A quantum chemical study of electronic circular dichroism in alanine oligopeptides

Sammanfattning: Circular dichroism (CD) spectroscopy, exploiting the wavelength-dependentdifferential absorption of left- and right-handed circularly polarized light, isa popular method of protein characterization. Theoretically computed CDspectra from quantum mechanical computer models of peptides can widen theapplicability of the method. In this work, the usefulness of Time-DependentDensity Functional Theory (TD-DFT) with the CAM-B3LYP functional and6-31+G(d) basis set in obtaining CD spectra of model alanine α-helices 3to 15 residues long is investigated.  It is found that including 10 excitedstates per residue in the TD-DFT calculation resolves the characteristic partof the spectra sufficiently.  However, the results suffer from blueshift andimproper weakness of the 222 nm peak of the characteristic α-helix spectracorresponding to the n → π∗ transition, despite extension of the basis set to6-311++G(d,p) and use of the Polarizable Dielectric Continuum Model totreat solvent effects. In this case, these issues are limiting the usefulness ofTD-DFT for prediction of peptide CD spectra. More advanced methods totreat solvent interaction and benchmarking the performance of the functionalwith higher-level ab initio methods are suggestions for future studies. A introduction to electronic structure theory and the use of time-dependentperturbation theory to treat spectroscopy is also given in this work.