CHEMICAL-SHIFT AND LINEWIDTH CHARACTERISTICS OF REVERSIBLY BOUND LIGANDS

Chemical-shift and linewidth characteristics of reversibly bound ligands provide a useful source of information about binding kinetics. Conversely, it is often useful to predict these parameters for a particular set of ligand resonances if the kinetic behavior is known, For a two-state system, an analytical expression for the chemical-shift maxima as a function of the fraction of complexed ligand, pB, valid if T2B, T2F ≫ τB, can be derived. The resulting expressions are useful for the analysis of experimental data in the intermediate-exchange case and can be applied to chelator-ion equilibria and, depending on the value of T2B, ligand-macromolecule equilibria. Since this expression can be used regardless of whether the exchange is fast or slow, no a priori assumptions regarding the exchange rate, other than the condition noted above, are needed. Effects of shorter T2 values on resonance shift have been evaluated numerically. In general, it is found that if the linewidths of the two species in the absence of exchange differ significantly, the observed shift is weighted toward the sharper resonance. The relationship of the exchange-broadening maximum to the ligand dissociation rate constant in the fast-exchange limit is also considered. Evaluation of the concentration dependence of the exchange broadening provides a useful basis for determining whether a simple kinetic scheme for reversible binding provides a valid description of the exchange process. Several illustrative applications are presented. © 1993 Academic Press. All rights reserved.