High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: Effects of spin-spin coupling

By combining magnetic field cycling in the range from 0.1 mT to 7 T with high-resolution NMR detection the T1 relaxation dispersion (nuclear magnetic relaxation dispersion (NMRD)) of protons in the nucleotides adenosine mono-phosphate and guanosine mono-phosphate was measured in a site-specific way. While at high field the individual spins have distinctly different T1 times, their scalar spin-spin interaction fulfills at low field the condition of strong coupling and leads to convergence of their T1 dispersion curves. In addition, the spin-spin coupling can lead to oscillatory components in the relaxation kinetics traceable to a coupling between spin polarization and coherence in the relaxation process. As a consequence the NMRD curves do not directly reflect the spectral density function of the motional processes, but the effects of motion and spin coupling must be separated for a reliable evaluation. A theoretical approach is described allowing such an analysis.