Paramagnetic Cobalt(II) as an NMR Probe for Dendrimer Structure, Mobility, and Cooperativity
Journal of the American Chemical Society 2001, 123, 85838592.

Jon D. Epperson, Li-June Ming,* Gregory R. Baker, and George R. Newkome*#
Department of Chemistry, the Institute for Biomolecular Science, and
the Center for Molecular Design & Recognition
University of South Florida
Tampa, FL 33620-5250

#Current address:
Department of Polymer Science
University of Akron
Akron, OH 44325-4717

Cobalt(II) has been utilized as an external paramagnetic 1H NMR probe for the study of the structure of dendrimers that possess specifically located metal recognition sites. The hyperfine-shifted 1H NMR signals of the Co(II) complexes of several 2,6-diamidopyridine-containing dendrimers have been fully assigned by means of 1D and 2D NMR techniques, including NOE difference, EXSY, COSY, and TOCSY. Temperature-dependent T1 values of the hyperfine-shifted signals were used to conclude that the Co(II)-dendrimer complexes are in the "liquid-like" regime, indicative of a shell-like structure instead of a "dense-core" structure. The presence of sizeable cavities within the dendrimers was observed including a loosely packed conformation for the 2,6-diamidopyridino moiety to bind to potential guest molecules. Cooperativity among the dendritic arms in metal binding is also observed, whereby two dendritic arms bind to the metal center at the same time. In the case of dendrimers with the metal binding site located near the surface of the molecule, such binding cooperativity is still observed despite the large degree of freedom of the metal-binding moiety. Cooperativity among the dendritic arms can thus be considered an intrinsic property, which has to be taken into consideration in future design of functional dendrimers for the purse of specific recognition and catalysis. The hydrodynamic radii of these dendrimers have been determined by means of nuclear Overhouser effect at low temperature. The study offers a method for the study of the dynamics of dendrimers in solution under different conditions and upon ligand binding and recognition. The study also provides a tool for monitoring systematic variation of the metal binding site in different dendrimer frameworks for specific applications, such as catalysis and molecular recognition.

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