Calculated isotropic chemical shift (a) or magnetic shielding (b) of crystalline trehalose dihydrate, anhydrous trehalose, lactose monohydrate, and sucrose vs experimental chemical shifts . The results for all carbons of the four dimeric sugars are presented together. Calculations were carried out on isolated sugar molecule with atomic coordinates taken from crystallographic data and hydrogen atoms placed at positions corresponding to energy minima of the BIO85 force field: (a) Semiempirical TNDO method: (b) Ab initio density functional theory with GIAO method at levels (●), (엯), and (▾).
Snapshot of a trehalose conformation showing the relevant angular parameters and the symmetry occurring in this dimeric sugar (in this conformation, angles are set to ). Hydrogen atoms have been omitted for clarity.
(a) GIAO magnetic shielding calculated at the level of theory on different molecular structures of trehalose. Except for (∎), these structures were obtained by fixing the glycosidic dihedral angles to (; ), then optimizing the other degrees of freedom using AM1 (●), (엯), (▾), and (▵). For (∎), the structure was first fully optimized using , then kept rigid after fixing (; ) prior to the NMR calculation. (b) GIAO magnetic shielding calculated at the level of theory on different molecular structures of trehalose. First, the structure was fully optimized using the BIO85 empirical force field, then either kept rigid after fixing (; ) (엯) or reoptimized using the same force field for each couple of values (; ) (●).
(a) Adiabatic Ramachandran map of trehalose calculated with the CHARMM-type BIO85 force field parameters. The fluctuations of the calculations have been averaged following the symmetry of the trehalose molecule: . The lowest energy was found for and was conventionally chosen for origin . (b) isotropic chemical shift of trehalose carbon calculated on the same conformations as (a) by GIAO method on a DFT basis. A similar average as in (a) was applied, using the symmetry property .
Best fit simulations of the glycosidic region of the CPMAS spectrum of trehalose compared to the experiment (thin solid line). The simulations were carried out according to Eq. (1). Upper line (a): , with calculated with the following different force fields: BIO85 (bold solid line) and , CHARMM27 (dashed line) and , or AMBER99 (dotted line) and . Lower line (b): , with and or (90°,90°), respectively.
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