Measurement of Chemical Exchange Rate Constants with Solvent Protons Using Radiation Damping

When chemical exchange occurs between a concentrated and a diluted species like the proton exchange between water and proteins in biochemical systems, it is difficult to use the conventional methods for a quantitative dynamical study. In this paper, a new approach which utilizes radiation damping of the strong water magnetization exchanging with the weak resonance of the mobile protons in biomolecules is proposed. It is demonstrated with a dilute guanidine solution, using the simple inversion-recovery pulse sequence, that the exchange rate constants between water and guanidine protons are evaluated by solving the Bloch equations modified with both the radiation damping and the chemical exchange effects.