The calculated circular dichroism of polyproline ii in the polarizability approximation

Abstract

The circular dichroism (CD) spectrum of polyproline II (PPII) has heretofore been moderately well calculated from exciton theory only at the expense of assuming unreasonable chain conformations and accepting a conservative spectrum in the 180–250-nm region (which is not observed). We have incorporated far uv transitions in the polarizability approximation and, together with the Π 2 Π* transition, have calculated the resulting correction to the exciton model. This has been accompanied by a modified assignment of the ΠΠ* transition in PPII, and a simultaneous calculation of the absorption and CD spectra of the Α-helix, Β structure, PPI, and PPII. We obtain good agreement with the observed CD spectrum of PPII in the 180–250-nm region for acceptable chain conformations. In addition, we predict a negative CD into the far uv, in agreement with recent experimental observations. Our calculations also reproduce features of the far uv CD spectrum of the Α-helix, and are in agreement with the CD spectra of the Β chain and PPI. The calculated CD of the unordered polypeptide chain is not significantly influenced by far uv contributions, indicating that our previous calculation is valid for such a system. These results demonstrate the importance of incorporating far uv transitions in order to achieve an adequate theoretical explanation of the CD spectra of polypeptides.