Transferred cross-relaxation and cross-correlation in NMR. Effects of intermediate exchange on the determination of the conformation of bound ligands

Exchange transferred effects in soln.-state NMR expts. allow one to det. the conformation of ligands that are weakly bound to macromols. Exchange-transferred nuclear Overhauser effect spectroscopy (TR-NOESY) provides information about internuclear distances in a ligand in the bound state. Recently the possibility of obtaining dihedral angle information from a ligand in the bound state by exchange-transferred cross-correlation spectroscopy (TR-CCSY) was reported. In both cases the anal. of the signal amplitudes is usually based on the assumption that rapid exchange occurs between the free and bound forms of the ligand. We show that the fast exchange condition is not easily attained for observing exchange-transferred cross-correlation effects even in systems where exchange-transferred NOE can be obsd. Extensive simulations based on anal. expressions for signal intensities corresponding to fast, intermediate, and slow chem. exchange were carried out on a test system to det. the exchange regimes in which the fast exchange condition can be fulfilled for successfully implementing TR-NOESY and TR-CCSY. [on SciFinder (R)]