Rotational dynamics of MRI paramagnetic contrast agents in viscous media

Electron paramagnetic resonance spectra of model magnetic resonance imaging contrast agents vanadyl ethylenediaminetetraacetate and vanadyl diethylenetriaminepentaacetate in 46% w/v sucrose solutions have been acquired from near physiological temperature to near freezing to study their rotational dynamics in a viscous environment. The sucrose solution simulates environments that may arise in vivo and also when the contrast agents are bound either covalently or noncovalently (e.g., via hydrogen bonds) to another molecule either by chance or by design to improve the relaxivity and the specificity. We report that an isotropic Brownian model, which described the rotational dynamics of these agents in water, does not fully portray the rotational dynamics in this environment, especially in the intermediate or slow tumbling regime. However, an axial anisotropic rotational model simulated the experimental spectra well and can account for the trends exhibited by the data. The dynamics results suggest that the motion deviates substantially from Stokes-Einstein behavior, and possible physical models to account for the behavior are discussed.