Comparison between UV Raman and circular dichroism detection of short α helices in bombolitin III

We have used UV resonance Raman (UVRR) and circular dichroism (CD) spectroscopies to examine the dilute solution-phase secondary structure of the 17 amino acid peptide Bombolitin III (BIII). Both UVRR and CD clearly observe the α-helix structure induced by the addition of trifluoroethanol (TFE) to BIII. In contrast, only UVRR is able to detect the single α-helical turn induced by increasing the pH of BIII from pH 1.8 to 6.4. This α-helical turn is formed because of a stabilizing salt bridge formed between Lys₂ and Asp₅. Further increases in the α-helix content occur as the pH is raised further. We compare the relative sensitivity of UVRR and CD to short α helices and find, as expected, that the CD cannot detect short α helices. This study demonstrates that UV Raman measurements can detect the formation of single α-helical turns which cannot be detected by CD measurements.