Interaction of D-amino acid incorporated analogues of pardaxin with membranes.

The influence of specific L- to D-amino acid substitutions on the interaction of pardaxin, a shark repellent neurotoxin polypeptide, with phospholipid vesicles and human erythrocytes is described. Twelve modified, truncated, or fluorescently labeled [with the fluorophore 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD) at their N-terminal amino acid] analogues of pardaxin were synthesized by a solid-phase method. Fluorescence measurements were used to monitor the interaction of the analogues with membranes [Rapaport, D., & Shai, Y. (1991) J. Biol. Chem. 266, 23769-23775]. Upon titration of solutions containing the NBD-labeled peptides with small unilamellar vesicles, the fluorescent emission spectra of all NBD-labeled peptides displayed similar blue-shifts, in addition to enhanced intensities, upon relocation of the probe to the more apolar environment. Binding isotherms were constructed from which surface partition constants, in the range of 10(4) M-1, were derived. The existence of an aggregation process, suggested by the shape of the binding isotherms, could be associated only with those analogues in which the N-helix (residues 1-9) was not perturbed. The alpha-helical content of the analogues was estimated by circular dichroism (CD) spectroscopy, both before and after binding to vesicles at neutral pH. The ability of the peptides to dissipate a diffusion potential and to cause calcein release, as well as to lyse human erythrocytes, served to functionally characterize the peptides. The results support a two alpha-helix model, with a bend at position 13, as best describing pardaxin in its membrane-bound state.(ABSTRACT TRUNCATED AT 250 WORDS)