EPR relaxation-enhancement-based distance measurements on orthogonally spin-labeled T4-lysozyme.

Lanthanide-induced enhancement of the longitudinal relaxation of nitroxide radicals in combination with orthogonal site-directed spin labeling is presented as a systematic distance measurement method intended for studies of bio-macromolecules and bio-macromolecular complexes. The approach is tested on a water-soluble protein (T4-lysozyme) for two different commercially available lanthanide labels, and complemented by previously reported data on a membrane-inserted polypeptide. Single temperature measurements are shown to be sufficient for reliable distance determination, with an upper measurable distance limit of about 5-6 nm. The extracted averaged distances represent the closest approach in Ln(III) -nitroxide distance distributions. Studies of conformational changes and of bio-macromolecule association-dissociation are proposed as possible application area of the relaxation-enhancement-based distance measurements.