Inter- and intra-molecular distances determined by EPR spectroscopy and site-directed spin labeling reveal protein-protein and protein-oligonucleotide interaction

Recent developments including pulse and multifrequency techniques make the combination of sitedirected spin labeling and electron paramagnetic resonance (EPR) spectroscopy an attractive approach for the study of protein protein or proteinoligonucleotide interaction. Analysis of the spin label side chain mobility, its solvent accessibility, the polarity of the spin label microenvironment and distances between spin label side chains allow the modeling of protein domains or proteinprotein interaction sites and their conformational changes with a spatial resolution at the level of the backbone fold. Structural changes can be detected with millisecond time resolution. Inter and intramolecular distances are accessible in the range from approximately 0.5 to 8 nm by the combination of continuous wave and pulse EPR methods. Recent applications include the study of transmembrane substrate transport, membrane channel gating, gene regulation and signal transfer.