Solid-state NMR analyses reveal the structure dependence of the molecular dynamics for ω-amino acids

The molecular dynamics of metabolites is structure dependent and vitally important for the interactive functions in their potential applications as natural materials. To understand the relationship between molecular structure and dynamics, the molecular motions of four structurally related ω-amino acids (β-alanine, γ-aminobutyric acid, 5-aminovaleric acid, and 6-aminocaproic acid) were investigated by measuring their proton spin-lattice relaxation times (T(1), T(1ρ)) as a function of temperature (180-440 K). (13)C CPMAS NMR and DSC analyses were performed to obtain complementary information. All of these ω-amino acids showed no phase transition in the temperature range studied but had outstandingly long proton T(1) at 300 MHz and even at 20 MHz for the deuterated forms. The molecular dynamics of all these ω-amino acids were dominated by the reorientation motions of amino groups and backbone motions except in β-alanine. The activation energies for amino group reorientations were positively correlated with the strength of hydrogen bonds involving these groups in the crystals and the carbon-chain lengths, whereas such energies for the backbone motions were inversely correlated with the carbon-chain lengths. These findings provided essential information for the molecular dynamics of ω-amino acids and demonstrated the combined solid-state NMR methods as a useful approach for understanding the structural dependence of molecular dynamics.