PGSE-WATERGATE : a new tool for NMR diffusion-based studies of ligand-macromolecule binding

A new pulsed gradient spin-echo NMR diffusion sequence, PGSE-WATERGATE, which is based on the extremely efficient WATERGATE solvent suppression sequence, is presented. The sequence is simple to set up and particularly suited to measuring the diffusion coefficients of small ligands in aqueous solution such as is commonly required in pharmaceutical and combinatorial applications. It also affords the possibility of measuring the diffusion of exchangeable resonances, which is often impossible in conjunction with other suppression schemes. Further, a trivial modification of the sequence affords the possibility of multiple solvent suppression, thereby increasing its suitability to LC-NMR applications. The utility of the sequence is demonstrated on the salicylate-bovine serum albumin system. The dissociation constant, K _d, and the number of binding sites were found to be 0.030 M and 33, respectively. Importantly, the extremely high degree of suppression provided by the new sequence allowed the salicylate diffusion coefficients to be measured over a very wide concentration range sufficient to show that the salicylate-bovine serum albumin system is not well described by a simple two-site model. Previous studies in the literature have been based on data from a smaller concentration range, for which this model gives an apparently good fit.