Reinvestigation of the Oxidation Properties of Nitroxides

The rates of reduction of the two amino-nitroxides, 3-amino-2,2,5,5-te- tramethylpyrrolidine-l-oxyl (I) and 4-amino-2,2,6,6-tetramethylpiperidine-l- oxyl (JI) were compared. Reducing agents, /?-mercaptoethanol and sodium ascorbate, were freshly prepared in the presence of nitrogen or air, in a buffer consisting of 0.1 M Na-cacodylate, 0.01 M NaCl, pH 7. Similar functions of the ESR signal decrease with time were detected for I and II with both reducing agents under nitrogen when the ratio of nitroxide molarity versus reducing agent molarity was kept the same for a given set of experiments. However, a significant difference in the ESR signal decrease with time was observed between I and II with ascorbate in the presence of air. Reproducible time dependence of the ESR signal decay required the use of freshly prepared reducing solutions. Our results strongly suggest that at least amino substituted five and six- membered ring nitroxides oxidize electron donors at the same rate. The apparent greater stability of I versus II seems to be due to the slower reoxidation rate of the hydroxylamine of II as compared to the hydroxylamine of I. Thus, under in vivo conditions, I and II will become ESR silent at the same rate unless the tissue is not under anaerobic conditions

An electron spin resonance study of DNA dynamics using the slowly relaxing local structure model

A systematic analysis is presented of the ESR spectra from DNA oligomers of a range of sizes, including a polymer, spin-labeled with nitroxide moieties attached by different tethers. The complexity of the DNA dynamics is dealt with by the use of the general slowly relaxing local structure (SRLS) model, wherein the nitroxide moiety is reorienting in a restricted local environment, which itself is relaxing on a longer time scale. The slower motion describes the global tumbling of the DNA lattice, and the faster motion, the internal dynamics. In the present analysis, the correlation times for the axially symmetric global tumbling were those obtained from hydrodynamic theory, while the correlation times for the internal dynamics and the order parameter, which directly measures its restricted range of motion, were determined by nonlinear least-squares fits to the spectra. The principal result is the observation and characterization of two types of spectra from these labeled DNA systems. These two spectra represent components that differ from each other with respect to their local environments, one a highly restricted site yielding a large order parameter (0.61) and slower internal motions and the other a much less restricted site (with order parameter of 0.18) and faster internal motions. Whereas the one-atom tethered DUTA exhibits both sites (in roughly 9:1 ratio with the more restricted site more prevalent), the two-atom tethered DUMTA and five-atom tethered DUAT exhibit just the more restricted site, but the five-atom tethered DUAP exhibits only the less restricted site. (DUAP differs fro