Activity of Water and Osmotic Coefficients of Histidine Derivatives in Aqueous Solutions at 310.15 K |
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Authors: | Elena N. Tsurko Roland Neueder Werner Kunz |
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Affiliation: | (1) Scientific Research Institute of Chemistry, V.N. Karasin National University, Svoboda Sq. 4, 61077 Kharkiv, Ukraine;(2) Institute of Physical and Theoretical Chemistry, University of Regensburg, 93040 Regensburg, Germany |
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Abstract: | 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). |
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Keywords: | Vapor 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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