Impact of the substitution pattern of xylyl-bridged 1,4-bis(1,4,7-triazacyclononane) ligands on the crystal structures of their zinc(II) complexes and their ability to bind to N-terminally tagged proteins

Abstract

Reactions of ortho-, para- and substituted meta-xylenyl bridged bis(tacn) ligands with two or three molar equivalents of Zn(NO3)2·6H2O at pH 5.0 in the presence of NaClO4 or NH4PF6 resulted in the formation of a range of crystalline zinc complexes. In the presence of perchlorate counter ions the Lpx and Lphx ligands gave rise to dinuclear Zn2+ coordination complexes [Zn2Cl2(Lpx)(H2O)2](ClO4)2(H2O) and [Zn2Cl2(Lphx)(H2O)2](ClO4)2(H2O)2 respectively, which formed infinite 1D polymeric chains. In contrast, in the presence of the hexafluorophosphate counter ion, the Lnix ligand formed a unique tetranuclear Zn2+ complex, [Zn2Cl2(Lnix)(H2O)3](Cl)(PF6)2(H2O)4, the Lcax ligand formed an octanuclear complex [Zn8Cl4(Lcax)4(H2O)10](PF6)6.7(NO3)1.3(H2O)3.3, whilst the Lox ligand formed a mononuclear sandwich structure, [Zn(Lox)](PF6)2(H2O). The protonated metal-free Lpx ligand crystallised as [H4(Lpx)](NO3)2(PF6)(H2O)2 with the stacked ligand cations forming a hydrogen-bonded network. These structural attributes provide an explanation for the significant differences in binding behaviour of recombinant N-terminally tagged proteins with the corresponding Zn2+-azamacrocyclic complexes, compared to the analogous Cu2+- or Ni2+-azamacrocyclic complexes, when immobilised onto support materials and used as immobilised metal affinity chromatographic (IMAC) adsorbents.