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United States Department of Agriculture

Agricultural Research Service


item Momany, Frank
item Willett, Julious - J L

Submitted to: United States Japan Natural Resources Protein Panel
Publication Type: Abstract Only
Publication Acceptance Date: 9/15/2003
Publication Date: 11/9/2003
Citation: Momany, F.A., Willett, J.L. 2004. Water transport and motion in starch based biodegradable materials. Abstract p. 410-423.

Interpretive Summary:

Technical Abstract: We recently reported[1] the results of molecular dynamics simulations (NPT ensembles, 1 atm.) as applied to a periodic cell containing maltodecaose amylose fragments using the all atom force field AMB99C[2,3]. Molecular dynamics simulations were carried out for different hydration levels and calculated Tg values were in very good agreement with experimental values. Here we analyze the water molecules for cluster formation and obtain diffusion coefficients for maltodecaose. New studies are included for DP30 amylose fragments. Upon decreasing the hydration levels to less than 1H20/Residue or less, a number of striking effects on the microscopic structure and dynamics of these polymer solutions are observed. For example, the water cluster formation is not extensive, with clusters larger than six waters rarely found. Approximately half of the water molecules are not hydrogen bonded to other water molecules, interacting only with the carbohydrate hydroxyl groups. Further, unlike Brownian motion of water in the bulk liquid phase, activated water molecules in the amylose matrix have translational motions that can be described as occasional "hopping" movements. The "jump-like" movements are observed with appreciable magnitude difference in distance and pathway depending upon the ratio of water to carbohydrate in the cell and to the temperature studied. Below the glass transition temperature the water molecules tend to rotate similarly to higher temperature states (i.e. above Tg), but the translational diffusion and "hopping" is greatly reduced below Tg. The "jump-like" or "hopping" movements are described here and it appears that the larger jumps of less than 7A are somewhat unique to carbohydrates. That is, the water may move across the face of a glucose residue as the amylose chain moves up or down to open a path and this cooperative motion allows large translational movement of the water molecules. Activation energies are compared to values derived from native starches.

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