The stability constants of cobalt(III) chelates of polyaminopolycarboxylic acids

Abstract

The cobalt( i i i ) chelates of ethylenediaminetetra-acetic acid, iminodiacetic acid, NN'-bis-(2-aminoethy1)ethylenediaminehexa-acetic acid, and A/'-hydroxyethylenediamine-A/"'-triacetic acid have been prepared and analysed.and the stability constants measured over a pH range. Reproducible redox potentials were obtained by use of a gold-plated, gold indicator electrode in conjunction with a pH meter. The relative values of the stability constants were in agreement with those expected from the chemical structures of the chelates.

Cobalt(III)‐Mediated Permanent and Stable Immobilization of Histidine‐Tagged Proteins on NTA‐Functionalized Surfaces

“…To form the Co 3+ ‐mediated complex, the well‐known Co 2+ ‐mediated complex with NTA and the His‐tag of the protein is formed and then the Co 2+ center is oxidized to Co 3+ with an oxidant, such as hydrogen peroxide. Replacement of the divalent mediator ions with the Co 3+ ion has two important advantages: 1) the Co 3+ ion forms thermodynamically more stable complexes relative to the divalent counterparts (e.g., the dissociation constants for the NTA complex of Co 3+ , Co 2+ , Ni 2+ , and Zn 2+ ions are 10 −25 , 10 −11 , 10 −12 , and 10 −11 m respectively); 2) the Co 3+ ion in general forms kinetically inert complexes toward intersphere ligand exchange (e.g., although the hexa‐aqua complex of the Co 2+ ion has a ligand exchange rate of 3×10 6 s −1 , the corresponding Co 3+ complex has an exchange rate of less than 10 −6 s −1 ). The large increase in the thermodynamic and kinetic stabilities upon the oxidation of Co 2+ to Co 3+ has also been used for drug delivery, mechanistic studies in bioinorganic chemistry, the functionalization of porphyrin–phospholipid membranes with His‐tagged peptides, and protein labeling …”

Expanding a Portfolio of (FO-) SPR Surface Chemistries with the Co(III)-NTA Oriented Immobilization of His6-Tagged Bioreceptors for Applications in Complex Matrices

“…To overcome these limitations, Wegner’s group proposed to introduce Co 3+ as the mediator ion between NTA and His 6 -tagged proteins, which creates a more stable and inert functionalized surface. ,, Compared with Co 2+ and Ni 2+ complexes, the Co 3+ complex exhibited substantially higher association and lower dissociation rate constants (about 20 and 12 orders of magnitudes, respectively), − without possibility to regenerate the surface due to very slow ligand exchange. The Co­(III)-NTA chemistry was applied and validated in the development of several different biosensing platforms, including QCM, biolayer interferometry, and fluorescent sensors .…”

Cobalt(III) as a Stable and Inert Mediator Ion between NTA and His6‐Tagged Proteins

“…Therefore, we propose to use Co 3+ as the mediator ion for this interaction, which, as a d 6 ion, forms low-spin octahedral paramagnetic complexes (e g 6 t 2g 0 ) that have two advantages: 1) Co 3+ complexes have significantly higher formation constants than Co 2+ and Ni 2+ complexes with similar coordination environments. In particular, Co 3+ complexes with standard aminopolycarboxylic acids have formation constants that are about 20 orders of magnitudes higher than the Co 2+ analogues; [25,26] and 2) Co 3+ complexes are exchange-inert as they undergo only very slow ligand exchange in their primary coordination sphere. For instance, the aqua complex of Co 2+ has an exchange rate of 3 10 6 s À1 while the Co 3+ complex has an exchange rate of less than 10 À6 s À1 (Scheme 1).…”