Spin-lattice relaxation of 13C in solid amino acids using the CP-MAS technique

Abstract

It is shown by a simple application of relaxation theory that the 13C magnetization decays nonexponentially, in principle, in the CP-MAS experiment because of the distribution of the spin-lattice relaxation times; however, the deviation from the exponential decay is quite small. The transient Overhauser effect also contributes appreciably to the nonexponential decay of the 13C magnetization when the protons are not saturated during the 13C T 1 measurements and the correlation time of the group rotational motion satisfies the condition, ω 2 τ c 2 ≦ 1. It is shown by both experiment and theory that the transient Overhauser effect in the solid state is much smaller than that expected for the liquid state. The 13C spin-lattice relaxation times of l-alanine, deutero- l-alanine, glycine, and l-serine were determined for the individual carbon atoms. The experimentally obtained 13C T 1 values agree well with calculated ones, showing that the CH 3 group rotation provides the main source of the relaxation in alanine, while the NH 3 + group motion plays an important role for the relaxation in glycine and serene.