Structure and reactivity of paramagnetic molecules by solid-state NMR – PARA-NMR

The characterization of catalytic metal ions and their environments is one of the greatest challenges of modern chemistry and biology. Precise understanding of the active site in a catalytic process is a key element for controlling these complex systems, modifying their behavior, and designing improved sites in a rational way. The cornerstone around which most of our insight into site structure/function correlations derives from atomic or molecular structures by X-ray diffraction on single crystal samples. Anyway, in many situations, as for example in the case of supported heterogeneous catalysts, or of large multisubunit metalloenzymes, or again of multidomain membrane systems, crystals large enough for diffraction cannot be obtained, and the structural characterization becomes very challenging. Solid-state NMR is an efficient tool for acquiring insight on micro crystalline and non-crystalline inorganic samples, including diamagnetic organometallic complexes. Many catalysts, however, depend on the use of paramagnetic metal ions. NMR potentially provides a method for determining paramagnetic shifts and relaxation rates, which are direct probes of the electronic structures in such important compounds. However, to date solid-state NMR studies of solid paramagnetic organometallic complexes have been hindered by these very same large orientation-dependent paramagnetic shifts and the paramagnetically-enhanced nuclear relaxation, which hamper both the critical steps of the acquisition of the NMR experiments and the following spectral assignment. The present proposal concerns the development of a new powerful set of solid-state NMR (SSNMR) techniques that will remove the key current barriers to progress in NMR spectroscopy on samples containing paramagnetic metal ions. This will enable previously inconceivable studies of new classes of molecules of high chemical and biological relevance containing paramagnetic metal ions, such as heterogeneous organometallic complexes, metalloproteins and large paramagnetic protein-protein assemblies. The project will be articulated along three parallel axes. First, we will develop a set of experimental approaches for obtaining NMR spectra from challenging paramagnetic materials. These techniques will allow the sensitive acquisition and the assignment of well-resolved NMR signals originating from nuclei close to a paramagnetic metal ion, for which traditional solid-state NMR approaches only afford extremely poor resolution and sensitivity. In a second part, we will develop new ways to determine structure and reactivity around a metal center using paramagnetic NMR effects. We propose to develop protocols for structure determination of powdered solids that simultaneously provide information not only on the conformation of the molecule in the lattice or on a surface, but also on the intermolecular arrangement in the solid phase in a sort of 'NMR crystallography'. Finally, we will explore protein-ligand and protein-protein interactions as determined via paramagnetic effects. The main objective of this application is to provide researchers in chemical and biological sciences with the possibility to routinely use NMR to obtain structural and reactivity information on immobilized catalysts or complex biological systems. The structure analysis of large macromolecular assemblies (membrane proteins, aggregates) presents the ultimate goal of this proposal. The subjects described here have wide ranging implications for molecular and biological sciences, and success in any of these areas will have an immediate impact on a potentially large community of users. We believe that this research will strongly contribute to the development of supramolecular, nano-scale and surface chemistry. Furthermore, the proposed advances in the study of metalloproteins correspond to one of the most exciting challenges of modern structural biology, as they would allow a deeper understanding of biochemical reactions, and thus providing chemists with models to engineer biomimetic processes. Finally, the possibility of determining interactions involving large (or insoluble) biological molecules, offered by paramagnetic labeling, will profile a new leading role for solid-state NMR in pharmaceutical drug research.