Effects of Substituents on the Properties of Metal-Free MRI Contrast Agents

[Image: see text] Materials possessing electron spin can shorten the T(1) relaxation times in magnetic resonance imaging (MRI). For example, gadolinium (Gd) complexes with seven f-orbital electrons are widely used as contrast agents in clinical applications. However, Gd has severe potential side effects, and thus metal-free alternatives are needed. Toward this end, we synthesized seven NO radicals consisting of a dioxa-azaspiro[4.5]decane framework having various substituents, DAD-X (X = methyl, ethyl, n-propyl, c-propyl, vinyl, phenyl, and 2-pyridyl), that functioned as metal-free MRI contrast agents. The relationship between (i) water–proton relaxivity and log P and (ii) reactivity for ascorbic acid and the spin density of the NO oxygen atom were established, which provided a basis for the rational design of practical metal-free contrast agents.