Relationship between hydrophobic interactions and secondary structure stability for Trpzip beta-hairpin peptides

The temperature-induced beta-hairpin stabilities of selected mutations of the Trpzip1 peptide, SWTWEGNKWTWK (WWWW), have been investigated by electronic circular dichroism (CD), Raman, and FT-IR spectroscopies. The tryptophan (Trp) residues in the original Trpzip1 sequence were systematically substituted with tyrosine (Tyr) in different positions to test the impact of Trp interactions on the beta-hairpin structure and stability. The CD intensity at approximately 228 nm, which arises from Trp-Trp interactions (tertiary structure), and the amide I' IR absorbance at approximately 1635 cm(-1) (secondary structure) have been measured over a range of temperatures to investigate the impact of Tyr substitution on beta-hairpin thermal stability in Trpzip peptides. Mutation from Trp to Tyr in the Trpzip1 sequence reduces the extent of beta-hairpin structure and monotonically decreases the beta-hairpin stability of Trpzip1 mutant peptides with an increasing number of Tyr substitutions. Substituted Trpzip peptides with just one pair of Trp-Trp interactions close to either the terminal residues (WYYW) or the turn (YWWY) have similar stabilities. Comparison of conformational transitions monitored by CD and IR reveals them to have multistate behavior in which the temperature-induced disruption of the Trp-Trp interaction (tertiary structure) occurs at a lower temperature than the unfolding of the secondary structure.