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

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 (dpn) of 521–4400. The analysis of labeled amylose on size-exclusion HPLC with refractive index and fluorescence detection enabled the determination of dpn 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 dpn 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 103 were predominant in the cereals while amyloses with a dp over 103 were predominant in the tubers, suggesting a difference in the biosynthetic process determining the dp distribution of amylose between cereals and tubers.