Cobalt cage complexes as mediators of protein electron transfer

Abstract

A selection of cobalt (III)/(II) macrobicyclic 'sarcophagine' (sar) cage complexes with NS mixed donor sets but differing in a single apical substituent has been chosen to span a redox potential range of +150 to -150 mV vs the normal hydrogen electrode and thus acts as redox buffers in protein spectroelectrochemistry and redox potentiometry. The cobalt(III) cage complexes are all based on the same parent structure [Co(XMeNSsar)], where X, the variable apical substituent, is -NO, -Cl, -OH, -NH or -NMe , and a methyl group occupies the opposite apical position. The X-ray crystal structures of selected members of this series are reported. Changes to the apical substituent X enable the Co redox potential to be tuned across a range of more than 200 mV by the inductive effects of the functional group. The pH dependence of the redox potential enabled the pK values of some functional groups to be determined. The complexes were successfully employed as electron transfer mediators in the spectroelectrochemical investigation of a variety of heme proteins.

Biophysical Techniques for Distinguishing Ligand Binding Modes in Cytochrome P450 Monooxygenases

“…The solutions were approximately 20 μM in CYP199A4 [in Tris buffer (50 mM, pH 7.4)]. To avoid interference from highly colored organic meditators, each experiment included the complexes [Co­(AMMEN 3 S 3 sar)] 5+ , [Co­(CLMEN 4 S 2 sar)] 3+ , [Co­(AMMEN 5 Ssar)] 3+ , [Co­(sep)] 3+ , [Co­(AMMEsar)] 3+ , [Co­(cis-diammac)] 3+ , and [Co­(trans-diammac)] 3+ at 20 μM each (see the Supporting Information), which provided redox buffering across the potential range of 0 < E h < −600 mV versus NHE. , These complexes have small molar extinction coefficients (ε < 300 M –1 cm –1 ), so they make no significant contribution to the visible spectra at micromolar concentrations.…”

A spectroelectrochemical investigation of the heme‐based sensor DevS from Mycobacterium tuberculosis: a redox versus oxygen sensor

“…from Merck were used as received. The following coordination compounds, whose syntheses can be found elsewhere , were used as mediators in this work [formal potentials, E 0′ , in parentheses vs normal hydrogen electrode (NHE)] at pH 8.0: [Co((NMe 3 ) 2 sar)]Cl 5 (+10 mV vs NHE), [Co(CLME‐N 4 S 2 ‐sar)]Cl 3 (−134 mV vs NHE), [Co(AMME‐N 5 S‐sar)]Cl 3 (−220 mV vs NHE), [Co(sep)]Cl 3 (−296 mV vs NHE), [Co(AMMEsar)]Cl 3 (−380 mV vs NHE), [Co( trans ‐diammac)](ClO 4 ) 3 (−551 mV vs NHE), and Fe(NOTA) (195 mV vs NHE). Ferrocenemethanol (447 mV vs NHE) was purchased from Sigma‐Aldrich Merck and used as received.…”

“…from Merck were used as received. The following coordination compounds, whose syntheses can be found elsewhere [72,73]…”

Active site architecture reveals coordination sphere flexibility and specificity determinants in a group of closely related molybdoenzymes

“…No attempt was made to separate this mixture or establish which isomer was the most reactive. 3+ were present each at a concentration of 20 μM; the structures and properties of these compounds have been published (82)(83)(84). These mediators span the oxidation-reduction potential range +200 to -200 mV vs NHE and have small molar extinction coefficients, so they make little contribution to the visible spectra at micromolar concentrations.…”