title: An investigation of the equilibria between aqueous ribose, ribulose, and arabinose.
abstract: The thermodynamics of the equilibria between aqueous ribose, ribulose, and arabinose were investigated using high-pressure liquid chromatography and microcalorimetry. The reactions were carried out in aqueous phosphate buffer over the pH range 6.8-7.4 and over the temperature range 313.15-343.75 K using solubilized glucose isomerase with either Mg(NO3)2 or MgSO4 as cofactors. The equilibrium constants (K) and the standard state Gibbs energy (delta G degrees) and enthalpy (delta H degrees) changes at 298.15 K for the three equilibria investigated were found to be: ribose(aq) = ribulose(aq) K = 0.317, delta G degrees = 2.85 +/- 0.14 kJ mol-1, delta H degrees = 11.0 +/- 1.5 kJ mol-1; ribose(aq) = arabinose(aq) K = 4.00, delta G degrees = -3.44 +/- 0.30 kJ mol-1, delta H degrees = -9.8 +/- 3.0 kJ mol-1; ribulose(aq) = arabinose(aq) K = 12.6, delta G degrees = -6.29 +/- 0.34 kJ mol-1, delta H degrees = -20.75 +/- 3.4 kJ mol-1. Information on rates of the above reactions was also obtained. The temperature dependencies of the equilibrium constants are conveniently expressed as R in K = -delta G degrees 298.15/298.15 + delta H degrees 298.15[(1/298.15)-(1/T)] where R is the gas constant (8.31441 J mol-1 K-1) and T the thermodynamic temperature.
mesh_terms: D019747:Aldose-Ketose Isomerases; D001089:Arabinose / Q000378:metabolism*; D002238:Carbohydrate Epimerases / Q000378:metabolism; D007700:Kinetics; D010429:Pentoses / Q000378:metabolism*; D012266:Ribose / Q000378:metabolism*; D012996:Solutions; D013302:Streptomyces / Q000201:enzymology; D013816:Thermodynamics
chemical_list: D010429:Pentoses; D012996:Solutions; C100182:ribulose; D012266:Ribose; D001089:Arabinose; D002238:Carbohydrate Epimerases; D019747:Aldose-Ketose Isomerases; C023298:xylose isomerase