Thermochemistry of α-D-xylose(cr)

Abstract

The thermochemistry of α-D-xylose(cr) was studied by means of oxygen bomb calorimetry and a Physical Property Measurement System (PPMS) in zero magnetic field. The sample of α-D-xylose(cr) used in this study was one well-characterized by HPLC, Karl Fischer analysis, NMR, and by carbon dioxide analysis. The standard molar enthalpy of combustion was found to be ΔcHmo=−(2342.2±0.8) kJ·mol−1 at T=298.15K and at the standard pressure p°=0.1MPa. The standard molar heat capacity for α-D-xylose(cr) was measured with the PPMS over the temperature range 1.9001⩽T/K⩽303.66. At T=298.15K, Cp,mo=(178.1±1.8)J·K−1·mol−1. The values of Cp,mo were fit as a function of T by using theoretical and empirical models for appropriate temperature ranges. The results of these fits were used to calculate values of Cp,mo, the entropy increment Δ0TSmo, Δ0THmo, and Φmo=(Δ0TSmo-Δ0THmo/T) from T=0.5K to T =300K. Derived quantities for α-D-xylose(cr) are the standard molar enthalpy of formation ΔfHmo=−(1054.5±1.1) kJ·mol−1, the third law standard molar entropy Smo=(175.3±1.9)J·K−1·mol−1, and the standard molar Gibbs energy of formation ΔfGmo=−(750.5±1.0) kJ·mol−1. A comparison of values of ΔcHmo and Smo for the five-carbon aldoses demonstrated a striking similarity in the values of these respective properties for α-D-xylose(cr), D-ribose(cr), and D-arabinose(cr). Thermochemical network calculations were performed that led to values of the standard formation properties at T=298.15K for a variety of biochemical substances: D-xylose(aq), D-xylose−(aq), D-xylose2−(aq), D-lyxose(cr and aq), D-lyxose−(aq), D-xylulose(aq), xylitol(aq), 1,4-β-D-xylobiose(am and aq), and 1,4-β-D-xylotriose(am andaq).