Retention behavior of amino acids and peptides on protoporphyrin-silica stationary phases with varying metal ion centers.
Various metalloprotoporphyrins (MProP) covalently linked to silica supports are examined as novel immobilized metal ion affinity chromatography (IMAC) stationary phases for separations of amino acids/peptides. Under reversed-phase HPLC conditions, the MProP-silicas exhibit high affinity toward L-histidine via metal-nitrogen axial ligation interactions, with an increasing degree of histidine retention highly dependent on the specific metal ion (M) in the center of the protoporphyrin (ProP) structure: Fe(III) > Ni(II) > Cu(II) > Zn (II) approximately Cd(II). Aromatic amino acids (i.e., L-trytophan and L-phenylalanine) are also retained on MProP columns through pi-pi interactions with the immobilized porphyrins, with the greatest affinity for L-trytophan observed on CuProP-silica columns. Peptides rich in L-histidine and L-tryptophan residues are selectively retained on most of the MProP-silica phases examined; however, the addition of an organic modifier and/or lowering the pH of the mobile phase can be used independently to attenuate the pi-pi and metal ion-nitrogen ligation interactions, respectively. Reproducible separations of His-Phe and trytophan releasing hormone are achieved on a FeProP-silica column even after extensive washing with 50 mM EDTA, demonstrating a fundamental advantage of the new MProP-silica over existing IMAC stationary phases, in which the metal ion is anchored weakly to the support via immobilized iminodiacetate and related ligands.