptionspectra, and modeling of the electrostatic intermolecular interactionsusing a charge density coupling approach. Compared to LH2, the upperexcitonic level is red-shifted from 755 to 790 nm and is associatedwith an approximate 2-fold decrease of B820 intrapigment coupling.The absorption properties of LH3 cannot be reproduced by only changingthe B850 site energy but also require a different scaling factor tobe used to calculate interpigment couplings and a change of histidineprotonation state. Several protonation patterns for distinct aminoacid groups are presented, giving values of 162–173 cm–1at 100 K for the intradimer resonance interactionin the B820 ring. [ABSTRACT FROM AUTHOR]