Low-Frequency Cooperative Dynamics in l-, d-, and dl-Alanine Crystals:  A ¹³C and ¹⁵N Cross-Polarization Magic-Angle-Spinning NMR Study

Abstract

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(1rho) of protons as a probe, we have investigated the low-frequency (approximately 60-100 kHz) dynamics in the crystal structures of L-, D-, and DL- alanine (C(12)H(28)O(8)N(4)) polymorphs. The proton relaxation times T(1rho) were obtained from (13)C <-- (1)H and (15)N <-- (1)H 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(1rho) 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.

Adsorption of Amino Acids on Oxide Supports: A Solid-State NMR Study of Glycine Adsorption on Silica and Alumina

“…In fact, CP signals strongly depend on the contact time, which has to be optimized experimentally. The problem is not trivial and has been discussed before, , including in the case of bulk glycine and the related alanine . For the carboxylate carbon, which does not directly bear a hydrogen, an approximate formula for the evolution of signal intensity ( I ) as a function of experimental CP contact time ( t c ) is, according to Sen et al I = I° true( 1 − T C H T 1 ρ true) − 1 ( e − t normalc / T 1 normalρ − e − t normalc / T normalC normalH ) …”

“…The problem is not trivial and has been discussed before, 13,38 including in the case of bulk glycine 18 and the related alanine. 39 For the carboxylate carbon, which does not directly bear a hydrogen, an approximate formula for the evolution of signal intensity (I) as a function of experimental CP contact time (t c ) is, according to Sen et al 39 Although this expression may be misleading in general for powder samples, 18 it has shown a satisfactory fit for the carboxylate carbon. The maximum intensity I°can only be approached if the { 1 H-13 C} polarization transfer characteristic time T CH is much smaller than the rotational frame relaxation time for hydrogen, T 1F .…”