A significant role for high‐energy transitions in the ultraviolet circular dichroism spectra of polypeptides and proteins

Abstract

The nπ* and ππ* transitions of polypeptides mix significantly with very high energy transitions in the poly(Pro)II conformation, as evidenced by the strongly nonconservative CD spectrum in the 170-250 nm region. Because of this, the exciton model, the standard quantum mechanical model for predicting absorption and CD spectra of polypeptides, gives poor results for the poly(Pro) II (P(II)) conformation, although it works well for the α-helix, β-sheet, and β-turns. The exciton theory has been extended to include the effects of mixing of discrete peptide transitions near 200 nm with the large number of uncharacterized transitions in the deep ultraviolet. These latter transitions dominate the polarizability, and their mixing with the discrete transitions can be described via bond and lone-pair polarizability tensors, derivable by ab initio methods. This extended exciton method gives a good description of the CD spectrum of (Ala)(n) oligomers in the P(II) conformation. For this conformation, the polarizability contributions lead to a strong negative band near 200 nm that dominates the calculated and observed CD spectrum. The model does not give a good description of the CD of (Pro)(n) oligomers, probably because of conformational heterogeneity or nonadditive contributions of the Pro side chains. The model improves the calculated CD spectra of α-helical (Ala)(n) oligomers. Although the high-energy transitions make only a small net contribution to the CD of the α-helix in the 200 nm region, they enhance the negative exciton band at ∼205 nm and largely cancel the negative exciton band near 175 nm, substantially improving agreement with experiment.

Molecular and supramolecular studies on polyglycine and poly-l-proline

“…These spectral features are diagnostic of a very stable left-handed PPII helix conformation which is the conformation adopted in aqueous solution by PLP homopolypeptide. [33][34][35][36] In TBS solution at 0 C the CD spectrum shows a strong negative band at 205 nm and a weak positive band at 229 nm (Fig. 3B).…”

Water interaction differences determine the relative energetic stability of the polyproline II conformation of the alanine dipeptide in aqueous environments

“…11 Although the TK assignment met with initial skepticism, 12 a further evaluation concluded that a range of studies now furnished ''strong evidence in favor of (this) proposal'' 13 and much subsequent research has supported this conclusion [14][15][16][17][18] including recent extended exciton-based theoretical CD studies. 19,20 Despite this consensus, two significant structural issues still remain to be clarified: the number of consecutive PPII conformations in a peptide sequence and the source of its relative stability. With respect to the first issue, two opinions have emerged.…”

UV Resonance Raman Finds Peptide Bond−Arg Side Chain Electronic Interactions

“…The peptide bond π→π* transitions have been characterized to a limited extent, as have been the transitions of the different isolated side chain chromophores. 1–13 However, there has been little attention paid to the potential coupling of the peptide bond transitions to the side chain electronic transitions or to coupling between side chain transitions. Coupling of side chain – peptide bond transitions could be important in areas such as electron transport, for example.…”

“…There is little understanding of the electronic transitions of peptides and proteins. − It is generally assumed that peptide and protein electronic transitions result from the summation of the electronic transitions of the constituent chromophores. The peptide bond π → π* transitions have been characterized to a limited extent, as have been the transitions of the different isolated side chain chromophores. − However, there has been little attention paid to the potential coupling of the peptide bond transitions to the side chain electronic transitions or to coupling between side chain transitions. Coupling of side chain− peptide bond transitions could be important in areas such as electron transport, for example.…”