Strong coupling effects during X-pulse CPMG experiments recorded on heteronuclear ABX spin systems:: artifacts and a simple solution

Simulation and experiment have been used to establish that significant artifacts can be generated in X-pulse CPMG relaxation dispersion experiments recorded on heteronuclear ABX spin-systems, such as C-13(i) -C-13(j)-H-1, where C-13(j) and C-13(j) are strongly coupled. A qualitative explanation of the origin of these artifacts is presented along with a simple method to significantly reduce them. An application to the measurement of H-1 CPMG relaxation dispersion profiles in an HIV-2 TAR RNA molecule where all ribose sugars are protonated at the 2' position, deuterated at all other sugar positions and C-13 labeled at all sugar carbons is presented to illustrate the problems that strong C-13-C-13 coupling introduces and a simple solution is proposed.