Paramagnetic cobalt(II) as an NMR probe of dendrimer structure: Mobility and cooperativity of dendritic arms

Cobalt(II) has been utilized as an external paramagnetic H-1 NMR probe for the study of the structure of dendrimers that possess specifically located metal recognition sites. The hyperfine-shifted H-1 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 differen ce, EXSY, COSY, and TOCSY. Temperature-dependent T-1 values of the hyperfin e-shifted signals were used to conclude that the Co(II)-dendrimer complexes are in the "liquidlike" regime, indicative of a shell-like structure inste ad of a "dense-core" structure. The presence of sizable cavities within the dendrimers was observed including a loosely packed conformation for the 2, 6-diamidopyridino moiety to bind to potential guest molecules. Cooperativit y among the dendritic arms in metal binding is also observed, whereby two d endritic arms bind to the metal center at the same time. In the case of den drimers with the metal binding site located near the surface of the molecul e, such binding cooperativity is still observed despite the large degree of freedom of the metal-binding moiety. Cooperativity among the dendritic arm s cart thus be considered an intrinsic property, which has to be taken into consideration in future design of functional dendrimers for the purpose of specific recognition and catalysis. The hydrodynamic radii of these dendri mers have been determined by means of nuclear Overhouser effect at low temp erature. The study offers a method for the study of the dynamics of dendrim ers in solution under different conditions and upon ligand binding and reco gnition. The study also provides a tool for monitoring systematic variation of the metal binding site in different dendrimer frameworks for specific a pplications, such as catalysis and molecular recognition.