Relaxivity enhancement of low molecular weight nitroxide stable free radicals: Importance of structure and medium

The longitudinal relaxivities of seven water‐soluble nitroxide derivatives ot low‐molecular weight have been measured at 5°C and 37°C in water and in serum between 0.01 and 200 MHz. The nuclear magnetic relaxation dispersion (NMRD) profiles show a clear relationship between the relaxivity observed in serum and the relative balance of the hydrophobichydrophilic character of the paramagnetic molecules. From the data analysis, contributions arising from a population of nitroxides characterized by reduced mobility can be extracted. The values of the correlation times are consistent with a system involving nitroxides adsorbed at the surface of albumin and magnetically interacting with the protons of hydrogen bonded water molecules.