Studies of Solution Paramagnetic-Substrate Nuclear and Electron Intermolecular Interactions

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Virginia Tech

Advanced nuclear and electron magnetic resonance techniques (i.e. nuclear magnetic resonance (NMR), dynamic nuclear polarization (DNP), and magnetic resonance imaging (MRI)) were used to study the attitude and dynamics of TEMPO (2,2,6,6-tetramethylpiperidinyloxy)-substrate systems and the relaxivity properties of water-soluble trimetallic nitride template functionalized endohedral metallofullerenes (TNT-fMF). The attitude and average distance of interaction for each TEMPO-substrate system was determined from comparing density functional theory (DFT) calculation results with experimental hyperfine coupling constants leading to an improved understanding of solution dynamics. The short-lived solvent-solute interactions of the TEMPO-substrate molecules, such as transient complex formation, are governed by weak hydrogen-bonding interactions. The collisions in solution were explained by determining the favored orientations of the two molecules interacting using calculated relative energy minima and reproducibility of the experimental results by the calculated coupling constants.

Water-soluble TNT-fMFs are studied as candidates for the next generation MRI contrast agents as diagnostic agents and also as possible therapeutic agents to kill cancer cells and decrease tumors. The TNT-fMFs are being studied as part of a multi-modal platform dependent upon which metal atoms are encapsulated inside: Gd — MRI contrast agent (diagnostic), Lu and Ho — radio labeled for use as a therapeutic agent, Tb – fluorescence, and Lu – x-ray contrast. The current commercial MRI contrast agent, OmniscanTM, contains one gadolinium atom; however, the metal is complexed to, not encapsulated in, the molecule. TNT-fMFs fully encapsulate three metal atoms to ensure the patient does not run the risk of metal poisoning. The r1 and r2 relaxivities of TNT-fMFs containing either Gd, Lu, Ho, or Sc metals were measured at 0.35T. The data for the Gd containing TNT-fMFs indicated the metallofullerene has significantly higher relaxivities than OmniscanTM, and can be the next generation MRI contrast agent. The Ho containing species has a high r2/r1 ratio compared to the other samples showing it is a potential T2 agent, and has therapeutic capabilities.

Relaxivity, TEMPO, Hyperfine Coupling Constant, Magnetic Resonance Imaging, Endohedral Metallofullerene, Contrast Agent