Kinetics of NMR spin-lock polarization transfer in crystalline glycine and spin-lattice relaxation of amino acids.

We report data determined from proton-carbon polarization-transfer kinetics at 23 degrees C for six common solid amino acids. Proton spin-lattice relaxation times in the rotating frame, T(1rhoH), for alpha-glycine, alanine, cysteine, leucine, isoleucine, and valine determined from the long-time decay of the carbon magnetization indicate that the presence of a mobile entity such as a methyl group shortens T(1rhoH) to a few milliseconds. Polarization transfer between protons and carbons in polycrystalline alpha-glycine is analyzed and compared to theoretical models, two of which account for the variation of polarization-transfer rate with orientation of the dipole-dipole vector in the magnetic field. A generalization of a model proposed by Mueller et al. (Phys. Rev. Lett. 32 (1974) 1402) reproduces the observed polarization transfer in alpha-glycine with reasonable accuracy, showing that the early time development reflects orientational variation of dipolar oscillations.