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.