Investigation of Chemical Modification of Peptide Amine Groups for Compatibility with Fluorescent Labeling and Capillary Electrophoresis

Abstract

Two means for chemical modification of amine groups are investigated as to their compatibility with capillary electrophoresis (CE) and fluorescent derivatization of polypeptides. The chemical modification reagents, which are currently under examination for protein modification and subsequent peptide mapping, are selected such that there is minimal impact (permanent modification of amine groups with little or no effect on pK_a, relative size, solubility, etc.) or no impact (clean removal of the reagent after fluorescent derivatization) on the subsequent electrophoretic behavior of the fluorescently labeled peptide. Capillary electrophoresis conditions that permit the separation of the three possible amine modification products of the model peptide studied, performic acid oxidized insulin B-chain (two primary amine groups), are demonstrated. The distribution of formed products can thus be probed by CE.

To assess the characteristics of the modification strategies the conjugation reaction between fluorescein isothiocyanate (FITC) and the peptide was examined. The model peptide was then chemically modified by reductive dimethylation or reductive dihydroxypropylation (DHP) prior to fluorescent labeling. Permanent modification by dimethylation results in derivatives that feature CE properties identical to the unmodified peptide; while conditions suitable for removing the DHP blocking group, leaving the attached fluorescent label intact, are shown. These reagents hold promise for directing and controlling the fluorescent labeling of peptides resulting from chemical or enzymatic digestions.