UV Raman studies of peptide conformation demonstrate that betanova does not cooperatively unfold

We used UV resonance Raman spectroscopy (UVRR) excited within the peptide b ond pi --> pi* electronic transitions and within the aromatic amino acid pi --> pi* electronic transitions to examine the temperature dependence of th e solution conformation of betanova, a 20-residue P-sheet polypeptide [Kort emme, T., Ramirez-Alvarado, M., and Serrano, L. (1998) Science 281, 253-256 ]. The 206.5 nm excited UVRR enhances the amide vibrations and demonstrates that betanova has a predominantly beta -sheet structure between 5 and 82 d egreesC. The 229 nm excited UVRR, which probes the tyrosine and tryptophan side chain vibrations, shows an increase in the solvent exposure of the try ptophan side chains as the temperature is increased. Our results are consis tent with the existence of an intermediate state similar to that calculated by Bursulaya and Brooks [Bursulaya, B. D., and Brooks, C. L. (1999) J. Arn . Chem. Soc. 121, 9947-9951] and exclude the previously proposed two-state cooperative folding mechanism. Betanova's structure appears to be molten gl obule over the 3-82 degreesC temperature range of our study.