Conformational properties of the spin-labeled tylopeptin B and heptaibin peptaibiotics based on PELDOR spectroscopy data

In this work, we extracted 3D-structural information on newly synthesized, medium-length, double spin-labeled peptaibiotics using PELDOR spectroscopy. We investigated the magnetic dipole-dipole interactions between spin labels and the orientation selectivity effects. In particular, the medium-length peptaibiotics tylopeptin B1,2 and heptaibin3, double spin-labeled with the nitroxyl probe TOAC (4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-4-carboxylic acid), were studied by means of X-Band PELDOR spectroscopy. This study was conducted on tylopeptin labeled at positions 3 and 13 (T313) and heptaibin labeled at positions 2 and 14 (H214) in frozen glassy methanol solutions at 77 ?. PELDOR data analysis was carried out using the theory developed for short interspin distances. The distance distribution functions between spin labels for ?313 (maximum at 1.78 nm, halfwidth of 0.08 nm) and ?214 (maximum at 2.30 nm, half-width of 0.05 nm) were determined. The intramolecular distances observed between the labels allowed us to assign an essentially ?-helical conformation to ?313 and a largely prevailing 310-helical structure to ?214 under the aforementioned experimental conditions.