Paramagnetic NMR relaxation in polymeric matrixes: sensitivity enhancement and selective suppression of embedded species (1H and 13C PSR filter)

Filtering the NMR Spectra of Mixtures by Coordination to Paramagnetic Cu2

The paramagnetic spin relaxation (PSR) filter allows the selective NMR signal suppression of components in mixtures according to their complexation ability to a paramagnetic ion. It relies on the faster relaxation of nuclei in paramagnetic environments and thus is complementary to classical diffusion and relaxation filters. So far, the PSR filter has established Gd³⁺ as the sole PSR agent, restricting the paramagnetic filtering repertoire. Herein, we present Cu²⁺ as a robust PSR agent with characteristic filtering properties. While Gd³⁺ depends on unspecific ion-pair interactions with anionic components, Cu²⁺ stands out for filtering species via ordered coordination complexes. An evaluation of the paramagnetic effect of Cu²⁺ over more than 50 small molecules and polymers has unveiled different sensitivities to Cu²⁺ (especially high for pyridines, diamines, polyamines, and amino alcohols) and precise filtering conditions for mixtures (¹H, COSY, and HMQC) that were challenged with a test bed of commercial drugs. The advantage of integrating Cu²⁺ and Gd³⁺ for the stepwise PSR filtering of complex mixtures is also shown.

Fast NMR Screening of Macromolecular Complexes by a Paramagnetic Spin Relaxation Filter

The paramagnetic spin relaxation filter is described for the rapid NMR screening of intermolecular interactions between ligands and macromolecular anionic receptors with large transverse relaxation enhancements (R 2p). The addition of micromolar concentrations of Gd3+ to the mixture produces the immediate broadening/suppression of the NMR signals of interacting species while leaving unaffected those of noncompetitive binders (one-dimensional and two-dimensional experiments). The method is highly sensitive, unveiling interactions that are too weak to generate changes in chemical shifts or relaxation times. It is operationally very simple and hence, it is amenable to ready implementation by nonspecialists. Examples of application such as detecting the formation of interpolymer complexes, cyclodextrin host-guest interactions, and the screening of DNA ligands are included that demonstrate the reliability and broad applicability of the method.

Emerging new strategies for successful metabolite identification in metabolomics

This review discusses strategies for the identification of metabolites in complex biological mixtures, as encountered in metabolomics, which have emerged in the recent past. These include NMR database-assisted approaches for the identification of commonly known metabolites as well as novel combinations of NMR and MS analysis methods for the identification of unknown metabolites. The use of certain chemical additives to the NMR tube can permit identification of metabolites with specific physical chemical properties.

Predicting PSR filters by transverse relaxation enhancements

The paramagnetic spin relaxation (PSR) filter allows the suppression of the NMR resonances of individual components in mixtures according to their Gd(3+)-complexing ability. The difficulty in predicting this property hampers, however, the widespread application of this filter. Herein we describe that the PSR filter is dominated by the transverse relaxation enhancement (R(2p)) experienced by nuclei in the presence of Gd(3+), so that R(2p) represents a reliable predictive tool of suppression in the 1D and 2D PSR filter of complex mixtures. The robustness of R(2p) as a predictive tool in PSR filters has been demonstrated at different magnetic fields and for the (1)H, (13)C, COSY, and HMQC filtering of commercial multicomponent compositions, including beverages and drugs.

NMR methods for unravelling the spectra of complex mixtures

The main methods for the simplification of the NMR of complex mixtures by selective attenuation/suppression of the signals of certain components are presented. The application of relaxation, diffusion and PSR filters and other techniques to biological samples, pharmaceuticals, foods, living organisms and natural products are illustrated with examples.