Comprehensive Solid-State NMR Analysis Reveals the Effects of N-Methylation on the Molecular Dynamics of Glycine

Abstract

Molecular dynamics of metabolites are important for their interactions and functions. To understand the structural dependence of molecular dynamics for N-methylated glycines, we comprehensively measured the ¹³C and ¹H spin–lattice relaxation times for sarcosine, <i>N</i>,<i>N</i>-dimethylglycine, betaine, and betaine hydrochloride over a temperature range of 178–460 K. We found that the reorientations of methyl groups were observed for all these molecules, whereas reorientations of whole trimethylamine groups were detected in betaines. While similar rotational properties were observed for methyl groups in <i>N</i>,<i>N</i>-dimethylglycine and those in betaine, three methyl groups in betaine hydrochloride had different motional properties (<i>E</i>_a ≈ 20.5 kJ/mol, τ₀ ≈ 1.85 × 10^–13 s; <i>E</i>_a ≈ 13.9 kJ/mol, τ₀ ≈ 2.1 × 10^–12 s; <i>E</i>_a ≈ 15.8 kJ/mol, τ₀ ≈ 1.1 × 10^–12 s). <i>N</i>,<i>N</i>-Dimethylglycine showed a phase transition at 348.5 K with changed relaxation behavior for methyl groups showing distinct <i>E</i>_a and τ₀ values. The DIPSHIFT experiments showed that CH₃ and CH₂ moieties in these molecules had dipolar-dephasing curves similar to these moieties in alanine and glycine. The activation energies for CH₃ rotations positively correlated with the number of substituted methyl groups. These findings provided useful information for the structural dependence of molecular dynamics for N-methylated glycines and demonstrated solid-state NMR as a useful tool for probing the structure–dynamics relationships