Purification of animal immunoglobulin G (IgG) using peptoid affinity ligands.

The availability of highly pure animal antibodies is critical in the production of diagnostic tools and biosensors. The peptoid PL16, previously isolated from an ensemble of peptoid variants of the IgG-binding peptide HWRGWV, was utilized in this work as affinity ligand on WorkBeads resin for the purification of immunoglobulin G (IgG) from a variety of mammalian sources and chicken immunoglobulin Y (IgY). The chromatographic protocol initially optimized for murine serum and ascites was subsequently employed for processing rabbit, goat and sheep, donkey, llama, and chicken sera. The PL16-WorkBeads resin proved able to recover all antibody targets with values of yield between 50 and 90%, and purity consistently above 90%. Notably, PL16 not only binds a broader spectrum of animal immunoglobulins than the reference ligands Protein A and G, but it also binds equally well with all their subclasses. Unlike the protein ligands, in fact, PL16 afforded excellent values of yield and purity of mammalian polyclonal IgG, namely murine (47 and 94%), rabbit (66.5 and 91.7%), caprine IgG (63 and 91-95%), donkey, and llama (93 and 97%), as well as chicken IgY (42 and 92%). Of notice, it is also the ability of PL16 to target monomeric IgG without binding aggregated IgG; when challenged with a mixture of monomeric and aggregated murine IgG, PL16 eluted <3% of fed aggregates, against 11-13% eluted by Protein A and G. Collectively, these results prove the potential of the proposed peptoid ligand for large-scale purification of animal immunoglobulins.

Translating antibody-binding peptides into peptoid ligands with improved affinity and stability.

A great number of protein-binding peptides are known and utilized as drugs, diagnostic reagents, and affinity ligands. Recently, however, peptide mimetics have been proposed as valuable alternative to peptides by virtue of their excellent biorecognition activity and higher biochemical stability. This poses the need to develop a strategy for translating known protein-binding peptides into peptoid analogues with comparable or better affinity. This work proposes a route for translation utilizing the IgG-binding peptide HWRGWV as reference sequence. An ensemble of peptoid analogues of HWRGWV were produced by adjusting the number and sequence arrangement of residues containing functional groups that resemble both natural and non-natural amino acids. The variants were initially screened via IgG binding tests in non-competitive mode to select candidate ligands. A set of selected peptoids were studied in silico by docking onto putative binding sites identified on the crystal structures of human IgG1, IgG2, IgG3, and IgG4 subclasses, returning values of predicted binding energy that aligned well with the binding data. Selected peptoids PL-16 and PL-22 were further characterized by binding isotherm analysis to determine maximum capacity (Qmax ˜ 48-57 mg of IgG per mL of adsorbent) and binding strength on solid phase (KD ˜ 5.4-7.8 10-7 M). Adsorbents PL-16-Workbeads and PL-22-Workbeads were used for purifying human IgG from a cell culture supernatant added with bovine serum, affording high values of IgG recovery (up to 85%) and purity (up to 98%) under optimized binding and elution conditions. Both peptoid ligands also proved to be stable against proteolytic enzymes and strong alkaline agents. Collectively, these studies form a method guiding the design of peptoid variants of cognate peptide ligands, and help addressing the challenges that, despite the structural similarity, the peptide-to-peptoid translation presents.