Carbohydrates internal motion

Owing to the complexity of the internal motion of carbohydrate molecules, the elucidation of their conformational properties by MO calculations requires a lessening of dimensionality to manageable proportions. Several small acyclic molecules have therefore been used as models for ab initio or semiempirical MO studies on the structural segments of carbohydrates. On the whole, calculations reproduce all of the main structural trends and conformational preferences observed experimentally in the crystal structures of carbohydrates and in solution. 

These investigators employed the CHARMm force field with the carbohydrate parameters of Ha et al. jn j s MD simulations in vacuo. They note particularly, as have others, >135 that the use of the isolated spin pair approximation (ISPA), which is often used to convert NOE intensities into interproton distances, can be extremely inaccurate. Within ISPA, the assumption is made that the NOE intensity NOEfj) between two protons arises only from spin relaxation between the two protons. This approximation neglects the effects of spin diffusion and internal motion. Taking for calibration a known interproton distance (r f) and its associated NOE (NO f), the ISPA distance two protons i and j) may be derived from Eq. [14].i35... 

These measurements can then be used to derive dynamic models of carbohydrates and oligosaccharides. It is now generally accepted that carbohydrates are dynamic molecules with internal motions that occur on a time-scale faster than the overall motion of the molecules. 

We present here coarse grain simulation results of two simple binary water-carbohydrate mixtures that shed light on the mechanism of water diffusion in the supercooled and glassy state, the coupling of water mobility in the glass to the sub-Tg dynamics of the matrix, and the effect of the internal modes of the saccharide on the decoupling of water motion from carbohydrate translation. 

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