Metal complexes of new bis(tacn) ligands: Syntheses and structures of copper(II) complexes

N-terminal processing of affinity-tagged recombinant proteins purified by IMAC procedures

The ability of a new class of metal binding tags to facilitate the purification of recombinant proteins, exemplified by the tagged glutathione S-transferase and human growth hormone, from Escherichia coli fermentation broths and lysates has been further investigated. These histidine-containing tags exhibit high affinity for borderline metal ions chelated to the immobilised ligand, 1,4,7-triazacyclononane (tacn). The use of this tag-tacn immobilised metal ion affinity chromatography (IMAC) system engenders high selectivity with regard to host cell protein removal and permits facile tag removal from the E. coli-expressed recombinant protein. In particular, these tags were specifically designed to enable their efficient removal by the dipeptidyl aminopeptidase 1 (DAP-1), thus capturing the advantages of high substrate specificity and rates of cleavage. MALDI-TOF MS analysis of the cleaved products from the DAP-1 digestion of the recombinant N-terminally tagged proteins confirmed the complete removal of the tag within 4-12 h under mild experimental conditions. Overall, this study demonstrates that the use of tags specifically designed to target tacn-based IMAC resins offers a comprehensive and flexible approach for the purification of E. coli-expressed recombinant proteins, where complete removal of the tag is an essential prerequisite for subsequent application of the purified native proteins in studies aimed at delineating the molecular and cellular basis of specific biological processes.

Bi-Nuclear Metal Complexes of 2,6-Bis(1,4,7-triazacyclonon-1-yl-methylene)pyridine with Zinc(II), Copper(II), and Nickel(II)

The coordination chemistry of 2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)pyridine (Lpyx, 1) has been investigated and shown to yield bi-nuclear metal complexes in the presence of zinc(ii), copper(ii), or nickel(ii) ions. The reaction of (Lpyx)·7HCl (2) with Zn(NO3)2 gave the monomeric [Zn2(μ-Cl)2(Lpyx)(H2O)](ClO4)2 (3) in which the ligand encapsulates a Zn2(μ-Cl)2 moiety. Similar treatment of 2 with Cu(NO3)2 gave a bi-nuclear complex cation which exists as a 1 : 1 co-crystal {[Cu(Cl)(Lpyx)Cu(μ-Cl)(Cl)](BF4)(H2O)}2 and {[Cu(Cl)(Lpyx)Cu(μ-Cl)(H2O)](BF4)2(H2O)}2 (4) with two discrete CuII centres bridged by the Lpyx ligand and dimerised through an unsymmetrical Cu2(μ-Cl)2 interaction. Similarly, reaction of 2 with Ni(NO3)2 also gave a dimeric complex {[Ni(Cl)(μ-Cl)(Lpyx)Ni(NO3)(H2O)](PF6)}2 (5) containing two discrete NiII centres with dimerisation occurring through a symmetrical Ni2(μ-Cl)2 interaction. In all cases, the Lpyx ligand binding is unsymmetrical between the two metal centres being tridentate to one and tetradentate to the other through an additional coordination of the pyridyl linker.

Studies with an immobilized metal affinity chromatography cassette system involving binuclear triazacyclononane-derived ligands: automation of batch adsorption measurements with tagged recombinant proteins

This study describes the determination of the adsorption isotherms and binding kinetics of tagged recombinant proteins using a recently developed IMAC cassette system and employing automated robotic liquid handling procedures for IMAC resin screening. These results confirm that these new IMAC resins, generated from a variety of different metal-charged binuclear 1,4,7-triaza-cyclononane (tacn) ligands, interact with recombinant proteins containing a novel N-terminal metal binding tag, NT1A, with static binding capacities similar to those obtained with conventional hexa-His tagged proteins, but with significantly increased association constants. In addition, higher kinetic binding rates were observed with these new IMAC systems, an attribute that can be positively exploited to increase process productivity. The results from this investigation demonstrate that enhancements in binding capacities and affinities were achieved with these new IMAC resins and chosen NT1A tagged protein. Further, differences in the binding performances of the bis(tacn) xylenyl-bridged ligands were consistent with the distance between the metal binding centres of the two tacn moieties, the flexibility of the ligand and the potential contribution from the aromatic ring of the xylenyl group to undergo π/π stacking interactions with the tagged proteins.