Raman Bands of Malto- and Isomalto-Oligosaccharides as Models for Starch

Abstract

Raman spectroscopy has been employed as a valuable analytical method for starch; however, assignments of starch Raman bands in the low wavenumber region are still not sufficiently discussed. The complicated starch structure is a major limitation for theoretical calculations. To further elucidate starch Raman bands based on an experimental approach, the present study measured glucose, malto-oligosaccharides (degree of polymerization 2–8), and isomalto-oligosaccharides (degree of polymerization 2–5) as models and compared them with starch. X-ray diffraction analysis revealed that starch, glucose, maltose, and malto-pentaose exhibited crystalline structures, whereas non-crystalline halos were detected for the other saccharides. In the Raman measurements, 12 bands were observed in crystalline starch in the range of 100–1000 cm⁻¹; however, several bands became barely observable or disappeared, and the intensities of all bands declined in non-crystalline starch. Similar Raman bands and spectral changes were observed for crystalline and non-crystalline malto-pentaose, suggesting that α (1 → 4) glycosidic chains with degree of polymerization ≥ 5 share common Raman bands. Furthermore, a distinctive starch band at 478 cm⁻¹ was observed in malto-oligosaccharides with degree of polymerization ≥ 2 and its intensity increased with chain length, whereas this band was absent in all isomalto-oligosaccharides. These results indicate that the Raman band at 478 cm⁻¹ is attributed to the vibration of α (1 → 4) glycosidic chains with degree of polymerization ≥ 2, and not to α (1 → 6) glycosidic linkages.