Optical extraction of the helical pitch angle of amylopectin in starch

Abstract

Starch is one of the major constituents of our everyday diet and it forms granules. Starch granules are basically consisting of amylose and amylopectin molecules and they are among the brightest nature-made second harmonic generation (SHG) converters. In this study, we take advantage of that and we perform polarization sensitive SHG (PSHG) imaging of starch granules. We fit the SHG signal variation of each pixel of the PSHG images into a biophysical model and we extract molecular information below the experimental resolution limit. Specifically, by assuming that the SHG source molecule is a helix with cylindrical symmetry along its long axis, the model extracts the helical pitch angle of the SHG source and the orientation of its supporting filament for every pixel of the image. Pixel by pixel fitting consequently creates new images which their contrast is based on the values of the fitting to the theoretical model. Then we chose a region of interest in the image and we create pixels' values histograms. We applied the above in wheat starch granules and we found a highly peaked pixels' histogram of helical pitch angles at θe = 36.2°. This pitch angle corresponds to the strand of the parallel double helical structure, called amylopectin (as measured by small angle X-ray scattering). Thus, using an optical technique we extracted the helical pitch angle of amylopectin in starch. This angle value can be used as a quantitative biomarker capable of characterizing the quality of starch based structures and products.