Estimation of the Intrinsic Birefringence of the A, B and V Crystalline Forms of Amylose
2009
Shogren, Randal L.
Optical birefringence (Δn) for A, B, Va, Vh amylose crystals were calculated by vector summation of bond polarizabilities. Orientation of the O-H bond vectors in these amylose crystals is not well understood so results were given for a range of O-H conformers. For B amylose, Δn was calculated to be 0.043-0.058 for the Wu and Sarko crystal structure, while Δn = 0.0-0.01 for the now widely accepted crystal structure of Imberty and Perez. This latter estimate is somewhat lower than experimental values of 0.031-0.037 determined previously for highly oriented amylose films. Reasons for the discrepancy may include uncertainty in empirical polarizability parameters or neglect of contributions from amorphous components and water. Predicted values of Δn for A amylose were similar to B amylose while Δn for V amyloses were smaller (-0.04-0.0). These estimates of the intrinsic birefringence of amylose crystals should be helpful in estimating the degree of orientation for starch films/fibers and in better understanding the molecular origins of birefringence of oriented starch.
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