UV Raman demonstrates that α-helical polyalanine peptides melt to polyproline II conformations

We examined the 204-nm UV Raman spectra of the peptide XAO, which was previously found by Shi et al.'s NMR study to occur in aqueous solution in a polyproline II (PPII) conformation (Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 9190). The UV Raman spectra of XAO are essentially identical to the spectra of small peptides such as alas and to the large 21-residue predominantly Ala peptide, AP. We conclude that the non-alpha-helical conformations of these peptides are dominantly PPII. Thus, AP, which is highly alpha-helical at room temperature, melts to a PPII conformation. There is no indication of any population of intermediate disordered conformations. We continued our development of methods to relate the Ramachandran Psi-angle to the amide III band frequency. We describe a new method to estimate the Ramachandran Psi-angular distributions from amide III band line shapes measured in 204-nm UV Raman spectra. We used this method to compare the Psi-distributions in XAO, ala(5), the non-alpha-helical state of AP, and acid-denatured apomyoglobin. In addition, we estimated the IF-angle distributions of peptide bonds which occur in non-alpha-helix and non-beta-sheet conformations in a small library of proteins.