Low-frequency cooperative dynamics in L-, D-, and DL-alanine crystals:: A 13C and 15N cross-polarization magic-angle-spinning NMR study

Knowledge of the dynamical changes in molecular configurations in various amino acid structures over a wide range of time scales is important since such changes may influence the structural transformations and the diverse biological functionalities of proteins. Using the temperature dependence of the rotating-frame NMR spin-lattice relaxation times T-1 rho of protons as a probe, we have investigated the low-frequency (similar to 60-100 kHz) dynamics in the crystal structures of L-, D-, and DL- alanine (C12H28O8N4) polymorphs. The proton relaxation times T-1 rho were obtained from C-13 <- H-1 and N-15 <- H-1 cross-polarization magic-angle-spinning NMR experiments over a temperature range of 192-342 K. The data reveal that the time scales of these low-frequency dynamical processes are distinctly different from the localized, high-frequency rotational motion of methyl and amine groups. The strongly asymmetric T-1 rho versus temperature curves and the subtle dynamical differences between the DL-alanine and the L- and D-enantiomorphs indicate that these low-frequency processes are cooperative in nature and are sensitive to molecular packing.