Solid state NMR studies of the molecular motions in the polycrystalline α-L-fucopyranose and methyl α-L-fucopyranoside

Molecular dynamics of polycrystalline α-L-fucopyranose and methyl α-L-fucopyranoside have been investigated using ¹³C and ¹H spin relaxation times over a range of temperatures (90-400 K). For methyl α-L-fucopyranoside at 300 K, both methoxyl and methyl groups had much shorter ¹³C T₁ than the carbons in the pyran ring. ¹³C T₁ relaxation measurements (at 74.56 MHz) as a function of temperature enabled characterization of the 3-fold rotations of the methoxyl (E_a ∼ 9 kJ/mol, τ_c ∼ 0.7 × 10^-13 s) and methyl (E_a ∼ 9 kJ/mol, τ_c ∼ 2 × 10^-13 s) groups. Proton T₁ (100 MHz) measurements of methyl α-L-fucopyranoside showed that the relaxation processes (E_a ∼ 11 kJ/mol, τ_c ∼ 0.7 × 10^-13 s), corresponding to rotations of the methoxyl and methyl groups, are not distinguishable and occurred together at about 131 K. For α-L-fucopyranose, proton T₁ showed a relaxation rate maximum at 224 K, attributed to the 3-fold rotation of methyl group (E_a ∼ 15 kJ/mol, τ₀ 2.6 × 10^-13 s). The rotation processes of the methyl groups dominated T_1ρ processes for both saccharides and appeared at the low-temperature end (90-150 K) of the temperature range. Contributions of hydroxyl groups to both T₁ and T_1ρ processes were obscured by the overwhelming relaxation efficiency of methyl groups. The proton second moment showed a reduction to a lower value at about 150 K due to the rotation of methyl groups.