Conformations of bombolitins I and III aqueous solutions: circular dichroism, proton NMR, and computer simulation studies

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Abstract
The heptadecapeptides bombolitin I and bombolitin III are two of a series of peptides posulated to be biologically active within a membrane environment. In the preceding paper [Bairaktari, E., Mierke, D. F., Mammi, S., and Peggion, E. (1990) Biochemistry (preceding paper in this issue)] the conformational preferences of these peptides in the presence of SDS surfactant micelles, a mimetic for biological membranes, were examined. During these studies the conformations of these peptides were investigated in aqueous solutions by circular dichroism and nuclear magnetic resonance. A large difference was observed for the two peptides. Bombolitin I lacks any observable secondary structure in aqueous solution, independent of temperature, pH, and concentration. In striking contrast, bombolitin III adopts a well-defined .alpha.-helix at concentrations greater than 1.3 mM. This is indeed surprising given the great similarity of the two peptides. The .alpha.-helix of bombolitin III is pH dependent, with a great decrease in the observed secondary structure at pH values below 3.5. This observation could only be due to the protonation of the Asp residue at the fifth position. These findings suggest that the secondary structure arises from molecular aggregation of bombolitin III through the formation of a salt bridge involving the Asp side chain. The .alpha.-helix observed at "high" concentration (>2.5 mM) has been characterized by CD and by the NOE''s measured throughout a majority of the peptide. The experimentally determined structure has been energy refined with restrained molecular dynamics. The conformational results from this study are then compared with the conformations found in the presence of surfactant micelles.