Relaxivity studies on dinitroxide and polynitroxyl functionalized dendrimers: effect of electron exchange and structure on paramagnetic relaxation enhancement

The 1H NMR relaxivity of o- and p-dinitroxide-substituted phthalate esters and a series of nitroxyl-functionalized poly(propyleneimine) dendrimers was measured in acetonitrile and methanol. Studies of dinitroxide relaxivity indicate that the electron exchange rate has only a small effect on relaxivity. Outer-sphere relaxivity was measured using benzene as a probe mol. In studies on dendritic polynitroxides, the per-nitroxide-based outer-sphere relaxivity nearly doubles for the generation 5 nitroxyl-functionalized dendrimer as compared to a mononitroxide model. This relaxivity enhancement may be due to crowding of dendrimer surface groups in higher generation dendrimers. Water relaxivity was measured for these polynitroxides as well, and a significant inner-sphere contribution to relaxivity is found. Dendritic polynitroxides exhibit higher per-nitroxide-based water relaxivity as compared to a mononitroxide model. This relaxivity enhancement is attributed to an increase in rotational correlation time (tc) for the dendritic polynitroxides. [on SciFinder (R)]