Publications

Abstract The thermochemistry of adenosine(cr) was studied by a variety of methods: the standard (pressure p degrees = 0.1 MPa) molar enthalpy of combustion in oxygen at T = 298.15 K was measured by means of combustion calorimetry and was found to be Delta (c)H(m)degrees = -(5139.4 +/- 3.3) kJ . mol(-1); the standard molar heat capacity C(p,m)degrees was determined by means of adiabatic calorimetry for the range 11 less than or equal to (T/K) less than or equal to 328 {C(p,m)degrees = (290.10 +/- 0.58) J . K-1 . mol(-1) at T = 298.15 K}; and the saturation molality in water was measured by means of high-performance liquid chromatography and was found to be m(sat) (1.849 +/- 0.015) . 10(-2) mol . kg(-1) at T = 298.15 K. Derived quantities at T = 298.15 K for adenosine(cr) are the standard molar enthalpy of formation Delta (f)H(m)degrees = -(653.6 +/- 3.6) kJ . mol(-1), the third law standard molar entropy S(m)degrees = (289.57 +/- 0.6) J . K-1 . mol(-1) and the standard molar Gibbs free energy of formation Delta (f)G(m)degrees = -(204.4 +/- 3.6) kJ . mol(-1). The function Phi (m)degrees - Delta TS(m)degrees - Delta (T)(0)H(m)degrees /T for adenosine(cr) was tabulated for 5 less than or equal to (T/K) less than or equal to 320. A thermochemical cycle was used to calculate Delta (f)G(m)degrees = -(194.5 +/- 3.6) kJ . mol(-1), Delta (f)H(m)degrees = -(621.3 +/- 3.6) kJ . mol(-1), and m S(m)degrees = (364.6 +/- 0.7) J . K-1 mol(-1) for adenosine(aq) at T = 298.15 K. These values in turn were used to calculate standard molar formation properties at T = 298.15 K and ionic strength I = 0 for the adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), and adenosine 5'-triphosphate (ATP) series of aqueous species. Standard molar transformed formation properties for the aqueous biochemical reactants adenosine, AMP, ADP, and ATP have also been calculated at T = 298.15 K, PH = 7.0, pMg 3.0, and I = 0.25 mol . kg(-1). (C) 2001 Academic Press.

Abstract Pancreatic ribonuclease A may be cleaved to produce two fragments: the S-peptide (residues 1-20) and the S-protein (residues 21-124), The S-peptide, or a truncated version designated as the S15 peptide (residues 1-15), combines with the S-protein to produce catalytically active complexes. The conformation of these peptides and many of their analogues is predominantly random coil at room temperature; however, they populate a significant fraction of helical form at low temperature under certain solution conditions. Moreover, they adopt a helical conformation when bound to the S-protein. A hybrid sequence, disulfide-stabilized peptide (ApaS-25), designed to stabilize the helical structure of the S-peptide in solution, also combines with the S-protein to yield a catalytically active complex. We have performed high-precision titration microcalorimetric measurements to determine the free energy, enthalpy, entropy, and heat capacity changes for the binding of ApaS-25 to S-protein within the temperature range 5-25 degreesC. The thermodynamic parameters for both the complex formation reactions and the helix-to-coil transition also were calculated, using a structure-based approach, by calculating changes in accessible surface area and using published empirical parameters. A simple thermodynamic model is presented in an attempt to account for the differences between the binding of ApaS-25 and the S-peptide, From this model, the thermodynamic parameters of the helix-to-coil transition of S15 can be calculated. (C) 2001 Wiley-Liss, Inc.

