Theory of the optical spectra of the bacteriochlorophyll a antenna protein trimer from Prosthecochloris aestuarii.

A reasonable theoretical fit to the experimental low-temperature absorption and CD spectra of the BChl a-protein from Prosthecochloris aestuarii has been obtained for the unaggregated protein trimer based on standard assumptions regarding QY transition moment directions. The fits depend to some extent upon varying the site wavelengths of the individual BChls not just in one subunit but in the entire trimer, a procedure not tried before. Features in both spectra, but especially in CD, at wavelengths longer than ≈810 nm are very strongly influenced by the intersubunit interactions of BChls 7 (in the Fenna-Matthews numbering). Unlike earlier theoretical models, which also gave reasonable fits but were based on unorthodox or incorrect assumptions, this exciton model gives every indication of being refineable by improved choices of site wavelengths and exciton-transition lineshapes. Calculated exciton-transition wavelengths and line widths are compared with values deduced from recent laser hole-burning experiments (Johnson and Small 1991). As in the case of the absorption and CD spectra, agreement is best for the long-wavelength half of the QY region.