RELAXIVITY ENHANCEMENT OF LOW-MOLECULAR-WEIGHT NITROXIDE STABLE FREE-RADICALS - IMPORTANCE OF STRUCTURE AND MEDIUM

The longitudinal relaxivities of seven water-soluble nitroxide derivat ives of low-molecular weight have been measured at 5 degrees C and 37 degrees C in water and in serum between 0.01 and 200 MHz. The nuclear magnetic relaxation dispersion (NMRD) profiles show a clear relationsh ip between the relaxivity observed in serum and the relative balance o f the hydrophobic/hydrophilic character of the paramagnetic molecules. From the data analysis, contributions arising from a population of ni troxides characterized by reduced mobility can be extracted. The value s of the correlation times are consistent with a system involving nitr oxides adsorbed at the surface of albumin and magnetically interacting with the protons of hydrogen bonded water molecules.