Thermodynamic data are available for the dimerization of small molecules containing peptide groups in water and tetrachloromethane. Dimerization is opposed by loss of translational and rotational entropy and the freezing out of in;ernal rotations and is favored by the formation of two amide-amide hydrogen bonds. The magnitude of the translational entropy lost in solution can be estimated from the Sackur-Tetrode equation in the gas phase, which is corrected by Trouton's rule for condensation to a liquid and a dilution term to a 1 M solution. If the effects of the translation, rotation, and internal rotation are removed from the thermodynamic parameters for complexation, the free energy, entropy, and enthalpy for the formation of peptide hydrogen bonds are obtained. This free energy will be close to the theoretical maximum intrinsic binding energy of a hydrogen bond. It is found that the formation of a hydrogen bond is highly favorable in both aqueous and nonpolar solvents by a mean value of -26 kJ mol-' in water and -32 kJ mol-' in tetrachloromethane. These values may be too large by up to
This paper describes the isolation and structure elucidation of two toxic tannins, named a-and ppunicalagin, from Terminalia oblongata, an Australian tree. Techniques used in the structure determination included Fast Atom Bombardment (FAB) mass spectrometry, and one-and twodimensional N M R, UV, and I R spectroscopy. A number of microscale derivatisations were performed, followed by FAB analysis.Methylation.-Component B (0.1 mg) was dissolved in diethyl ether (1 ml). A solution of diazomethane in diethyl ether Paper 0/00673D
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