The proton affinities and deprotonation enthalpies of β-D-fructopyranose and α-L-sorbopyranose

Abstract

The proton affinities (PAS) and deprotonation enthalpies (DPEs) were calculated for the pyranoid forms of two naturally occumng sugars, D-fructose and L-sorbose. In both molecules the PAS of the primary hydroxyl group (HO-1), the anomeric hydroxyl group (HO-2), and the ring-oxygen atom (0-6) were calculated, as were the DPEs of HO-1 and HO-2. The stabilities of the conjugate acids and bases of these sugars are enhanced by the presence of intramolecular hydrogen bonding, a feature that is significant in explaining the differences in sweetness and the rates of mutarotation of the title compounds, as well as the differences in the rates of acid-catalyzed hydrolysis of ketopyranosides.Key words: proton affinity, deprotonation enthalpy, ab initio calculations, AMl, hexuloses. IntroductionThe most direct computational approach to the measurement of atomic and molecular acidities and basicities is through evaluation of the deprotonation enthalpies (DPEs) and proton affinities (PAS) of the acids and bases, respectively. Ab initio computational methods are well suited for the evaluation of the PAS and DPEs of small molecules, and have been employed extensively for such calculations (1). Del Bene (2) has reported the effects of basis set choice and electron correlation on ab initio calculations of the PAS of several oxygen and nitrogen bases, consisting of compounds containing no more than two non-hydrogen atoms. The most accurate results reported by her were within 1-3 kcal/mol of the experimental values, and were obtained from calculations employing a triple-split valence basis set (6-311G) with two sets of polarization functions and one set of diffuse functions on all non-hydrogen atoms, and one set of polarization functions on the hydrogen atoms. It should be noted that it has been recently reported (3) that the 6-3 11G basis

Radicalization and Radical Catalysis of Biomass Sugars: Insights from First-principles Studies

“… 19 , realizing the “one-pot” synthesis of HMF from glucose with relatively high conversion and selectivity. On the other hand, glucose has lower acidity than fructose 20 21 22 23 and hence is less reactive under basic conditions. Recently, Liu et al .…”

“…D-glucose (monomer of cellulosic biomass) and D-fructose (pivotal intermediate for cellulose utilization), in both α- and β-anomers, have been considered. To best of our knowledge, all previous sugar conversions focused on O sites, which are preferred over C sites during protonation 16 17 18 36 37 38 and deprotonation 20 21 22 23 . Here a systematic study was also conducted for O-centered sugar radicals, and comparisons with the results of C-centered radicals clearly indicated that the formation of C-centered radicals is always preferential, thus suggesting the possibility to activate and convert the C-H bonds of biomass sugars.…”