Conformation and interactions of all-D-, retro-all-D- and retro-bombolitin III analogues in aqueous solution and in the presence of detergent micelles.

Bombolitins are five structurally related heptadecapeptides which lyse erythrocytes and liposomes and enhance the activity of phospholipase A2. The conformational properties of the all-D-, retro-all-D, and retro-L-analogues of bombolitin III were investigated by circular dichroism (CD) and two-dimensional 1H-nuclear magnetic resonance (NMR) techniques. In water, all three sequences are in a random conformation and aggregate to various extents depending on pH and concentration. The two enantiomeric retro-sequences exhibit a higher propensity to form beta-aggregates, while the D-analogue of the native sequence tends to form aggregates of alpha-helices. In the presence of an excess of sodium dodecyl sulfate (SDS), the peptides fold into an amphiphilic alpha-helical conformation (left-handed in the case of the all-D sequences). NMR studies on the L-retro-analogue indicate that the helical segment is localized in the central part of the sequence. Combined CD and surface tension measurements indicate that the critical micellar concentration of SDS is raised in the presence of peptide and that helical folding occurs before micelle formation. These results are interpreted in terms of formation of peptide-detergent complexes. It is estimated that the helical structure forms when 40-50 molecules of detergent are bound to each peptide molecule. These data and our previous results in vitro biological tests confirm that the ability to form amphiphilic helices is the major determinant of biological activity of bombolitins.