Electron paramagnetic resonance spectra of model magnetic resonance im
aging 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 solut
ion simulates environments that may arise in vivo and also when the co
ntrast agents are bound either covalently or noncovalently (e.g., via
hydrogen bonds) to another molecule either by chance or by design to i
mprove the relaxivity and the specificity. We report that an isotropic
Brownian model, which described the rotational dynamics of these agen
ts in water, does not fully portray the rotational dynamics in this en
vironment, especially in the intermediate or slow tumbling regime. How
ever, an axial anisotropic rotational model simulated the experimental
spectra well and can account for the trends exhibited by the data. Th
e dynamics results suggest that the motion deviates substantially from
Stokes-Einstein behavior, and possible physical models to account for
the behavior are discussed.