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

[Image: see text] 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 Gd(3+) 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.