Separation of optical isomers of fac-triscobalt(III) chelates of several L-α-amino acids and dissolution behavior of their isomers in water

Abstract

fac-Triscobalt(III) chelates of L-α-amino acids such as L-alanine ( L-ala), L-2-aminobutanoic acid ( L-ambt), L-valine ( L-val), and L-leucine ( L-leu) were synthesized and the separation of the optical isomers of these chelates was attempted. For the chelates of L-ala and L-leu, (+)- and (−)-isomers were obtained separately, while for the chelates of L-ambt and L-val only the (+)-isomer was isolated. The solubilities of the chelates thus obtained in water were determined over the temperature range of 5 to 55 °C. The thermodynamic parameters of solution were estimated from the temperature dependence of the solubilities and the dissolution behavior was considered in terms of the thermodynamic parameters. A contribution of hydrophobic effect of these chelates to their solubilities was found to be cancelled out by a compensation relation between the enthalpy and the entropy changes accompanied by the hydrophobic hydration. As a first approximation it is concluded that the relatively low solubilities of these chelates are attributed to highly negative entropy of solution and that the chelates with relatively smaller enthalpy of solution are more soluble.