Cobalt(II) Substituted Derivatives ofCarcinus maenasHemocyanin:  Magnetic Characterization, Magnetooptic, and Kinetic Studies Regarding the Geometry of the Active Site

Abstract

Mononuclear (CoII−Hc) and dinuclear (CoII 2−Hc) Co(II) substituted derivatives of Carcinus maenas hemocyanin (Hc) are characterized by magnetic susceptibility measurements, magnetic circular dichroism spectroscopy, and kinetic experiments with cyanide. Magnetization measurements up to 8 T reveal that the mononuclear CoII−Hc posseses a S = 3/2 ground state, demonstrating that the optical properties are consistent with a distorted tetrahedral coordination geometry. The CoII 2−Hc, CoII 2−N3−Hc, and CoII−Hc derivatives exhibit visible and near-infrared magnetic circular dichroism (MCD) signals and magnetic characteristics which also reflect tetrahedral coordination geometry. Magnetic susceptibility measurements of dinuclear CoII 2−Hc and CoII 2−N3−Hc reveal strong antiferromagnetic coupling with |J| > 100 cm-1, consistent with the existence of a ligand bridging the metal centers. The results of the magnetic susceptibility measurements together with the MCD data imply that the putative bridging ligand of the dinuclear CoII 2−Hc can be substituted by small anions like azide. Otherwise a coordination number higher than 4 would have been achieved. No evidence for an octahedrally coordinated Co(III) species was obtained from magnetic or magnetooptical studies, ruling out the oxidation of CoII 2−Hc to form a CoIII−O2 -−CoIII complex and hence dioxygen-binding to CoII 2−Hc. Kinetic investigation of the reaction of dinuclear CoII 2−Hc with excess cyanide by stopped-flow spectrophotometry reveals that the protein adds cyanide in a fast initial equilibrium reaction to form an adduct. This is followed by slow cyanide-assisted removal of the metal ions from the active site in two steps.

Interaction and coordination geometries for Ag(I) in the two metal sites of hemocyanin

“…That is, different coordination geometries on the potential energy surface become accessible. Similar observations were made with Co(II)‐substituted mononuclear and dinuclear derivatives of Carcinus maenas hemocyanin, showing that the coordination geometry of one metal site is influenced by that of the other metal site [22].…”

Converting a Maltose Receptor into a Nascent Binuclear Copper Oxygenase by Computational Design

“…Therefore, it is likely that the Cu 2 ‚MBP.Hc.E species do not adapt a structure that is similar to a oxy-Hc core. Binuclear cobalt-substituted proteins (41)(42)(43) and model complexes (38)(39)(40)(44)(45)(46) (47,48). Taken together, these data are consistent with formation of metal-oxygen cores that are structurally similar to the oxyhemocyanin core.…”

“…Binuclear cobalt-substituted proteins ( − ) and model complexes ( 38−40 , 44−46 ) can mimic many of the important aspects of the binuclear Cu II 2 centers. No spectral changes were observed upon addition of O 2 to Co II 2 ·MBP.Hc.E, whereas addition of H 2 O 2 forms a spectroscopically active complex (Co II 2 ·MBP.Hc.E·H 2 O 2 ).…”

High-spin Co(II) in monomeric and exchange coupled oligomeric structures: Magnetic and magnetic circular dichroism investigations

“…Naturally occurring cobalt metalloenzymes are not so common as iron metalloenzymes, but cobalt can replace other metal ions in some native metalloenzymes with partial or full retention of their activity, as, e.g., Zn(II) in carbonic anhydrase [14], in horse liver alcohol dehydrogenase [15] or in insulin hexamer [16]. By substituting Cu, binuclear Co centers can be formed as in the case of cobalt hemocyanin [17,18] and can be investigated by MCD spectroscopy [18]. For such binuclear complexes, magnetic interactions are possible.…”