Role of the vancomycin-ristocetin heterodimerization on the enantioselectivity of d,l-tryptophan and d,l-dansyl tryptophan

In this paper, a chromatographic system using immobilized ristocetin as chiral stationary phase and vancomycin as chiral mobile phase additive (CMPA) was described in order to investigate the role of the glycopeptide heterodimerization on the retention and enantioselectivity of d,l-tryptophan and d,l-dansyl tryptophan. A simplified interaction model was derived considering the formation of heterodimers between immobilized ristocetin and vancomycin. This theoretical approach was convenient to describe adequately the retention behavior. When the CMPA concentration increased, the solute retention factor increased for all the solute enantiomers studied indicating that the vancomycin adsorbed on the immobilized ristocetin played a preponderant role in the retention. The d,l-tryptophan enantioselectivity on the dynamically modified stationary phase was improved by a factor of 1.3, probably due to a glycopeptide conformational change upon heterodimerization. On the other hand, a decrease in the chiral discrimination of d,l-dansyl tryptophan was observed. Such a behavior seems to result from the antagonist enantioselective properties of the two glycopeptides for the dansyl amino acids.