Salt effects on the interaction of an amphiphilic model molecule with immobilized phosphatidylcholine monolayers

Immobilised artificial membrane chromatography coupled with molecular probing. Mimetic system for studying lipid-calcium interactions in nutritional mixtures

Immobilised artificial membrane (IAM) chromatography was utilised to study the interactions of usual membrane probes with grafted phosphatidylcholine silica support, in relation to the presence of calcium ions introduced in the mobile phase as they are present in nutritional mixtures. IAM acts as a mimetic membrane of lipid emulsion globules, a major component of nutritional mixtures. The tested probes were 1,6-diphenyl-1,3,5-hexatriene (DPH), 9-diethylamino-5H-benzo[alpha]phenoxazine-5-one or nile red (NR) and 2-(p-toluidinyl)naphtalene-6-sulfonate (TNS). For each probe, partition coefficients and thermodynamic parameters of transfer from the mobile phase to the IAM stationary phase have been measured. Our results suggested that the interactions of neutral probes (i.e. DPH and NR) with phosphatidylcholine are driven by hydrophobic forces. Addition of calcium chloride to the mobile phase slightly decreased the retention of these neutral probes and dramatically increased that of anionic TNS. Moreover, an enthalpy-entropy compensation study revealed that the mechanism of interaction between TNS and IAM is independent of the calcium concentration. Results argued for the existence of electrostatic repulsion forces exerted by IAM phase towards anionic TNS. Addition of calcium ions into the mobile phase led to the establishment of an ionic double layer at the zwitterionic stationary phase surface weakening the electrostatic barrier and increasing TNS retention. Consequently, it was demonstrated that IAM appears as a suitable model to get a better insight on the lipid-calcium interactions taking place in nutritional mixtures.

Streptavidin chiral stationary phase for the separation of adenosine enantiomers

In this paper, a microbore column packed with streptavidin particles was used, at various temperatures (0-24 degrees C), to separate the adenosine enantiomers by HPLC. Using an aqueous mobile phase, the apparent enantioseparation was high for a small molecule, varying from 11.5 at 0 degrees C to 6.2 at 24 degrees C. From the experiments carried out with a streptavidin-biotin complex stationary phase, it was demonstrated that the blockage of the biotin sites of the immobilized streptavidin was responsible for a strong decrease in the enantioselectivity via a direct and/or an indirect effect. From the analysis of the concentration dependencies of the solute retention factor, it was also shown that a reduction of the D-adenosine specific binding sites occurred at the lowest temperature. The thermodynamic parameters determined from the van't Hoff plots indicated that the D-adenosine binding to the streptavidin specific sites was enthalpically driven.

A DNA aptamer as a new target-specific chiral selector for HPLC

In this paper, a DNA aptamer, known to bind stereospecifically the D-enantiomer of an oligopeptide, i.e., arginine-vasopressin, was immobilized on a chromatographic support. The influence of various parameters (such as column temperature, eluent pH, and salt concentration) on the L- and D-peptide retention was investigated in order to provide information about the binding mechanism and then to define the utilization conditions of the aptamer column. The results suggest that dehydration at the binding interface, charge-charge interactions, and adaptive conformational transitions contribute to the specific D-peptide-aptamer complex formation. A very significant enantioselectivity was obtained in the optimal binding conditions, the D-peptide being strongly retained by the column while the L-peptide eluted in the void volume. A rapid baseline separation of peptide enantiomers was also achieved by modulating the elution conditions. Furthermore, it was established that the aptamer column was stable during an extended period of time. This work indicates that DNA aptamers, specifically selected against an enantiomer, could soon become very attractive as new target-specific chiral selectors for HPLC.