“…This fact, together with the high sensitivity of electron paramagnetic resonance (EPR) spectroscopy, the ability to determine structural properties of proteins in their native environments (i.e. in solution, in membranes, etc), and the ability to observe the time course of structural transitions on msec and longer timescales (Shin et al, 1993;Qu et al, 1997;Sienkiewicz et al, 1999) have all contributed to the emergence of double site directed spin labeling (DSDSL) as the method of choice for many different structural problems which are not amenable to characterization using classical techniques such as nuclear magnetic resonance or X-ray crystallography. The applications of DSDSL reported to date have demonstrated the utility and flexibility of the approach and have opened the door for a vast array of future applications as discussed at length in recent reviews (Millhauser, 1992;Hubbell and Altenbach, 1994b;Hubbell et al, 1998;Feix and KIug, 1999;Hustedt and Beth, 1999) and by other authors in this volume.…”