Reversible dioxygen binding and phenol oxygenation in a tyrosinase model system

The complex [Cu2(L-66)]2+ (L-66 = a,a'-bis?bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino?-m-xylene) undergoes fully reversible oxygenation at low temperature in acetone. The optical [lambda(max) = 362 (epsilon 15000), 455 (epsilon 2000), and 550 nm (epsilon 900M(-1)cm(-1))] and resonance Raman features (760 cm(-1), shifted to 719cm(-1)(-1) with 18O2) of the dioxygen adduct [Cu2(L-66)(O2)]2+ indicate that it is a mu-eta2:eta2-peroxodicopper(II) complex. The kinetics of dioxygen binding, studied at - 78 degrees C, gave the rate constant k1 = 1.1M(-1) 5(-1) for adduct formation, and k(-1) =7.8 x 10(-5)s(-1), for dioxygen release from the Cu2O2 complex. From these values, the O2 binding constant K= 1.4 x 10(4)M(-1) at -78 degrees C could be determined. The [Cu2(L-66)(O2)]2+ complex performs the regiospecific ortho-hydroxylation of 4-carbomethoxyphenolate to the corresponding catecholate and the oxidation of 3,5-di-tert-butylcatechol to the quinone at -60 degrees C. Therefore, [Cu2(L-66)]2+ is the first synthetic complex to form a stable dioxygen adduct and exhibit true tyrosinase-like activity on exogenous phenolic compounds.     

Electronic aspects of dioxygen binding to cobalt-schiff-base-complexes: Anab initio calculation

It is shown that there are four possible electronic configurations for the ground-state of the dioxygen complex of Co(acacen)L (with the fifth ligand L = none, H₂ O, Imidazole, CN^? and CO). Fromab-initio calculations with a minimal basis set, close energy values are indeed computed for these four configurations with the lowest one (ground state configuration) corresponding to a charge-transfer configuration Co(III)-O ₂ ^? ₂, in agreement with the results of ESR spectroscopy from the literature. The enthalpy of oxygenation is related to the σ-donor ability of the fifth ligand and to the ease of oxydation of Co(II) to Co(III).     

Reactions of iridium hydride pincer complexes with dioxygen: new dioxygen complexes and reversible O2 binding

The reaction of molecular oxygen with iridium pincer hydride complexes, ((tBu)PCP)Ir(H)(X) [(tBu)PCP = kappa(3)-C(6)H(3)(CH(2)P(t)Bu(2))(2), X = Ph, H, CCPh], results in O(2) induced reductive elimination and formation of the novel dioxygen complexes ((tBu)PCP)Ir(O(2))(n) [n = 1 (), 2 ()].     

Reversible binding of dioxygen by the copper(I) complex with tris(2-dimethylaminoethyl)amine (Me6tren) ligand

At low temperatures, the mononuclear copper(I) complex of the tetradentate tripodal aliphatic amine Me(6)tren (Me(6)tren = tris(2-dimethylaminoethyl)amine) [Cu(I)(Me(6)tren)(RCN)](+) first reversibly binds dioxygen to form a 1:1 Cu-O(2) species which further reacts reversibly with a second [Cu(I)(Me(6)tren)(RCN)](+) ion to form the dinuclear 2:1 Cu(2)O(2) adduct. The reaction can be observed using low temperature stopped-flow techniques. The copper superoxo complex as well as the peroxo complex were characterized by resonance Raman spectroscopy. The spectral characteristics and full kinetic and thermodynamic results for the reaction of [Cu(I)(Me(6)tren)(RCN)](+) with dioxygen are reported.     

Reversible photoregulation of binding of alpha-chymotrypsin to a gold surface

An ability to optically modulate the interactions of surfaces with functional biomolecules provides an important basis for generating new technologies including reversible biosensors, advanced medical implants, and biomolecular computers. Here we report the first example of reversible photoregulation of binding of a protease to a functional surface. A modular approach is presented with a surface-bound inhibitor containing a photoisomerizable azobenzene core to which is attached (i) appropriate protease binding functionality and (ii) a tether for surface attachment. The principle is demonstrated for alpha-chymotrypsin using a phenylalanine-based trifluoromethylketone inhibitor containing an azobenzene core and an alkyne-functionalized ethylene glycol tether, which is attached to the surface using click chemistry. UV/vis irradiation of the functional surface leads to a significant, reversible change in the amount of alpha-chymotrypsin that attaches to the surface, as measured by surface plasmon resonance.     

Energetics of dioxygen binding into graphene patches with various sizes and shapes

Density functional theory (DFT) calculations were employed to investigate the electronic properties of an H-atom terminated graphene patch (hydrographene) smaller than a rhombic C96H26 structure with zigzag edges. Depending on shapes and sizes of hydrographenes, some hydrographenes have the triplet ground state where unpaired electrons are localized on their zigzag edges. The stability of the triplet spin state is diminished, decreasing the hydrographene sizes. The existence of the localized spin densities allows triplet dioxgen to bind into a hydrographene. According to the DFT calculations, the energetics of the dioxygen bindings is negatively influenced by downsizing hydrographenes, as well as depends on their shapes. The size-and shape-dependences of the dioxygen bindings reflect from the stability of the triplet state of a hydrographene, because its localized unpaired electrons can be utilized to be attached to an unpaired electron of triplet dioxygen.     

