Redfield and coworkers exploited this method to further characterize interactions of small molecules with proteins. Redfield and Roberts also introduced the use of electron spin labels on proteins to identify the binding modes of small molecules thanks to paramagnetic relaxation enhancements at low field. Gradziel et al. demonstrated that spin labelling combined with It was shown that the analysis of the relaxation profile can also allow for determining the size of ligand-protein complexes as well as that of metabolite-protein complexes in human blood serum. In the case of a water solution of alanine, sodium 3-(trimethylsilyl)propionate-2,2,3,3-d4 (TSP) and bovine serum albumin, the relaxation profiles measured for the methyl protons of alanine and TSP are quite different as a result of the absence (for alanine) or of the presence (for TSP) of binding with albumin (Figure ). The position of the low field dispersion in the relaxation profile of TSP is consistent with a correlation time of ca. 40 ns, in good agreement with the reorientation time that has been estimated for albumin. On the other hand, a reorientation correlation time of 36 ps was obtained from the relaxation rates of alanine. 67 macromolecular assemblies, offering the potential to dissect the contributions of various interactions and dynamic processes at an unprecedented level of detail.