A molecular dynamics study and solution-phase 1H and 13C chemical shift anisotropy determination of a symmetric cryogenic disaccharide, α,α-trehalose, has been performed in a temperature range between 264 and 350 K. Negligible temperature dependence of proton-carbon couplings of the asymmetrically [1-13C]-labeled trehalose suggest that the averaged conformation of the interglycosidic linkage is centered around dihedral angles φ = ψ, = -41°with ±5°uncertainty. Homonuclear NOE-s in the labeled trehalose support the dominance of similar conformation. Close to the slow and fast motional regime, α,α-trehalose can be considered as a spherical top, and global correlation times can be determined easily at extreme temperatures. This allows the construction of an Arrhenius plot for the whole range of temperature. The approach, which we call 'trouble-free', yields E(a) = 28.2 kJ/mol for the activation energy of molecular reorientation. The model-free analysis of 13C T1, T2, and NOE data showed a local maximum of the generalized order parameters with S2 = 0.9 around 273 K. Monte-Carlo error analysis corroborated that this effect could be real; however, effective correlation times have relatively high error limits. Thermodynamically, the S2 data can be interpreted in terms of changes in the Gibbs free energy due to increased or diminished spatial restriction of rapid CH fluctuations. Liquid state 1H and 13C chemical shift anisotropies were determined from the interference of dipole-dipole and chemical shift anisotropy relaxation. In solution, chemical shift anisotropies cannot be separated from an inherent geometrical factor, so a combined CSA(g) factor was used. Cross-correlated spectral densities could be well fitted for the C-1,H-1 vector over the entire temperature range with the 'trouble-free' global correlation times. The resulting numerical values for CSA(g) were smaller compared to the model-free evaluation, due to the omission of internal fluctuations. The measured shift anisotropies were found to be independent from the selection of isotropic or anisotropic dynamical models. Apparent CSA(g) factors were nearly constant in the entire temperature range except C-3, H-2, and H-3. Comparison with deuterium labeled [2,4,6-2H]trehalose proved that temperature-induced changes of the ABX-type strong coupling pattern (caused by the change of differential chemical shift of vicinal H-2 and H-3 protons) interfere with asymmetric multiplet relaxation and potentially lead to misinterpretation of CSA/DD relaxation rates.
ASJC Scopus subject areas
- Colloid and Surface Chemistry