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 alpha-relaxation peaks with the same shape for the same alpha-relaxation time tau(alpha), and the same steepness index for the T(g)-scale T-dependence of tau(alpha). However, the two isomers have slightly different glass transition temperatures T(g)'s, and their secondary gamma-relaxation times also differ slightly from the previously observed gamma-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 alpha-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 (beta(KWW)=0.46+/-0.01) than the latter (beta(KWW)=0.55+/-0.01). The width of the alpha-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 beta-relaxation, the data of D- and L-arabinose show no such feature for identification of the supposedly universal JG beta-relaxation. Nevertheless, on comparing the loss spectra of D-arabinose with that of D-ribose, the presence of the JG beta-relaxation in D-arabinose has been rationalized.