Activity of Water and Osmotic Coefficients of Histidine Derivatives in Aqueous Solutions at 310.15 K
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From the data of vapor pressure osmometry the activity of water, osmotic coefficients, and the values of activity coefficients of two derivatives of histidine: N-Boc-L-histidine (Boc-His-OH, m=0.005–0.14 mol⋅kg−1) and N-Boc-L-histidine-methyl ether (Boc-His-OMe, m=0.005–0.05 mol⋅kg−1) are obtained in aqueous solutions at 310.15 K. From the comparison of water activity and osmotic coefficient values it follows that Boc-His-OMe shows a more pronounced deviation from ideality than Boc-His-OH. Both components exhibit a stronger non-ideality than histidine and a weaker one than His⋅HCl. By means of potentiometric titration the acid-base properties of Boc-His-OMe are investigated and the ionization constant at 298.15 K is determined. The pK value related to the acid-base equilibrium of the nitrogen atom in the imido group of the imidazole ring is higher (6.47) than the corresponding value of histidine (6.00).
KeywordsVapor pressure osmometry Emf Activity of water Activity coefficients Osmotic coefficients N-Boc-L-histidine N-Boc-L-histidine methyl ether Zwitterions Ionic form of amino acid ⋅
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