Structural evaluation of an alternative Protein A biomimetic ligand for antibody purification.

Affinity chromatography is one of the most common techniques employed at the industrial-scale for antibody purification. In particular, the purification of human immunoglobulin G (hIgG) has gained relevance with the immobilization of its natural binding counterpart-Staphylococcus aureus Protein A (SpA) or with the recent development of biomimetic affinity ligands, namely triazine-based ligands. These ligands have been developed in order to overcome economic and leaching issues associated to SpA. The most recent triazine-based ligand-TPN-BM, came up as an analogue of 2-(3-amino-phenol)-6-(4-amino-1-naphthol)-4-chloro-sym-triazine ligand also known as ligand 22/8 with improved physico-chemical properties and a greener synthetic route. This work intends to evaluate the potential of TPN-BM as an alternative affinity ligand towards antibody recognition and binding, namely IgG, at an atomic level, since it has already been tested, after immobilization onto chitosan-based monoliths and demonstrated interesting affinity behaviour for this purpose. Herein, combining automated molecular docking and molecular dynamics simulations it was predicted that TPN-BM has high propensity to bind IgG through the same binding site found in the crystallographic structure of SpA_IgG complex, as well as theoretically predicted for ligand 22/8_IgG complex. Furthermore, it was found that TPN-BM established preferential interactions with aromatic residues at the Fab domain (Trp 50, Tyr 53, Tyr 98 and Trp 100), while in the Fc domain the main interactions are based on hydrogen bonds with pH sensitive residues at operational regime for binding and elution like histidines (His 460, His 464, His 466). Moreover, the pH dependence of TPN-BM_IgG complex formation was more evident for the Fc domain, where at pH 3 the protonation state and consequently the charge alteration of histidine residues located at the IgG binding site induced ligand detachment which explains the optimal elution condition at this pH observed experimentally.