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The calculated circular dichroism of polyproline ii in the polarizability approximation

Wiley Blackwell (John Wiley & Sons)
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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.

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