Towards synthetic models for trinuclear copper active sites of ascorbate oxidase and laccase: self-assembly, crystal structure and magnetic properties of the copper(II) complexes of 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene †

Abstract

Reaction of the hydrobromide salt of 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene, Lؒ9HBr, with copper() nitrate followed by cation-exchange chromatographic purification affordedThe ESR and magnetic susceptibility data indicated that the complex consists of three identical non-interacting copper() centres. Reaction of 1 with a phosphate source produces {[Cu 3 L(µ-OH)(µ 3 -HPO 4 )(H 2 O)][PF 6 ] 3 ؒ3H 2 O} n 2 the polymeric lattice of which contains trinuclear copper() sites with structural similarities to laccase (Lc) and ascorbate oxidase (AO). These trinuclear sites consist of two type 3 copper() centres, at a separation of 3.557(4) Å, linked by an hydroxo bridge and two phosphate oxygens while another phosphate oxygen links these two centres to the further removed type 2 copper() centre, establishing separations of 4.561(4) and 5.474(4) Å. The magnetic properties of 2 were investigated in the temperature range 4.2-300 K and they revealed an S = ¹ 2 molecular ground state arising from antiferromagnetic coupling. A number of models have been employed in order quantitatively to fit the µ eff versus temperature data including those applicable to the resting oxidised state of laccase and ascorbate oxidase, vis-à-vis dimer plus uncoupled monomer, but the best fits were obtained using a symmetrical trinuclear approximation with J 12 for the doubly bridged [CuL(µ-OH)(µ-HPO 4 )Cu] moiety of Ϫ53 or ca. Ϫ80 cm Ϫ1 combined respectively with J for the two equal three-atom phosphato bridges of Ϫ90 or Ϫ77 to Ϫ90 cm Ϫ1 . The 77 K solid-state ESR spectrum of 2 is unusual for S = ¹ 2 ground-state systems and shows six components of a probable seven-line copper hyperfine multiplet between 2500 and 3100 G and a strong x, y resonance at ca. 3200 G, most probably due to weak copper() pair interactions or the superimposition of S = ¹ 2 signals from both molecular doublets. The 77 K solution ESR spectrum (dimethylformamide, water-glycol 1 : 1) for 2 is typical of a monomeric copper() centre and closely resembles the ESR spectrum of the type 2 site in Lc and AO. The ESR, electrospray mass spectrometric and NMR data indicate that 2 dissociates in solution to give a trinuclear unit consisting of two type 3 copper() centres (ESR silent), which are linked by the hydroxo and phosphate group (phosphate is released from the complex only on addition of acid), and an isolated type 2 copper() centre which is probably responsible for the ESR features.

Separation of hexahistidine fusion proteins with immobilized metal ion affinity chromatographic (IMAC) sorbents derived from MN+‐tacn and its derivatives

“…Compared to other tri-or tetradentating chelating ligands, tacn and its derivatives have much larger stability constants for many borderline metal ions, with values typically higher than IDA by at least 4-10 orders of magnitude, that is, in the range of 16 log b 30 (Haidar et al, 1997;Spiccia et al, 1997;Tanaka et al, 1977;Zhang et al, 1995). The homologues, bis(1,4,7-triazacyclononyl) ethane (dtne) and bis(1,4,7-triazacyclononyl)propane (dtnp), involve two tacn macrocyclic rings linked via either an ethyl or a propyl bridge respectively (Fig.…”

Cleavage of double-strand DNA by linear and triangular trinuclear copper complexes

“…Among all of the reported metal complexes, the most attractive class is copper(II) complexes, which have been extensively applied as catalysts due to their efficient cleavage of nucleic acids last two decades [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. In view of a number of nucleases with two or three metal ions in their catalytic centre [28], considerable efforts have been made to develop multiple metal models [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44].…”

Trinuclear CuII Complexes Containing Peripheral Ketonic Oxygen Bridges and a μ3‐OH Core: Syntheses, Crystal Structures, Spectroscopic and Magnetic Properties

“…The EPR spectra of trinuclear S = 1/2 compounds have been analyzed in terms of three active states (one quartet and two doublets), with temperaturedependent population. [18][19][20][21][22] In the light of the superexchange parameter from the magnetic measurements, at low temperatures only the doublets should be populated, whereas at room temperature signals due to the transitions inside the quartet may also be observed. This transition may show noticeable zero-field splitting (ZFS) (S = 3/2).…”