Funds are requested for an electron paramagnetic resonance (EPR) spectrometer. The new spectrometer will replace an existing IBM/Bruker ER 200D series spectrometer that was purchased in 1982 and has recently broken down. Although we have been able to temporarily resurrect the machine by borrowing a microwave bridge, an increasing number of components no longer function, e.g., we can no longer use the fast sweep. The problems are exacerbated by the fact that Bruker no longer supports spectrometers that are this old. This instrument has been in continual use for more than 20 years, generating more than 40 publications and is clearly at the end of its useful lifetime. Thus, we are requesting a new machine. The spectrometer requested, a Bruker EMX series, represents a replacement of the existing spectrometer. An important benefit of the new machine is that it will increase sensitivity by about a factor of 4-10. Although originally purchased by the Cooke laboratory the existing spectrometer has now become a truly shared instrument that is currently being used by the laboratories of Dr. Roger Cooke, Dr. Ron Vale, Dr. Robert Fletterick, Dr. Jonathan Weissman, Dr. Edward Pate, Dr. David Agard, Dr. Paul Otiz de Montellano, and Dr. Howard Maibach. All of these investigators are a part of this proposal, and all but one are currently funded by the NIH. Applications of the group include structural studies of: myosin, the microtubule-based motor proteins, human estrogen receptors, and prions. In addition we will also investigate the properties of epidermis and generation of free radicals by heme proteins, The core users will form an advisory committee to oversee use of the instrument and to set policy. UC San Francisco will provide space for operating the instrument, and funds to cover unexpected repairs. ? ?