Optimization of peptide and protein separation with a monolithic reversed-phase column and application to arsenic-binding studies

A separation method for a mixture of eight sulfur-containing peptides and proteins characterized by a wide molar mass (1-18.4 kDa) and pI range (4.5-10.7) was developed onto a monolithic phenyl phase. Based on the first optimization steps that revealed an increase of the acetonitrile content to 45 vol.% as sufficient for the elution of all biomolecules and the addition of the ion pairing reagent trichloroacetic acid (TCA) as preferable over the eluent additives formic acid or ammonium acetate buffer, the critical variables TCA concentration, gradient time, and eluent flow rate were optimized using a Box-Behnken experimental design. To achieve optimum values for separation factors of all peak pairs, a TCA content of 0.025% (m/v), a gradient time of 10 min, and a flow rate of 3.5 mL min(-1) were selected. Arsenic binding studies were undertaken under conditions optimized with respect to the crucial separation factor of the nonapeptides vasotocin (Vtc) and vasopressin (Vpr) in a shortened gradient time of 7.5 min. A complete separation of phenylarsenic-substituted and unmodified forms of these peptides allowed the calculation of both consumptions and apparent equilibrium constants K from HPLC-UV peak areas. The nonapeptide consumptions by the reaction with phenylarsine oxide (PAO) increased from 7% up to 100% in dependence on the molar ratio of the reaction components. Due to an enhanced UV absorption of the phenylarsenic-substituted biomolecules, the calculation of apparent equilibrium constants led to increasing K values with rising PAO molarities from 9.6×10(5) to 1.2×10(8) in case of Vtc and from 2.2×10(6) to 1.4×10(9) in case of Vpr. For α-lactalbumin, a consumption of 59.2±6.1% by the reaction with molar excesses of PAO varying from 1.4 to 21 can be derived from the chromatograms. The quantitative evaluation of the reaction of the small protein aprotinin with PAO was hindered by a pronounced peak broadening that occurred after reduction of the disulfide bridges.