Carbon-13 nuclear magnetic relaxation and motional behavior of carbohydrate molecules in solution

This chapter describes the carbon-13 nuclear magnetic relaxation and the motional behavior of carbohydrate molecules in solution. The principal source of 13C relaxation is intramolecular dipole–dipole (DD) interaction between a 13C nucleus and neighboring protons. Local magnetic fields are generated not only by DD interactions, but also by other sources such as electron dipoles, electric quadrupoles, anisotropy in the chemical shielding tensor, modulation of scalar coupling, and spin rotation. Each of these interactions represents a separate relaxation mechanism, which may contribute to the measured relaxation parameters. For small or medium-sized molecules undergoing rotational motion in the extreme narrowing limit, the spectral density is field-independent and the nuclear overhauser effect (nOe) attains its maximum value. On the other hand, for a rigid spherical or nearly spherical molecule undergoing diffusive rotational motion in the extreme narrowing limit, a single correlation time is adequate to describe the overall motion. When the overall motion is not isotropic, the diagonal elements of the rotational diffusion tensor are no longer equivalent and the rotation about the three principal axes of the diffusion tensor may be described by different diffusion coefficients or correlation times. It is suggested that the treatment of internal motion of a flexible segment superposed on overall motion requires an appropriate time-correlation function describing that motion. © 1995 Academic Press, Inc.