Dielectric properties of two diastereoisomers of the arabinose and their equimolar mixture

Dielectric relaxation measurements were performed on two enantiomers, d- and l-arabinose and their equimolar mixture, and compared to dielectric data obtained for d-ribose. d-Arabinose differs from d-ribose by having the opposite configuration at C2. This study reveals that both d- and l- of arabinose exhibit α-relaxation peaks with the same shape for the same α-relaxation time τ_α, and the same steepness index for the T_g-scale T-dependence of τ_α. However, the two isomers have slightly different glass transition temperatures T_g’s, and their secondary γ-relaxation times also differ slightly from the previously observed γ-relaxation in d-ribose at the same temperature. However, when samples of both investigated monosaccharides are annealed at higher temperatures, their glass transition temperatures become nearly identical. This is an effect of the mutarotation process, which leads to the formation of pairs of the enantiomers and accordingly they should have the same physical properties. The width of the α-relaxation of d- and l-arabinose is broader than that of d-ribose, as reflected by the smaller stretch exponent in the Kohlrausch-Williams-Watts function used to fit the data of the former (β_KWW = 0.46 ± 0.01) than the latter (β_KWW = 0.55 ± 0.01). The width of the α-relaxation of racemic mixture of the d- and l-arabinose is slightly broader than that of the pure isomers. While the dielectric loss data of d-ribose in the glassy state at ambient and elevated pressures show an inflexion indicating the presence of the JG β-relaxation, the data of d- and l-arabinose show no such feature for identification of the supposedly universal JG β-relaxation. Nevertheless, on comparing the loss spectra of d-arabinose with that of d-ribose, the presence of the JG β-relaxation in d-arabinose has been rationalized.