Aggregation and conformational transition in aqueous solution of a bombolitin III analogue containing a photoreactive side-chain group.

The peptide toxin bombolitin III [B(III)], originally isolated from bumblebee venom, has been shown to undergo a concentration-dependent conformational change from a random structure to an alpha-helix induced by aggregation. 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 concentration. In order to gain insight on the structure of the aggregates, and in particular, on the aggregation number and relative orientation of helices in the molecular complexes, the following analogue of bombolitin III was designed and synthesized: Ile-Lys-Bpa-Met-Asp-Ile-Leu-Ala-Lys-Leu-Gly-Lys-Val-Leu-Ala-His-Val-NH2 Bpa3-B(III) where Bpa is benzoylphenylalanine. Bpa3-B(III) aggregates were investigated by CD and nmr techniques. 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 in spatial proximity. After irradiation, the reaction mixture was analyzed by high performance liquid chromatography and electrospray mass spectrometry. The results confirmed the presence of dimeric and trimeric complexes of bombolitin III formed upon interhelix cross-linking.