relax web

Relaxometry is a technique that has been developed to obtain structural and dynamical information on nuclear spin systems. In the presence of a paramagnetic metal ion in the compound under investigation, relaxometry may provide information on the coordination of the nuclear spin with respect to the paramagnetic metal and, indirectly, information on the electron spin system. In fact, if the water proton exchange rate is fast or of the same order as the NMR timescale, the magnetic properties of the paramagnetic center are carried over from the water in bound position to the bulk. Experiments are performed by measuring the relaxation rate of bulk water protons as a function of the external magnetic field. In this way, Nuclear Magnetic Relaxation Dispersion (1H NMRD) profiles are obtained, using field cycling relaxometers from 0.01 to 50 MHz of proton Larmor frequency, and high-resolution spectrometers for larger frequencies.

Parameters affecting the paramagnetic enhancement in 1H NMRD profiles are the electron spin quantum number, the number and distance of protons in the first and second metal coordination sphere, the presence of freely diffusing waters and their distance of closest approach to the metal ion, the rotational time of the complex, the electron relaxation time and its field dependence, the lifetime of coordinated water protons, the anisotropy of the g factor, the presence of zero field splitting (and its rhombicity), and the presence of electron-metal nucleus hyperfine coupling (and its rhombicity).

For the first time, direct relaxation profiles of protein protons in D2O at mM concentration were measured at CERM in 2004 through a prototype of the Stelar high sensitivity fast field cycling relaxometer. We have shown that direct protein proton relaxation dispersion data yield information on the lack of rigidity, through the order parameter, which is directly related to foldedness (including the extreme situation of a completely unfolded protein), and on protein aggregation, through a safe estimate of the re-orientational time of the protein.

To access software for the calculation of paramagnetic enhancements of nuclear relaxation rates, click here.