Chiral mobile phase in ligand-exchange chromatography of amino acids: Exploring the copper(II) salt anion effect with a computational approach

With the use of a chiral ligand-exchange chromatography (CLEC) system operating with the O-benzyl-(S)-serine [(S)-OBS] [1,2] as the chiral mobile phase (CMP) additive to the eluent, the effect of the copper(II) anion type on retention (k) and separation (alpha) factors was evaluated, by rationally changing the following experimental conditions: salt concentration and temperature. The CLEC-CMP analysis was carried out on ten amino acidic racemates and with nine different cupric salts. While the group of analytes comprised both aliphatic (leucine, isoleucine, nor-leucine, proline, valine, nor-valine, and alpha-methyl-valine) and aromatic (1-aminoindan-1,5-dicarboxylic acid, phenylglycine, and tyrosine) species, representative organic (formate, methanesulphonate, and trifluoroacetate) and inorganic (bromide, chloride, fluoride, nitrate, perchlorate, and sulphate) Cu(II) salts were selected as the metal source into the eluent. This route of investigation was pursued with the aim of identifying analogies among the employed Cu(II) salts, by observing the variation profile of the selected chromatographic parameters, upon a change of the above experimental conditions. All the data were collected and analyzed through a statistical approach (PCA and k-means clustering) that revealed the presence of two behavioural classes of cupric salts, sharing the same variation profile for k and alpha values. Interestingly, this clustering can be explained in terms of ESP (Electrostatic Surface Potential) balance (ESPbal) values, obtained by an ab initio calculation operated on the cupric salts. The results of this appraisal could aid the rational choice of the most suitable eluent system, to succeed in the enantioseparation of difficult-to-resolve compounds, along with the eventual scale-up to a semi-preparative level.