The Effect of Molecular Size on Molecular Mobility in Amorphous Oligosaccharides

The physical properties and especially the molecular mobility of amorphous carbohydrate matrixes directly influence the stability of foods, feeds, and pharmaceuticals and the dessication tolerance of animals and plants during anhydrobiosis. Phosphorescence of the sodium salt of erythrosin B was used to investigate the local molecular mobility in pure amorphous solids of a homologous series of malto-oligosaccharides (maltose, G₂; maltotriose, G₃; maltotetraose, G₄; maltopentaose, G₅; maltohexaose, G₆; and maltoheptaose, G₇); sucrose and maltodextrin DE18 (a hydrolytic fraction of starch) were investigated for comparison. Measurements of the temperature-dependence of the phosphorescence emission energy, an indicator of the extent of local dipolar relaxation, and the phosphorescence emission lifetime, an indicator of the rate of collisional quenching of the excited state by matrix molecules, demonstrate that the local matrix molecular mobility increases with molecular size, and thus, with an increase in T _g, in these glucose oligosaccharides. Master curves of the spectroscopic measures of matrix mobility for each oligosaccharide, plotted against T–T _g, were not superimposable, suggesting that local properties of the amorphous sugar matrixes, rather than T _g per se, influence local matrix mobility. Indicators of spectral heterogeneity also varied with molecular size, indicating that dynamic site heterogeneity also increased with molecular size and thus, T _g. These results emphasize the importance of additional research in developing appropriate “molecular rules” for designing amorphous matrix systems with better long-term stability for foods, feeds, or pharmaceuticals.