Thermodynamics of transfer of amino acids from water to ethanol-water mixtures

Abstract

Dissociation constants of <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-tryptophan, <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-histidine, <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-aspartic acid, <img src='http://www.niscair.res.in/jinfo/small.gif' border=0>-glutamic acid and t-serine have been determined in water and ethanol-water mixtures at 298 K potentiometrically by the Calvin-Bjerrum titration technique. These values have been used to calculate the free energies of transfer <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>G</em>(i) involved in the dissociation equilibria. <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>G</em><img src="http://www.niscair.res.in/jinfo/zerot.gif" alt="" border="0" /> (RH<img src="http://www.niscair.res.in/jinfo/twoplustwo.gif" alt="" border="0" />) and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>G</em><img src="http://www.niscair.res.in/jinfo/zerot.gif" alt="" border="0" />(R^2-) have been calculated (RH^&plusmn;) being the zwitter ion) knowing the solubilities of these acids in ethanol-water mixtures. Contribution due to amino acid side chain to the free energy of transfer from 100% ethanol to water has been estimated in acidic, neutral and basic media. The equivalence of these values confirms the hydrophobicity scale first proposed by Nozaki and Tanford [<em>J bioi Chem, </em>246 (1971) 2211].