SIMULATIONS OF Prosthecochloris BACTERIOCHLOROPHYLL a-PROTEIN OPTICAL SPECTRA IMPROVED BY PARAMETRIC COMPUTER SEARCH

Recently, reasonable spectral simulations of the experimental low-temperature absorption and circular dichroism (CD) spectra of the bacteriochlorophyll (BChl) a protein from Prosthecochloris aestuarii were produced for the unaggregated protein trimer based on standard assumptions regarding Q_Y transition moment directions. We report here significant improvements of these simulations, obtained by computer search of unknown parameters including the site wavelengths of the individual BChl and exciton-transition line widths. Absorption and CD spectra are fit with a common set of search parameter values. As before, calculated exciton-transition wavelengths and line widths also compare favorably with values deduced from laser hole-burning experiments. However, the improved simulations are only obtained when comparison is made to 77 K data obtained with a cryosolvent containing glycerol but not glycerophosphate. Simulations of 77 K data obtained with a mixture of glycerol and glycerophosphate are poorer than those recently reported. (The latter are based on 5 K absorption and 77 K CD data, with transition line widths independently selected for each simulation.) Computer tests of the search routine demonstrate that these results are independent of the routine itself. A possible explanation is that the glycerophosphate-containing cryosolvent perturbs the protein structure enough to alter slightly the inter-BChl geometry.