Examination of number-average degree of polymerization and molar-based distribution of amylose by fluorescent labeling with 2-aminopyridine

Abstract

Suitable conditions for the fluorescent labeling of the reducing residue of amylose with 2-aminopyridine were examined. Amylose of up to 38.5 nmol was labeled with a constant labeling efficiency. The same efficiencies were obtained for amyloses having a number-average degree of polymerization (dp n) of 521–4400. The analysis of labeled amylose on size-exclusion HPLC with refractive index and fluorescence detection enabled the determination of dp n and dp distribution on a molar basis. The analysis of eight amylose specimens from seven botanical sources (potato, sweet potato, barley, wheat, indica rice, japonica rice, and maize) gave dp n values in good agreement with those determined by a conventional colorimetric method. The molar-based distributions of these amyloses were characteristic of botanical source and revealed the presence of several molecular species with different dp not detectable in the distribution on a weight basis. Small amyloses with a dp less than 10 3 were predominant in the cereals while amyloses with a dp over 10 3 were predominant in the tubers, suggesting a difference in the biosynthetic process determining the dp distribution of amylose between cereals and tubers.