title: Selective injection of magnetization by slow chemical exchange in NMR.
abstract: In a system in slow dynamic equilibrium two NMR methods are shown to be suitable for injecting magnetization from one resonance to another by means of slow chemical exchange. The combined outputs of the methods may be employed to measure the value of the off-rate constant kappa(off) in the complex. The methods are implemented experimentally using the complex of molecules composed of the enzyme Escherichia coli dihydrofolate reductase (DHFR) and the ligand folate. In an equilibrium solution with DHFR, folate is known to undergo chemical exchange between a free state and a bound state. The modified synchronous nutation method is applied to a spin of the folate molecule in the free and bound states; magnetization transfer occurs between the two sites due to the underlying exchange process. As a preliminary step for the application of the synchronous nutation method, a new one-dimensional 1H NMR technique is proposed which facilitates the assignment of the resonance of a spin in the bound state, provided the resonance of its exchange partner in the free state is known. This experiment is also used to obtain quantitative estimates of the transverse relaxation rate constant of the bound resonance. The numerical procedure necessary to analyze the experimental results of the synchronous nutation experiment is presented.
authors.affiliation: The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California 92037, USA. bboulat@hahn.coh.org
authors.forename: B
authors.initials: B
authors.lastname: Boulat
authors.forename: D M
authors.initials: DM
authors.lastname: Epstein
authors.forename: M
authors.initials: M
authors.lastname: Rance
mesh_terms: D004926:Escherichia coli / Q000201:enzymology; D005492:Folic Acid / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D007700:Kinetics; D008024:Ligands; D008280:Magnetics; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D013113:Spin Labels; D013762:Tetrahydrofolate Dehydrogenase / Q000737:chemistry* / Q000378:metabolism
chemical_list: D008024:Ligands; D013113:Spin Labels; D005492:Folic Acid; D013762:Tetrahydrofolate Dehydrogenase