The role of bonded phase composition on the ligand-exchange chromatography of dansyl- D, L-amino acids

Abstract

This paper examines the role of bonded phase composition in the ligand-exchange chromatographic separation of dansyl amino acids (Dns- D, L-AAs). As the bonded optically active chelate, we have synthesized N-ϖ-(dimethylsiloxyl)undecanoyl- L-valine and as the metal, we used Cu(II). Bonded phases, where the chelate coverage is 2.5 μmol/m 2, have been compared with mixed or diluted phases, in which the contribution of the bonded chelate to the total coverage is less than 0.2 μmol/m 2. Using a decyl group as diluent, it has been shown that capacity factors and enantio selectivities of Dns- D, L-AAs are much higher on the diluted phases relative to the concentrated phase, in spite of the fact that the amount of copper loaded on the latter column is roughly 40 times greater. This result has been interpreted in terms of the formation of his complexes between the metal ion and the bonded phase chelate, i. e., 2:1 chelate to metal, in the case of the concentrated phase. For the diluted phases, the mono complex, i.e., 1:1 metal to chelate, is the predominant form. Not only does the diluted phase yield improved separation, but it is in principle a simpler system to manipulate and optimize. We have been able to develop a simple model to analyze the chromatographic behavior on diluted phases relative to the concen trated phase. Based on this model, we have compared four diluents: a polyether, n- butyl, n-decyl and n-eicosyl ligands. By normalizing for the amount of copper loaded on the stationary phase, it is shown that the apparent complexation constants in crease from the ether to the C 20 phase. Moreover, due to differences in the character of the side chains of the amino acids, additional selectivity is observed between the different solutes. This selectivity is useful in separation applications. In addition, it is shown that the diluted bonded phases yield high performance, approximately 4000 plates for 15 cm columns, with good peak symmetry. Also included is a detailed discussion of the characterization of the bonded phases along with its optical purity.