The papers related to the theoretical background and experimental investigations by pulsed electron double resonance (PELDOR) are reviewed. The main aim of this pulsed ESR application is to study the dipole-dipole spin interaction. In PELDOR the ESR spectrum is excited by two ESE pulses at frequency w, and additional pumping pulse at tob. Decay function V(T) of the ESE signal, when the time interval T between the first ESE pulse and pumping pulse is varied, contains the information on dipole-dipole couplings in the spin system. The kinetics of V(T) decay strongly depends upon distance, mutual orientation inside interacting spin pairs and space distribution of radicals throughout the sample. The distances between spins which were measured or estimated using PELDOR in the papers reviewed are in the range of 15= 130 A. This pulsed ESR technique turns now to be a powerful supplement to conventional ESE in studying the free radicals space distribution..

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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.