Calculations of the CD spectrum of bovine pancreatic ribonuclease.

CD spectra of bovine pancreatic ribonuclease A (RNase A) and its subtilisin-modified from (RNase S) have been calculated, based upon high-resolution structures from x-ray diffraction. All known transitions in the peptide and side-chain groups, especially the aromatic and disulfide groups, have been included. Calculations have been performed with both the matrix method and with first-order perturbation theory. A newly developed method for treating the electrostatic interactions among transition charge densities and between static charge distributions and transition charge densities is used. The effects of local electrostatic fields upon the group transition energies are included for all transitions. Rotational strengths generated by the matrix method were combined with Gaussian band shapes to generate theoretical CD spectra. The calculated spectra reproduce the signs and approximate magnitudes of the near-uv CD bands of both RNase A and S. Agreement is most satisfactory for the negative 275 nm band, dominated by tyrosine contributions. In agreement with two previous studies by other workers, coupling between Tyr 73 and Tyr 115 is the single most important factor in this band. The positive band observed near 240 nm is dominated by disulfide contributions, according to our results. The far-uv CD spectrum is poorly reproduced by the calculations. The observed 208 nm band, characteristic of alpha-helices, is absent from the calculated spectrum, probably because the helices in RNase are short. A strong positive couplet centered near 190 nm is predicted but not observed. Possible reasons for these incorrect predictions of the current theoretical model in the far-uv are discussed.