AGGREGATION AND CONFORMATIONAL TRANSITION IN AQUEOUS-SOLUTION OF A BOMBOLITIN-III ANALOG CONTAINING A PHOTOREACTIVE SIDE-CHAIN GROUP

The peptide toxin bombolitin III [B(III)], originally isolated from bu mblebee venom, has been shown to undergo a concentration-dependent con formational change from a random structure to an a-helix induced by ag gregation. The aggregation process and the consequent folding results from a delicate balance of electrostatic and hydrophobic interactions. The conformational change is strongly dependent on pH and salt concen tration. In order to gain insight on the structure of the aggregates, and in particular, opt the aggregation number and relative orientation of helices in the molecular complexes, the following analogue of bomb olitin III was designed and synthesized: t-Asp-Ile-Leu-Ala-Lys-Leu-Gly -Lys-Val-Leu-Ala-His- Val-NH2 Bpa(3)-B(III) where Bpa is benzoylphenyl alanine. Bpa(3)-B(III) aggregates were investigated by CD and nmr tech niques. The observed nuclear Overhauser effect pattern accounts for an antiparallel orientation of two distinct helices. The Bpa side chain allows for the photoinduced cross reaction with any aliphatic proton i n spatial proximity. After irradiation, the reaction mixture was analy zed by high performance liquid chromatography and electrospray mass sp ectrometry. The results confirmed the presence of dimeric and trimeric complexes of bombolitin III formed upon interhelix cross-linking. (C) 1997 John Wiley & Sons, Inc.