Comparison of distance information in [TOAC(1), Glu(OMe)(7,18,19)] alamethicin F50/5 from paramagnetic relaxation enhancement measurements with data obtained from an X-ray diffraction-based model

Peptaibol antibiotics are membrane-active linear peptides of fungal origin that are characterized by a high population of the C-alpha-tetrasubstituted, strongly helicogenic, alpha-amino acid, alpha-aminoisobutyric acid, an N-terminal acetyl group, and a C-terminal 1,2-amino alcohol. Alamethicins (Alms), among the longest peptaibiotics, are a group of closely sequence-related peptides composed of 19 amino acid residues. [TOAC(1), Glu(OMe)(7,18,19)] Alm and [TOAC(16), Glu(OMe)(7,18,19)] Alm are synthetic, nitroxide free-radical labeled analogs of [Glu(OMe)(7,18,19)] Alm F50/5. In this work, nitroxide to peptide NH proton distance information obtained from paramagnetic relaxation enhancement (PRE) studies on [TOAC(1), Glu(OMe)(7,18,19)] Alm is compared with distances derived from an X-ray diffraction-based model. The methodology for PRE determination, as well as the generation of the X-ray diffraction-based model three-dimensional structures, is discussed. The distances obtained from PRE measurements are in close agreement with the information derived from the X-ray diffraction-based model. This finding suggests that this type of information could be implemented as long-range distance restraints in NMR-based structure determination.