Electron-paramagnetic-resonance studies on cobalt(II) carbonic anhydrase. Low-spin cyanide complexes

Abstract

The low-spin cyanide complexes of three Co(II) carbonic anhydrases were investigated by electron paramagnetic resonance (e.p.r.) at 9 and 35GHz. Well-defined and closely axial spectra were obtained only in the absence of oxygen. Several mole equivalents of cyanide were required for complete formation of the complexes in frozen solution, although large excesses caused abstraction of the cobalt. Experiments with [(13)C]cyanide showed that the low-spin complexes contained two CN(-) groups in an environment similar to that of the in-plane ligands in [Co(CN)(5)](3-). A combined e.p.r. and spectrophotometric titration confirmed the presence of two CN(-) ligands. A 5-co-ordinate square pyramidal structure involving three protein ligands was proposed. The dicyanide complex could be oxygenated reversibly, producing a characteristic new e.p.r. spectrum. The O(2) molecule was thought to occupy the remaining octahedral metal site in a formally Co(III) species. The optical spectrum of the dicyanide lacked the prominent d-d bands of the high-spin monocyanide. Both e.p.r. and optical data indicated that the low-spin complex was formed much more fully in frozen solution than at room temperature. Differences in behaviour between the high- and low-activity enzymes suggested some variation in conformational flexibility at the metal binding site.

An electrochemical study of cobalt-salen (N,N′-bis(salicylidene)ethylenediaminocobalt(II) in the oxidation of syringyl alcohol in acetonitrile

“…With the addition of pyridine (1.1:1, pyridine to cobalt), the appearance of 8 hyperfine (hf) structures are resolved which indicate the complexation of 1 pyridine with 1 cobalt. This is in agreement with previous studies on cobalt complexation which also displayed 8 hf features after axial base complexation with the d 7 cobalt (II) (I = 7/2) [48,49]. An additional EPR experiment was performed in an oxygen saturated environment to determine how the Co(salen)pyr-O 2 system would perform under electrolysis conditions.…”

Electrocatalytic Properties of Metal Phthalocyanine Tetrasulfonate Intercalated in Metal Layered Simple Hydroxides (Metal: Co, Cu, and Zn)

“…This peak is characteristic of a superoxide Co(III)-O 2 adduct 43,44 and could be reproduced using the following set of parameters: g // = 2.070 ± 0.001, g ⊥ = 1.974 ± 0.002, A ⊥ = 0.6 mT (for about 3% of the total EPR signal). This characteristic feature has been observed for Co(II) anhydrases, 43 porphyrines, 45 cytochrome C, 46 or phthalocyanines inserted into zeolithes. 47 The formation of this additional feature is perfectly reversible upon removing the oxygen atmosphere.…”

Electron-paramagnetic-resonance studies on cobalt(II) carbonic anhydrase–sulphonamide complexes

“…Native carbonic anhydrase was isolated from human and bovine erythrocytes by established procedures (Henderson & Henriksson, 1973;Lindskog, 1960;Nilsson & Lindskog, 1967). Some bovine enzyme was also purchased from Miles-Seravac (Pty) Ltd., CH-1006 Lausanne, Switzerland. The cobalt enzymes were prepared as described by Cockle (1974).…”

“…We have previously applied e.p.r. to the study of a unique low-spin (S = -) cyanide adduct of Co(II) carbonic anhydrase (Cockle, 1973(Cockle, , 1974, but in other situations the metal ion adopts the high-spin (S= -) configuration (Lindskog & Ehrenberg, 1967). Owing to rapid electron spin relaxation, satisfactory measurements on high-spin Co(II) require temperatures below about 20°K, which in the past has impeded experimentation on molecular species in frozen solution.…”