Permeability Studies of Redox-Sensitive Nitroxyl Spin Probes Through Lipid Membranes Using an L-Band ESR Spectrometer

Electron spin resonance (ESR) studies were carried out for 2 mM ¹⁴N-labeled deuterated 3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (MC-PROXYL) and 3-carboxy-2,2,5,5,-tetramethyl-pyrrolidin-1-oxyl (carboxy-PROXYL) in pure water and various concentrations of liposomal solution by using an L-band ESR spectrometer. The ESR parameters, such as the line width, hyperfine coupling constant, g-factor, rotational correlation time and partition parameter, were reported for the samples. The changes in the line width were observed for ¹⁴N-labeled deuterated MC-PROXYL and carboxy-PROXYL in liposomal solution. The hyperfine coupling constant was observed for both nitroxyl spin probes. The permeable and impermeable nature of nitroxyl radicals was demonstrated using the ESR L-band spectra. The rotational correlation time increases with increasing concentration of liposome. The partition parameter for ¹⁴N-labeled deuterated MC-PROXYL in liposomal solution increases with increasing concentration of liposome, which reveals that the nitroxyl spin probe permeates into lipid membrane. The lipid peaks were observed for 2 mM ¹⁴N-labeled deuterated MC-PROXYL in 200, 300 and 400 mM liposomal concentration. The lipid peaks were not observed for ¹⁴N-labeled deuterated carboxy-PROXYL. These results indicate the permeable and impermeable nature of ¹⁴N-labeled deuterated nitroxyl spin probe.