The secondary structure of a membrane-modifying peptide in a supramolecular assembly studied by PELDOR and CW-ESR spectroscopies

The new technique of pulsed electron-electron double resonance in electron spin-echo (PELDOR) in combination with the CW-ESR method has been used to i nvestigate the secondary structure of a double spin-labeled peptide (the [T OAC-1,8]-analogue of the peptaibol antibiotic trichogin GA IV ) that is hid den into a tetrameric supramolecular assembly of unlabeled peptide molecule s. The magnetic dipole-dipole relaxation of spin labels has been experiment ally studied in glassy solutions of the double-labeled peptide frozen to 77 K in a mixture of chloroform-toluene with an excess of unlabeled peptide. The PELDOR signal oscillations have been observed at high degrees of diluti on with unlabeled peptide. The intramolecular distance between the spin lab els of the peptide molecule in the aggregate has been determined from the o scillation frequency to be 15.7 Angstrom which is close to the value of con gruent to 14 Angstrom calculated for a 3(10)-helical structure. Estimation of the G action of this ordered secondary structure shows that about 19% of the peptide molecules in aggregates are folded in the 3(10)-helical confor mation. The present experimental results are consistent with our molecular model presented in J. Am. Chern. Sec. 2000, 122, 3843-3848, wherein four am phiphilic 3(10)-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. The experimental data were compared with the results obtained with other techniques.