Isotropic contact interaction shifts in the proton NMR spectra of pyridine-type molecules coordinated with paramagnetic nickel (II) and cobalt (II) acetylacetonates have been observed. The role of π- and σ- bonding frameworks in the transfer of unpaired electron spin density from the metal ion to ligand protons is considered. Pseudocontact interactions appear to be significant in the cobalt systems but not in the nickel systems. When the pseudocontact contribution to isotropic shifts in cobalt systems is factored out there remain contact shifts which are in good agreement with the observed nickel shifts. It is concluded that the unpaired electrons in metal eg orbitals of both cobalt and nickel systems cause contact shifts in the pyridine-type ligands whereas the t2g electron in the cobalt system does not. The t2g electron gives isotropic shifts by contributing to anisotropy in the electronic g factor.

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The cooked meat mutagen 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) is metabolized in vivo to electrophilic intermediates that covalently bind to DNA guanines. Here we address the mechanism of PhIP's non-covalent interaction with DNA by using spectroscopic and computational methodologies. NMR methodologies indicated that upon addition of DNA, PhIP aromatic protons underwent a small, 0.11-0.12 p.p.m. upfield shift. DNA phosphorus resonances of non-covalent PhIP-DNA complexes broadened and slightly shifted upfield, while DNA base imino proton resonances shifted slightly downfield relative to DNA alone. UV and fluorescence spectra of PhIP titrated with DNA showed no detectable shifting and hypochromism of absorbance or fluorescence bands. In the presence of DNA, PhIP fluorescence was efficiently quenched by acrylamide, but not by silver ion. Further, the NMR spectra suggest that PhIP is in fast exchange with the DNA, and is slightly specific for adenine-thymine (A-T) sequences. Finally, structural arguments based on quantum chemistry calculations suggested that PhIP and its metabolites are unlikely to intercalate into DNA. These data collectively indicate that PhIP non-covalently binds in a groove of DNA.