Ultraviolet raman examination of the environmental dependence of bombolitin I and bombolitin III secondary structure

Bombolitin I and III (BI and Bill) are small amphiphilic peptides isolated from bumblebee venom. Although they exist in predominately nonhelical conformations in dilute aqueous solutions, we demonstrate, using UV Raman spectroscopy, that they become predominately alpha-helical in solution at pH > 10, in high ionic strength solutions, and in the presence of trifluoroethanol (TFE) and dodecylphosphocholine (DPC) micelles, In this paper, we examine the effects of electrostatic and hydrophobic interactions that control folding of BI and BIII by systematically monitoring their secondary structures as a function of solution conditions. We determine the BI and Bill secondary structure contents by using the quantitative UV Raman methodology of Chi et al. (1998. Biochemistry. 37:2854-2864), Our findings suggest that the cr-helix turn in Bill at neutral pH is stabilized by a salt bridge between residues Asp(2) and Lys(5). This initial alpha-helical turn results in different BI and Bill alpha-helical folding mechanisms observed in high pH and high salt concentrations: Bill folds from its single alpha-helix turn close to its N-terminal, whereas the BI alpha-helix probably nucleates within the C-terminal half. We also used quasielastic light scattering to demonstrate that the BI and BIII alpha-helix formation in 0.2 M Ca(ClO4)(2) is accompanied by formation of trimers acid hexamers, respectively.