Tyrosine, phenylalanine, and disulfide contributions to the circular dichroism of proteins: Circular dichroism spectra of wild-type and mutant bovine pancreatic trypsin inhibitor

Improved descriptions of the lowest energy excited states of tyrosine and phenylalanine side chains have been developed in order to extend the capabilities of calculating the circular dichroism (CD) spectra of proteins. Four transitions (L-b, L-a, B-b, and B-a) for each of the side-chain chromophores were considered, and the transition monopole charges were obtained from a CNDO/S calculation on models representing the individual groups. Monopole charges at midpoints of the bonds, corresponding to the maximum transition charge densities in the L-b band, and monopole charges representing the vibronic coupling with the B transitions for the L-a transition were also included. The aromatic transitions were combined with the peptide transitions (n pi*, pi(0)pi* n'pi*, and pi+pi*) and disulfide transitions (n(1)sigma* and n(4)sigma*) in the framework of the origin-independent matrix method to compute the CD spectra of different crystal forms and Y --> L and F --> L mutants of bovine pancreatic trypsin inhibitor (BPTI). The structures of the mutants were obtained by replacing the appropriate tyrosine or phenylalanine residue by leucine in the wild-type crystal structure. The CD calculations were performed on the energy-minimized structures. The CD spectrum calculated for the form II crystal structure of BPTI showed the best agreement with experiment. In the far UV, the calculated and experimental CD spectra agree to various extents for the wild-type and mutant BPTI. Among the mutants, the calculated CD spectra of Y4L, Y10L, Y23L, and F45L showed reasonable a,agreement with experiment, while those of Y21L and F22L, the two residues interacting with most aromatic groups, showed poor agreement. In the near UV, the negative bands predicted for the wild-type and mutant BPTI have much less intensity than observed experimentally.