Spin lattice relaxation in an AM{Xn} system: Application to 13C relaxation in enriched molecules

Abstract

In 13C-enriched molecules, 13C–13C dipolar interactions can contribute significantly to the relaxation of 13C nuclei which are not directly bonded to protons. Comparison of the relaxation behavior of the 13C–13C multiplet lines with that of the singlet for which there are no 13C–13C dipolar interactions allows a quantitative analysis of this interaction. However, since 13C–13C dipolar interactions are necessarily weak, substantial contributions to the relaxation of nonprotonated carbon atoms are made by protons attached to adjacent atoms. The general features of the spin lattice relaxation behavior of an AM{X} system in which {X} represents the decoupled protons, A, the nonprotonated carbon, and M, the adjacent protonated carbon, have been derived. In general, the A relaxation will be nonexponential and dependent on the initial state of the Mz magnetization. A particularly useful application of these measurements on 13C-enriched systems is found in their sensitivity to internal and/or anisotropic motions. Such rotations affect the 13C–13C and 13C−1H dipolar interactions differently, depending on the particular geometry involved and the axis of rotation. These techniques have been applied to the observed relaxation behavior of 6-phosphogluconate enriched to the 79% level. In this case the contributions to the intramolecular dipolar relaxation of the carboxyl carbon consist primarily of the 13C–13C interaction with its adjacent carbon, the 13C−1H interaction with the proton on the adjacent carbon, and a significant contribution from the proton attached to the third carbon in the chain. For this case, it is found that the carboxyl relaxation will be, to a very good approximation, exponential and independent of the initial value of Mz. Furthermore, a significantly improved fit of the data results if anisotropic rotation effects are considered.