Fifth Anniversary of ACERT

EPR newsletter 2007 vol.16 no.4 | 11 pelling reasons to invest in the development of modern ESR methods for applications in biophysical and biomedical research. The potential of ESR may best be evaluated by considering (1) its absolute sensitivity, (2) its spectral resolution, (3) its sensitivity to different types of molecular motion, including translation, rotation, and intermolecular relative diffusion, (4) its ability to measure both short and long distances in biomolecules as well as distance distributions, (5) its ability to measure the spatial distribution of a probe molecule with micron resolution, (6) the limited degree to which the measurement perturbs the “native” system, and (7) the convenience of the measurement. Methods that excel according to one or more of these criteria have found widespread use in biomedical research, for example nuclear magnetic resonance (NMR) including NMR microscopy, electron microscopy, and fluorescent or radioactive tracer molecule labeling. Although these methods may individually surpass ESR in one of these capabilities, in many ways ESR provides an optimal combination of all of these features. ESR provides good spectral and spatial resolution as does NMR, but with much higher absolute sensitivity per spin, a timescale more appropriate for examining molecular dynamics, the ability to measure longer distances, and the ability to achieve finer spatial resolution on smaller numbers of spins. Despite these significant advantages, ESR had been lagging far behind NMR in its application to biomedical research. The reasons for this were mainly technical: the much larger magnetic moment of the electron requires higher frequencies and faster timescales, where Jack H. Freed Cornell University, Ithaca, New York, U.S.A. Fifth Anniversary of ACERT