This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. It was recently found that nitroxide biradicals are efficient in low concentrations in DNP, a method that has long been used to improve SNR in NMR. The modern DNP experiment at MIT is based on irradiation of nitroxides with 140 or 250 GHz gyrotron sources, leading to signal enhancements of up to 400. The underlying physics is described by a three spin process, with the two electron spins spaced at the nuclear frequency. The use of biradicals is a new approach to an old technique based on paramagnetic species bearing a single electron spin in relatively high concentration. In a recent study bis-TEMPO n-ethyleneglycol (BTnE) biradicals (n=2, 3, or 4) were employed. The largest enhancement was observed for n=2 with 5 to 10 mM of electron spins. The purpose of the service request was two-fold: 1) to provide distances for several nitroxide biradicals of short length, ranging from 10 to 18 A; 2) to obtain mutual orientations of nitroxide hf tensors. A practical short distance limit for DEER is ca. 17-20 A, due to weak B1. On the other hand, DQC featuring B1 of 30 G or greater could be used to measure shorter distances. We applied DQC to BTnE biradicals in DMSO and glycerol-water solutions. Also, three other biradicals of rigid structure were studied. These include BTXA, BTOX, and BTUrea. BTOX and BTXA DQC gave clearly resolved dipolar spectra, from which the distances were easily inferred. BTnE proved to be more flexible, and its dipolar spectra were unresolved. The distances ranged from 11 to 16 and were well within the margin of error for the distances reported by W-band CW-ESR spectroscopy in perduterated solvents, although DQC did allow determination of distances in all cases. The study will be continued to obtain 2D-DQC spectra in order to determine the orientation of the hf tensors.
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