Use of nuclear magnetic resonance in probing ligand-macromolecule interactions

Nuclear magnetic resonance techniques can be used to study ligand-macromolecule interactions under a wide variety of conditions. There are a number of different approaches, which are generally applicable under different conditions of exchange rate and binding constant. In the tight binding limit direct structural studies using NOE are possible using labeled ligand (13C, 15N) and/or labeled macromolecule. In the intermediate exchange regime, line broadening complicates the analysis of bound conformations of ligands. However, a good deal of information can be obtained about the binding kinetics and about the number and nature of binding modes when several are present. In such cases it is often best to use an NMR active nucleus which can be directly detected, with high sensitivity. Tritium has good characteristics in this respect, and has been effective in a number of cases. In the fast exchange limit it is often best to use the rapid dissociation of the ligand to transfer structural information from the bound form to the free, where it can be sensitively and selectively detected, often by direct observation of 1H values. There are a number of conditions on the binding kinetics which must be met, and systems with multiple binding sites generally cannot be analyzed. The large number of different NMR approaches which can be used, often only under special conditions of exchange, complicates planning of NMR studies of ligand-macromolecule interactions. However, once the characteristics of binding affinity and exchange rate have been determined, it is usually possible to apply NMR spectroscopy to obtain information about the binding. The instrumental demands for carrying out ligand-macromolecule interactions are generally not more than those required for studies of the macromolecules alone.