Exploring the conformational energy landscape of glassy disaccharides by cross polarization magic angle spinning C13 nuclear magnetic resonance and numerical simulations. II. Enhanced molecular flexibility in amorphous trehalose

The aim of this article is to assess the ability of chemical shift surfacesto provide structural information on conformational distributions ofdisaccharides in glassy solid state. The validity of the general method leadingto a simulation of inhomogeneous 13C chemical shift distributions is discussedin detail. In particular, a proper consideration of extrema and saddle pointsof the chemical shift map correctly accounts for the observed discontinuitiesin the experimental CPMAS spectra. Provided that these basic requirements aremet, DFT/GIAO chemical shift maps calculated on relaxed conformations lead to avery satisfactory description of the experimental lineshapes. On solid-statetrehalose as a model of amorphous disaccharide, this simulation approachdefines unambiguously the most populated sugar conformation in the glass, andcan help in discriminating the validity of different models of intramolecularenergy landscape. Application to other molecular systems with broadconformational populations is foreseen to produce a larger dependence of thecalculated chemical shift distribution on the conformational map