Date of Award
Chemistry and Biochemistry
Sandra S. Eaton
Diradical, EPR relaxation, Isotope enrichment, J coupling, Nitroxide, Trityl
Trityl and nitroxide radicals are widely used as probes in chemical and biochemical systems. Understanding of factors that impact the electron spin relaxation for these radicals is important for selection of probes and experimental operating conditions. It has been previously reported that 13C isotope enrichment of the central carbon of the trityl yields a highly sensitive micro viscosity probe. It was important to characterize the impact of this substitution on the relaxation processes in fluid solution and immobilized samples.
Electron spin relaxation times for perdeuterated Finland trityl 99% enriched in 13C at the central carbon (13C1-dFT) and perchlorinated triarylmethyl tricarboxylic acid radical enriched in 13C at the central carbon (13C1-PTMTC) were measured in phosphate buffered saline (pH = 7.2) (PBS) solution at X-band. The 13C1 increased both spin-lattice and spin-spin relaxation rates in fluid solution, but the 13C1 and chlorine substituents on the phenyl rings have little impact on T1 for immobilized samples. In fluid solution 1/T1 was modeled as the sum of contributions from spin rotation, modulation of the anisotropic 13C hyperfine, modulation of g anisotropy, and a local mode. The temperature dependence of 1/T1 for immobilized samples was modeled with contributions from the direct, Raman, and local mode processes.
Understanding of the mechanisms of relaxation in diradicals is needed to improve applications in dynamic nuclear polarization and in vivo imaging. EPR line shapes in fluid solution and electron spin relaxation times and lineshapes in frozen solution were measured at X-band for two dinitroxides. For proxyl dinitroxides PxCONHPx and PxCOenPx the fluid solution spectra are characteristic of dynamic exchange between conformations with strong exchange (J > 850 MHz) and conformations with weaker exchange interaction. The relaxation rates for immobilized samples were within experimental uncertainty of values for structurally similar mono-radicals. These data demonstrate that even for these relatively flexible linkages dipolar interaction with an interspin distance of about 9 Å does not provide an effective spin-lattice relaxation mechanism for dinitroxides in glassy water:glycerol.
Simulation of X-band and Q-band spectra in phosphate buffered saline (PBS) fluid solution of a trityl-nitroxide connected by an amide linkage found J = 15 (83%) and 5 G (17%) at 293 K. For a second trityl-nitroxide with a hydroxyl-ether substituent on the amide linkage there were two conformations in fluid solution PBS with J = 113 G (67%) and 59 G (33%) at 293 K. X-band values of spin lattice relaxation rates (1/T1) at 80 to 120 K for the trityl-nitroxides are similar to values for mono-nitroxides, and faster than for trityl radicals. Values of 1/Tm for the trityl-nitroxides are similar to values for nitroxides between 80 and 120 K.
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Moore, Whylder, "EPR Relaxation of Modified Triphenylmethyl and Nitroxide Radicals" (2021). Electronic Theses and Dissertations. 1963.
Received from ProQuest
Physical chemistry, Chemistry