The Mechanism of Water‐Proton Relaxation in Enzyme · Paramagnetic‐Ion Complexes

The water proton relaxation times, T 1 and T 2 , have been measured in solutions of free Gd(III) ions and in Gd(III) solutions containing lysozyme over a wide range of temperatures (0–40°C) and nuclear frequencies (4–60 MHz) using pulsed nuclear magnetic resonance techniques. The equilibrium between lysozyme and Gd(III) has also been studied as a function of temperature. Water relaxation rates in both the free and enzyme‐bound metal sites have been determined and closely fitted to existing theories over the complete range using a combination of graphical and computing techniques. Values for the water co‐ordination number ( q ) and the correlation time (τ c ) for the dipolar relaxation processes have been obtained and the rate processes contributing to τ c have been analysed as a function of temperature and frequency. The values obtained for τ c in the enzyme complex differ considerably from some earlier estimates. In the aquo‐ion relaxation is dominated by the rotational correlation time (τ R ) but in the enzyme‐metal complex the electron spin relaxation time (τ s ) becomes more important than other processes at low frequencies. Possible sources of error in the data and assumptions made in the theoretical treatment are discussed. The implications of the results for mapping studies of lysozyme and other macromolecules, using magnetic resonance techniques, are stressed.