Enantioselective adsorption dynamics of leucyl-leucine in a Chirobiotic R column

The dynamics of adsorption of the Leu-Leu stereoisomers in a chromatographic column packed with the Chirobiotic R chiral stationary phase bearing grafted antibiotic ristocetin A was studied by means of measurement and analysis of van Deemter plots. Similar measurements were carried out with weakly retained Gly-Gly for the sake of comparison. The bulk diffusion coefficients of the investigated dipeptides were also determined. It is found that the van Deemter plots of both the Leu-Leu stereoisomers and Gly-Gly have an uncommon convex-upward shape. Besides, the van Deemter B coefficients for the Leu-Leu stereoisomers, but not for Gly-Gly, have unusually high values. It is suggested that a high transcolumn contribution to eddy dispersion, which turned out to be enantioselective, accounts for these findings. Adsorption kinetics of all the dipeptides considered is relatively slow, the adsorption rate constant (k_ads) being of order of magnitude 20-60 s^-1. k_ads does not depend on the configuration of Leu-Leu stereoisomers, although their affinity toward the chiral selector depends on this factor. This supports the above hypothesis that eddy dispersion is mainly responsible for the observed peculiarities in the dynamic behavior of dipeptides, and adsorption kinetics has secondary importance in this phenomenon.