Sorbitol conformation

Also, in 1890 Fischer had proven that the reduction of fructose with sodium amalgam yields a mixture of mannitol and sorbitol and pointed out that this conformed with the van t HofF-Le Bel theory (19). It seems, therefore, that the idea of asymmetric induction was clearly in a state of incubation prior to his publication of the relative configurations of the sugars in 1891. 

To date, only a few solution calculations for carbohydrates have been attempted (one such study of mannitol and sorbitol is described in the chapter by Grigera in this volume), but the results of these early studies bear out the expectation that solvation effects in carbohydrate systems can be both significant and difficult to predict. In the case of pyranoid rings, molecular solvation is further complicated by the close juxtaposition of these groups in essentially fixed relative orientations (assuming no conformational changes in the ring). Under such circumstances, molecular stereochemistry could play important physical roles, as is... 

Molecular dynamics (MD) simulations show that the conformations of sorbitol and mannitol depend on the typ e of solvent. The predicted conformations agreed well with experiment, supporting the view that MD has a good predictive value for solutions of carbohydrates. Preliminary dynamics results for methoxy-tetrahydropyran (MTHP) show that the methoxy group moves more in water than in vacuum. 

Sorbitol and mannitol represent a pair of hexytols that differ only in the configuration of one hydroxy group at C2. This slight difference in their configurations gives both compounds differing physicochemical properties. For example, sorbitol is three and one half times more soluble than mannitol in water. Previous MD simulation of these hexytols (2) pointed out some characteristics that warrant further discussion. In particular their conformations depended on the solvent system. 

Most enzyme powders are prepared by lyophilisation (freeze drying). However, the lyophilization procedure might inactivate the enzyme to some extent. To avoid this and thereby increase the activity of lyophilized enzymes in dry organic solvents, the lyophilization can be carried out in the presence of lyoprotectants such as sorbitol (Dabulis and Klibanov, 1993). The inactivation is believed to be caused at least partly by a reversible conformational change in the enzyme. This process can be reversed and the enzyme reactivated by the addition of small amoimts of water (Dabulis and Klibanov, 1993). 

Ketoprofen and 17 commonly used excipients were chosen as the probe systems for this study [81]. The molecular modelling has two parts prediction of crystal morphology followed by the prediction of binding energy of a probe molecule on the surfaces of the ketoprofen crystal. Three conformers for each of these excipients (probe molecules) were taken into consideration. For those excipient molecules which have more than one molecule in the asymmetric unit (e.g. sorbitol and a-lactose monohydrate), both the complete asymmetric unit as well as three conformers of a single molecule were considered as the probe conformations for the systematic search study. For those excipients that are polymers, a representative monomer unit was used as a probe molecule. 

Protein stability may be regarded as the opposite of denaturation. The stability of enzymes (and proteins) can be increased in many ways, e.g., by microenvironmental changes, immobilization, and protein engineering (78). Enzymes are more stable in the presence of polyols (ethylene glycol, glycerol, erythritol, and sorbitol), polymers (PEG, dextrans), and carbohydrates (sucrose, lactose, and trehalose). Hydrophilic enzymes are stabilized by the presence of salts (LiCl, NaCl, and KCl), whereas hydrophobic enzymes are hardly affected by salts. Proteins are also stabilized by compounds that bind specifically to the folded conformation. Most of the metalloenzymes and the enzymes that have an anion-binding site fall into this category. 

The trioxadecalin core 113 of mycalamide B 112 (which is structurally related to pederin) has been synthesized from D-mannitol (by Roush ) and D-sorbitol (by Hoffinann ). Hoffmann s approach is outlined in Scheme 23 (see next page), but both pieces of work incorporate interesting points. The unusual (and unstable) C(10) aminal unit of 112 has the opposite stereochemistry to that required by the natural product, but prior work has shown that equilibration at this centre is feasible. Also, both groups found that 112 exists as the ring-flipped c/s-decalin conformer in contrast to the conformational preference expressed by mycalamide B itself. 

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