Effect of ionic strength on folding and aggregation of the hemolytic peptide melittin in solution.

Melittin is a cationic, amphipathic, hemolytic peptide composed of 26 amino acid resi- dues. It is intrinsically fluorescent due to the presence of a single tryptophan residue, which has been shown to be crucial for its hemolytic activity. It undergoes a structural transition from a random coil monomer to an a-helical tetramer at high ionic strength. Although the aggregation behavior of melittin in solution is well characterized, dynamic information associated with the aggregation of melittin is lacking. In this paper, we have monitored the effect of ionic strength on the dynamics and aggregation behavior of melittin in aqueous solution by utilizing sensitive fluores- cence approaches, which include the red edge excitation shift (REES) approach. Importantly, we demonstrate that REES is sensitive to the self-association of melittin induced by ionic strength. The change in environment experienced by melittin tryptophan(s) is supported by changes in fluores- cence emission maximum, polarization, and lifetime. In addition, the accessibility of the tryptophan residue was probed by fluorescence quenching experiments using acrylamide and trichloroethanol as soluble and hydrophobic quenchers, respectively. Circular dichroism studies confirm the ionic strength-induced change in the secondary structure of melittin. Taken together, these results consti- tute the first report showing that REES could be used as a sensitive tool to monitor the aggregation behavior of melittin in particular and other proteins and peptides in general. # 2006 Wiley Peri- odicals, Inc. Biopolymers 83: 111-121, 2006 This article was originally published online as an accepted preprint. The ''Published Online'' date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com