Electron spin resonance spectroscopy studies on ¹⁵N-labeled and their deuterated nitroxyl spin probes used in molecular imaging

Electron Spin Resonance (ESR) studies were carried out for 2 mM concentration of ¹⁵N labeled carbamoyl-PROXYL, carboxy-PROXYL, MC-PROXYL and their deuterated derivatives using X-band ESR spectrometer. The ESR parameters such as linewidth, hyperfine coupling constant, g-factor, spin-spin relaxation time were determined. The lineshape analysis was also carried out. The observed linewidth values are ̃50% higher for undeuterated nitroxyl radicals compared with their deuterated derivatives. The lineshape analysis reveals that the observed ESR lineshape is a voigt lineshape, which is the convolution of a lorentzian and guassian profile. ESR lineshape analysis results that the lorentzian contribution is ̃20% higher for deuterated nitroxyl radicals compared with undeuterated nitroxyl radicals. The observed spin-spin relaxartion time for deuterated nitroxyl radicals is ̃50% longer compared with undeuterated nitroxyl radicals. The g value indicates the isotropic nature of nitroxyl radicals in pure water. From these results, the deuterated nitroxyl radicals are suitable spin probes for ESR/Overhauser- enhanced magnetic resonance (OMR) studies.