Abstract
Electron spin resonance spectroscopy (ESR) studies on the reduction process of nitroxyl spin probes were carried out for 1mM concentration of <sup>14</sup>N-labeled pyrrolidine nitroxyl spin probes, 3-carbamoyl-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (carbamoyl-PROXYL) and 3-carboxy-2,2,5,5-tetramethyl-pyrrolidine -1-oxyl (carboxy-PROXYL), <sup>14</sup>N-labeled piperidine nitroxyl spin probes, 4-methoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (methoxy-TEMPO) and 4-acetamido-2,2,6,6-tetramethyl-piperidine-1-oxyl (acetamido-TEMPO) in 1 mM concentration of ascorbic acid as a function of time. The half-life time and decay rate were estimated. The piperidine nitroxyl spin probes show a short half-life time compared with that of pyrrolidine nitroxyl spin probes. This result indicates that the higher stability of pyrrolidine nitroxyl spin probes. The ESR was also recorded for 1mM concentration of pyrrolidine nitroxyl spin probes and piperidine nitroxyl spin probes in pure water using X-band ESR spectrometer. The ESR parameters such as line width, hyperfine coupling constant, g-factor, signal intensity ratio and rotational correlation time were determined. These results indicate that the pyrrolidine nitroxyl spin probes have narrow line width and fast tumbling compared with the piperidine nitroxyl spin probes. Therefore, this study reveals that the pyrrolidine nitroxyl spin probes can act as good redox sensitive spin probes for free radical imaging.
Highlights
The nitroxyl radicals are well-known stable organic radicals
The decrease in rotational correlation time was observed for pyrrolidine nitroxyl spin probes, carbamoyl-PROXYL and carboxy-PROXYL compared with the piperidine nitroxyl spin probes, methoxy-TEMPO and acetamido-TEMPO in pure water. These results show that the pyrrolidine nitroxyl spin probes has fast tumbling motion compared with the piperidine nitroxyl spin probes
Among the pyrrolidine nitroxyl spin probes, the carbamoyl-PROXYL can act as a good permeable spin probe for invivo free radical imaging
Summary
The nitroxyl radicals are well-known stable organic radicals. Most frequently used nitroxyl radicals invivo are piperidine and pyrrolidine classes. The stability of nitroxyl radicals is attributed to the three π-bonding system with a delocalized unpaired electron on the N-O bond. These nitroxyl radicals are less toxic, and thereby preferable for invivo applications. Nitroxyl radicals are susceptible to oxygen concentration [1], reactive oxygen species (ROS) [2,3], and biological redox systems [4,5,6,7,8], and are widely used as probes for invivo electron spin resonance (ESR) measurement [9,10,11]. The most active use of invivo ESR spectroscopy and its imaging has been in the measurement of redox status, radical generation and partial pressure of oxygen. In the ESR X-band frequency the magnetic interactions of nitroxide spin labels are extremely sensitive to motion on the nanosecond time scale, which is relevant to the dynamics of bio-molecules [14]
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