Self-assembling properties of membrane-modifying peptides studied by PELDOR and CW-ESR spectroscopies

A new technique is described that is suitable to determine the formation of aggregates from monomeric biomolecules. This technique has been tested in the study of the self-assembling properties of the antibiotic trichogin GA IV which belongs to the class of peptaibols. We have investigated the self-assembling properties of three trichogin analogues by pulsed double resonance in electron spin-echo (PELDOR) spectroscopy combined with conventional continuous wave ESR spectroscopy. In the peptides examined Aib has been substituted by its spin-labeled analogue TOAC at three specific positions of the sequence. More specifically, the magnetic dipole-dipole relaxation of the spin-labeled peptides is measured in glassy polar and apolar solvents at 77 K. Specific assemblies of trichogin molecules are formed in an apolar solvent but addition of a more polar solvent leads to dissociation of the aggregates, The estimates based on experimental data show that each aggregate cluster contains four peptide molecules, Some of the distances between spin labels in the cluster have been determined. In addition, CW-ESR data suggest the occurrence of aggregated species in the same solutions at room temperature. The experimental results are consistent with a model wherein four amphiphilic helical peptide molecules form a vesicular system with the polar amino acid side chains pointing to the interior and the apolar side chains to the exterior of the cluster.