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation of crystalline bis(N-benzoyl-N',N'-diethylureato)copper(II), Cu(BETU)(2), and bis(N-benzoyl-N',N'-diisobutylureato)copper(II), Cu(BIBU)(2), were measured, at T = 298.15 K, by solution-reaction isoperibol calorimetry, respectively, as -(833.7 +/- 9.7) and -(1050.6 +/- 12.1) U mol(-1). The standard molar enthalpy of sublimation, at T = 298.15 K, of Cu(BETU)(2) Was determined by a Knudsen effusion technique as (180 +/- 37) kJ mol(-1). These values were used to derive the standard molar enthalpy of formation of Cu(BETU)(2) in gaseous phase, and to evaluate the mean metal-ligand dissociation enthalpy of the complex relative to that of the ligand with hydrogen.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline 3,5-dimethylpyrazole (Me(2)Pyr), 3,5-dimethyl-4-nitrosopyrazole (Me(2)PyrNO), 1,3,5-trimethyl-4-nitrosopyrazole (Me(3)PyrNO), and 3,5-dimethyl-1-phenyl-4-nitrosopyrazole (Me(2)PhPyrNO) were measured by static-bomb calorimetry. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds. The standard molar enthalpies of sublimation for these four compounds were measured by microcalorimetry. [GRAPHICS] The experimental results obtained allow us to derive the values of the standard molar enthalpies of formation, in the gaseous state, for the monomeric compounds involved in this work. These last values are discussed comparatively with results previously obtained for some aromatic nitroso derivatives. (C) 2001 Academic Press.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline picolinamide (2-NH2COPy), nicotinamide (3-NH2COPy), isonicotinamide (4-NH2COPy), nicotinamide N-oxide (3-NH2COPyNO), and isonicotinamide N-oxide (4-NH2COPyNO) were measured by static-bomb calorimetry. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three pyridinecarboxamide isomers were measured by microcalorimetry and the standard molar enthalpies of sublimation for the two pyridinecarboxamide N-oxide compounds were measured by a mass-loss effusion technique. From the enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies D-m(o) of the (N+-O-) covalent bonds were derived. Comparison has been made with D-m(o) (N-O) values in pyridine N-oxide derivatives. [GRAPHICS] (C) 2001 Academic Press.

Abstract The electrochemical behavior of the hydrolysis products of oxadiazon was studied by cyclic and square-wave voltammetry using a glassy carbon electrode. Maximum currents were obtained at pH 12.8 in an aqueous electrolyte solution containing 30% ethanol and the current did not decrease with time showing that there was little adsorption of the reaction products on the electrode surface. The hydrolysis products of oxadiazon were identified after isolation and purification, as 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl)-2-ethoxy-carbonylhydrazine (Oxa1) and 1-trimethylacetyl-2-(2.4-dichloro-5-isopropoxyphenyl) hydrazine (Oxa2) with redox potentials + 0.6V and -0.1V (vs. Ag/AgCl), respectively. Based on the electrochemical behavior of 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl) hydrazine (Oxa2) a simple electroanalytical procedure was developed for the determination of oxadiazon in commercial products used in the: treatment of rice crops in Portugal that contain oxadiazon as the active ingredient. The detection limit was 1 x 10 (-4) M, the mean content and relative standard deviation obtained for seven samples of two different commercial products by the electrochemical method were 28.4 +/- 0.8 % (Ronstar) and 1.9 +/- 0.2 % (Ronstar GR), and the recoveries were 100.3 +/- 5.4 % and 101.1 +/- 5.3 %, respectively.

Abstract The electrochemical behaviour of propanil and related N-substituted amides (acetanilide and N,N-diphenylacetamide) was studied by cyclic and square wave voltammetry using a glassy carbon electrode. Propanil has been found to have chemical stability under the established analytical conditions and showed an oxidation peak at +1.27 V versus Ag/AgCl at pH 7.5. N,N-diphenylacetamide has a higher oxidation potential than the other compounds of +1.49 V versus Ag/AgCl. Acetanilide oxidation occurred at a potential similar to that of propanil, +1.24 V versus Ag/AgCl. These results are in agreement with the substitution pattern of the nitrogen atom of the amide. A degradation product of propanil, 3,4-dichloroaniline (DCA), was also studied, and showed an oxidation peak at +0.66 V versus Ag/AgCl. A simple and specific quantitative electroanalytical method is described for the analysis of propanil in commercial products that contain propanil as the active ingredient, used in the treatment of rice crops in Portugal.

Abstract The oxidative stability of refined sunflower oil in the presence and in the absence of propyl caffeate (PC), propyl hydrocaffeate (PHC), propyl ferulate (PF), and propyl isoferulate (PI) has been evaluated according to the Rancimat method. The antioxidant activity of the phenolic derivatives was compared with that obtained with native [alpha -tocopherol (alpha -TOH)] and synthetic [propyl gallate (PG)] antioxidants. The results allow the establishment of a decreasing order of antioxidant power: PG > PHC > PC much greater than alpha -TOH > PI > PF. The oxidative stability was improved neither by the addition of PF nor by a supplement of alpha -TOH. Moreover, a positive antioxidant effect was obtained for PC that was placed between those of alpha -TOH and PG. The antioxidant activity of PHC was higher than that of its analogue (PC). A dose-dependent effect was observed for PG, PHC, and PC. A chain-breaking mechanism was proposed for the antioxidant activity of propyl phenolic esters because the same ranking order of efficacy was obtained for their antiradical activities evaluated by using the 2,2-diphenyl-1-picrylhydrazyl radical method.