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

Improved descriptions of the lowest energy excited states of tyrosine and p henylalanine side chains have been developed in order to extend the capabil ities 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 chromophore s 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 vib ronic coupling with the B transitions for the L-a transition were also incl uded. 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* an d n(4)sigma*) in the framework of the origin-independent matrix method to c ompute the CD spectra of different crystal forms and Y --> L and F --> L mu tants of bovine pancreatic trypsin inhibitor (BPTI). The structures of the mutants were obtained by replacing the appropriate tyrosine or phenylalanin e residue by leucine in the wild-type crystal structure. The CD calculation s were performed on the energy-minimized structures. The CD spectrum calcul ated for the form II crystal structure of BPTI showed the best agreement wi th experiment. In the far UV, the calculated and experimental CD spectra ag ree 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 muc h less intensity than observed experimentally.