Three-dimensional nuclear overhauser effect/ J-resolved spectroscopy

Abstract

Two-dimensional NMR has revolutionized materials characterization and solution structure determination mainly due to the increase in resolution that accompanies the introduction of a second frequency dimension (Z-3). Through-space nuclear Overhauser effect spectroscopy (NOESY) and through-bond J-correlated spectroscopy (COSY) reveal interactions that were previously obscured by peak overlap; these interactions can now be resolved and used to determine molecular connectivities and to measure internuclear distances and dihedral angles. However, for complex synthetic and natural macromolecules, ambiguities may still arise from the unfortunate overlap of cross peaks. This problem is particularly severe for synthetic materials where the magnetic environments for the carbon and hydrogen atoms along the main chain of the polymer are similar (3, 4). Small perturbations in the chemical shifts arise from stereochemical isomerism, but the changes are often not large enough to allow observation at a separate chemical-shift position. More frequently it is observed that the linewidths are inhomogeneously broadened from the chemical-shift dispersion. NMR experiments with three frequency dimensions (3D NMR) have recently been introduced as a method for resolving the ambiguities that arise from peak overlap in the NMR spectra of biopolymers (5-9). Three-dimensional NMR holds great promise for structure analysis and materials characterization but is experimentally and computationally demanding. Acquisition of the spectra requires large blocks of spectrometer time, and the massive data sets require high speed computers with large data storage capacities and sophisticated graphics terminals (5-7). As a compromise, 3D NMR experiments are collected and processed with lower digital resolution than is standard for most 2D NMR experiments ( 7). We found this lower digital resolution to be the limiting factor and present here the analysis of synthetic macromolecules using a combination of nuclear Overhauser effects and J-resolved spectroscopy. While many of the 2D and 3D NMR experiments developed for biomolecule structure elucidation can be used for materials characterization without modification, others are not directly applicable for synthetic polymers because of the differences between the two classes of polymers ( I, 3, 4, 10). The proteins and nucleic acids usually studied by NMR are in the molecular weight range of l-20 kilodaltons, they have a unique sequence of monomer units and a well-defined three-dimensional structure, and the proton resonances can frequently be assigned to specific sites on the polymer