A SOLID-STATE N-15 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC AND QUANTUM-CHEMICAL INVESTIGATION OF NITROSOARENE-METAL INTERACTIONS IN MODEL SYSTEMS AND IN HEME-PROTEINS

We have obtained the solid-state N-15 nuclear magnetic resonance isotr opic chemical shifts and/or shielding tensor elements for a range of n itrosoarene complexes: p-[N-15]nitroso-N,N-dimethylaniline, p-[N-15]ni troso-N,N-dimethylaniline hydrochloride monohydrate, PdCl2(p-[N-15]nit roso-N,N-dimethylaniline ZnCl2(p-[N-15]nitroso-N,N-dimethylaniline)(2) , 2(CH3)(2)(p-[N-15]nitroso-N,N-dimethylaniline)(2), PdCl2([N-15]nitro sobenzene)(2), [Fe(CO)(3)([N-15]nitrosobenzene)](2), and the [N-15]nit rosobenzene adducts of horse heart myoglobin and adult human hemoglobi n. The isotropic chemical shifts range from 171 to 802 ppm downfield f rom NH3(ext,1). Using a density functional method, we have computed th e isotropic shifts, the shielding tensor elements, and the absolute sh ieldings, for each of these compounds. There is excellent accord betwe en theory and experiment. In addition, the orientations of the tensors have been calculated, and for the dimer of PhNO, cis-dioxyazodibenzen e, there is good accord with an experimental determination of the shie lding tenser. Our results indicate that the shielding patterns observe d from compound to compound are overwhelmingly dominated by the behavi or of all, the least shielded element of the shielding tensor, which i s oriented close to the N-O bond vector (perpendicular to the PhNO pi orbital). We also find an excellent correlation between sigma(11) and the N-O Mayer bond order, with hemoglobin, myoglobin and all model com pounds fitting the correlation well (R-2 = 0.963). The nitrosoarenes h ave among the largest known N-15 shielding tensor widths, but by using density functional methods,it is possible to accurately compute them, even when they are bonded to transition metals. Overall, these result s thus represent the first comprehensive NMR and quantum chemical stud y of RNO bonding to heme proteins and model systems, and should form t he basis for future comparative studies of the biologically important isoelectronic species, dioxygen.