Dynamic directional carbohydrates

With the advent of molecular dynamics simulations applied to carbohydrates, one can anticipate the direct computation of more conceptually appealmg surfaces of V in 0s) space from a given U( qint,qext)) in the near future. Monte Carlo integration over (qext) and (b,x, 0h) for fix (0s) provides an alternative procedure, but one which is probably less attractive in terms of efficiency than the molecular dynamics approach. A second alternative, known as adiabatic mapping, provides an approximation to V((0s ), and applications of this method to carbohydrates have recently begun to appear. 12,13 in this approach the conformational... 

Molecular dynamics (MD) simulations are a class of molecular mechanics calculation which directly model the motions of molecular systems, often providing considerable information which cannot be obtained by any other technique, theoretical or experimental. MD simulations have only recently been applied to problems of carbohydrate conformation and motions, but it is likely that this technique will be widely used for modeling carbohydrates in the future. This paper introduces the basic techniques of MD simulations and illustrates the types of information which can be gained from such simulations by discussing the results of several simulations of sugars. The importance of solvation in carbohydrate systems will also be discussed, and procedures for including solvation in molecular dynamics simulations will be introduced and again illustrated from carbohydrate studies. 

It should be noted that none of these dynamic models is related directly to polysaccharide structures as a basis for describing possible modes of reorientation in a carbohydrate chain. Nevertheless, some of them appear to offer a good approximation for linear polysaccharides (for instance, amylose) as seen in the following sections. 

Because direct dynamic measurements of materials state within the barrel are impossible, the structural and molecular changes relevant to extrusion must be measured off-line and related to real process conditions. In principle, we need to explain not only the effect of applied physical parameters of heat, shear and pressure, but also the effect of formulation. This is not yet possible, and for reasons stated above, the behaviour of carbohydrate (starch) dominated systems will be quite different from proteinaceous systems, since their heat denaturation behaviour and glass to rubber transitions are different in detail during their conversion from a moist powder to a continuous melt. ... 

The unique advantages of NMR in the analysis of carbohydrate structure are only fully apparent in consideration of points 3 and 4. In contrast to the unbranched polymeric nature of polypeptide and nucleic acid chains, carbohydrates may be branched structures, capable of substitution at several points. The monosaccharide constituents are polymerized in nature by a non-template directed, enzymatic process the resulting oligosaccharides are often heterogeneous, differing in detail from a consensus structure. NMR is particularly efficient at investigating the solution conformations, and the dynamic properties of such molecules. 

In the case of Ti02, the dynamics of hole transfer has been studied for several adsorbates, e.g., anions (SCN [34, 35], 1 [26]), various alcohols [36, 37], carbohydrates [37], and 4-phenylbenzoic acid [38]. The timescales were found to span from 100 fs to 100 ns, depending on the adsorbate and the conditions. These timescales suggest that direct oxidation of molecules via holes can be an efficient pathway in photocatalysis. 

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