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.