Spin–spin and spin-lattice contributions to the rotating frame relaxation of 13C in L-alanine

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Abstract
The spin-lattice relaxation times in both the Zeeman (TC1) and rotating (TC1ρ) frames were determined for three chemically distinct carbon atoms (13C) in polycrystalline L-alanine by combining crosspolarization and magic angle spinning techniques together with proton decoupling. The spin-lattice and spin–spin contributions to the experimentally measured TC*1ρ could be separated by an experiment in which the 13C spin-locking field was varied. The spin-lattice contributions (TC1ρ), which contain motional information, were determined to be 21.7, 23.4, and 138 ms for the Cα, CH3, and COO− carbons, respectively. The spin–spin contribution (TDCH) was found to be exponential, namely, (TDCH)−1 ∝exp(−2πνeCτD) in the low 13C spin-locking field. Therefore, the assumption of a Lorentzian correlation function for the proton dipolar fluctuations is adequate for L-alanine. Furthermore, the proton dipolar correlation times τD were found to be the same (31±1 μs) for all three carbons in L-alanine. The spin-lattice relaxation times in the Zeeman frame TC1 were determined to be 4.0 s, 38 ms, and 13 s for the Cα, CH3, and COO− carbons, respectively. The experimental values of TC1ρ and TC1 can be explained quantitatively by considering the internal rotations of the methyl and amino groups.