Self-assembling properties of a membrane-modifying lipopeptaibol in weakly polar solvents studied by CW ESR

The self-assembling properties of TOAC spin-labeled analogues of the lipopeptaibol antibiotic trichogin GA IV have been studied by CW ESR in solvents of different polarity, ranging from CHCl3 to a mixture of CHCl3-toluene and CCl4 at different concentrations. At room temperature, evidence has been obtained that, in all of the solvents examined, a supramolecular assembly exists which is in equilibrium with monomeric molecules. The ESR spectral lines of the aggregates of double-labeled peptides are wider than those obtained from single-labeled peptides. Line widths were used to assess the rotational mobility of both the aggregates and the monomers. Their hydrodynamic radii were estimated to determine the number of peptide molecules per aggregate. This value (close to 4) almost coincides with the previously reported value for spin-labeled peptide clusters in frozen glassy solutions. Spectral broadenings, due to the modulation of the intra- and intermolecular dipole-dipole interactions of spin labels by the rotational mobility of aggregates, were separated by analyzing the ESR solution spectra of spin-labeled and nonlabeled peptide mixtures. The intramolecular distance between spin labels of the double-labeled peptide in the aggregate was estimated to be im A, which most closely corresponded to the alpha -helical conformation. It has been further established that the broadening due to the intermolecular coupling of spin labels is substantially greater in aggregates formed by a mixture of peptides labeled at different positions than it is in aggregates made only of one kind of single-labeled peptide. The intermolecular distances between labels were estimated to be about 9.2 and 16 Angstrom in the two cases, respectively. The results agree with a helix bundle where the helices are oriented in an antiparallel fashion.