Conformational Properties of the Spin-Labeled Tylopeptin B and Heptaibin Peptaibiotics Based on PELDOR Spectroscopy Data

X-Band pulsed electron–electron double resonance (PELDOR) spectroscopy was used to investigate for the first time the magnetic dipole–dipole interaction between spin labels for frozen glassy methanol solutions at 77 K of double spin-labeled, medium-length peptaibiotics, namely, tylopeptin B and heptaibin. This study was conducted on tylopeptin labeled at positions 3 and 13 (T313) and heptaibin labeled at positions 2 and 14 (H214). PELDOR data analysis was carried out using the theory developed for short inter-spin distances. The distance distribution functions between spin labels for T313 (maximum at 1.76 nm, halfwidth of 0.07 nm) and H214 (maximum at 2.30 nm, half-width of 0.065 nm) were determined. It is found that the distance distribution function for peptide T313 has the Gaussian shape. The main part of the distance spectrum for H214 has Gaussian shape and additional less intensive broad lines are shifted to high distances range 2.5–3.5 nm. The upper limit of distance spectrum in this case corresponds approximately to the length of extended peptide molecule and the number of such configurations is low. Intramolecular distances between the labels at maxima observed allowed us to assign a-helical conformation to T313 and 310-helical structure to H214 in methanol solution.