Thermochemistry of α-D-xylose(cr)

The thermochemistry of alpha-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 alpha-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 Delta H-c(m)o = -(2342.2 +/- 0.8) kJ.mol(-1) at T = 298.15 K and at the standard pressure p degrees = 0.1 MPa. The standard molar heat capacity for alpha-D-xylose(cr) was measured with the PPMS over the temperature range 1.9001 <= T/K <= 303.66. At T = 298.15 K, C-p,m(o) = (178.1 +/- 1.8) J.K-1.mol(-1). The values of C-p,m(o) 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 C-p,m(o), the entropy increment Delta S-T(0)m(o), Delta H-T(0)m(o), and Phi(o)(m) = (Delta S-T(0)m(o) - Delta H-T(0)m(o)/T) from T = 0.5 K to T = 300 K. Derived quantities for alpha-D-xylose(cr) are the standard molar enthalpy of formation Delta H-f(m)o = -(1054.5 +/- 1.1) kJ.mol(-1), the third law standard molar entropy S-m(o) = (175.3 +/- 1.9) J.K-1.mol(-1), and the standard molar Gibbs energy of formation Delta(f)G(m)(o) = -(750.5 +/- 1.0) kJ.mol(-1). A comparison of values of Delta H-c(m)o and S-m(o) for the five-carbon aldoses demonstrated a striking similarity in the values of these respective properties for alpha-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 = 298.15 K for a variety of biochemical substances: D-xylose(aq), D-xylose(aq), D-xylose(2) (aq), D-lyxose(cr and aq), D-lyxose (aq), D-xylulose(aq), xylitol(aq), 1,4-beta-D-xylobiose(am and aq), and 1,4-beta-D-xylotriose(am and aq). (c) 2012 Elsevier Ltd. All rights reserved.