Reversible binding of multivalent ions by surfactant self-assembly

High molecular weight nonionic surfactants have been chemically modified to bind multivalent ions reversibly by using a moderate temperature stimulus as an on/off mechanism. Only above the critical micellization temperature (CMT) does binding of multivalent ions take place, whereas below the CMT, no binding occurs to the free surfactant molecules. Different calorimetric techniques have been used to prove the reversible binding of multivalent ions. This tunable binding of multivalent metal ions allows for the improvement of many ion-exchange processes and offers attractive opportunities in the biomedical field.     

Mixed nitrogen/oxygen ligand affinities for bipositive metal ions and dioxygen binding to cobalt(II) complexes

The complex formation of Co(II) and Cd(II) with mixed N/O ligands in dimethylsulfoxide (dmso) at 298 K is investigated by means of potentiometric, UV-Vis, calorimetric, FT-IR, NMR and electrochemical techniques. The linear and cyclic ligands investigated are: 2,2'-oxydiethylamine (NON), N-(2-hydroxyethyl)-ethylenediamine (NNO), 1,4,10-trioxa-7,13-diaza-cyclopentadecane (2,1) and 1,4,7,10-tetraoxa-13-azacyclopentadecane (2,2). The results are discussed by taking into account the donor strength of donor atoms, strain effects in the formation of chelate rings, steric and inductive effects and the size of the cavity when macrocyclic ligands are concerned. DFT studies are also performed to gain an insight into the stabilization of the lower oxidation state of the CoL2(2+/3+) couple in order to understand the different reactivity of the Co(II) complexes towards dioxygen. The kinetics of dioxygen uptake has been studied by UV-Vis measurements and the results reveal an interesting strong solvent effect, which is active in lowering the rate constant for formation of the initial superoxo complex.     

Surface grafting of cobalt complexes on polymeric supports: Evidence for site isolation and applications to reversible dioxygen binding

Imprinted polymers were synthesized using the surface‐grafting technique with [Co(III) 1 (vpy)(dmap)]PF₆ { 1 , bis[2‐hydroxy‐4‐(4‐vinylbenzyloxy)benzaldehyde]ethylene‐diimine; vpy: 4‐vinylpyridine; dmap: N,N′‐dimethyl‐4‐aminopyridine} as the template. The metallated sites were probed using spectroscopic techniques including UV–vis, Fourier transform infrared, and electron paramagnetic resonance (EPR) spectroscopies to investigate the site architecture and isolation of the immobilized sites in the surface‐grafted polymers. EPR studies showed a distribution of four and five coordinated sites similar to the bulk copolymers, and the surface‐grafted polymer showed reversible binding to dioxygen in multiple cycles. Both results indicated site isolation in the surface‐grafted polymers analogous to the bulk polymers. Although the dioxygen binding in surface‐grafted polymers is reversible, the spin density decreases to 50% in the third cycle as opposed to bulk copolymers. This indicates that the sites are more heterogeneous and more exposed to the environment than the analogous sites in bulk copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 888–897, 2001     

Reversible binding of molecular oxygen to catecholate and o-amidophenolate complexes of SbV: energy approach

The theoretical study of the reactivity of catecholate and o-amidophenolate complexes of Sb^V in the reversible binding of molecular oxygen was performed. The evaluation of the strength of intramolecular interactions in pre-reaction complexes provides the assessment of the strength of oxygen binding in the coordination sphere of Sb. The initial complexes, their transition states, and final spiroendoperoxide complexes were modeled. The calculated activation energies of the reaction and ionization can serve as the energy criteria providing the explanation and prediction of interactions of these complexes with molecular oxygen.     

Reversible regulation of protein binding affinity by a DNA machine

We report a DNA machine that can reversibly regulate target binding affinity on the basis of distance-dependent bivalent binding. It is a tweezer-like DNA machine that can tune the spatial distance between two ligands to construct or destroy the bivalent binding. The DNA machine can strongly bind to the target protein when the ligands are placed at an appropriate distance but releases the target when the bivalent binding is disrupted by enlargement of the distance between the ligands. This "capture-release" cycle could be repeatedly driven by single-stranded DNA without changing the ligands and target protein.     

Reversible organogels triggered by dynamic K+ binding and release

In this study, a new lipophilic guanosine derivative was synthesized as an organogelator. The self-aggregation behavior of this organogelator was investigated by NMR, XRD and AFM. In solution, the lipophilic guanosine derivative can form a stable ribbon-like structure through NH(1)–N(7) and NH(2)–O(6) hydrogen bonds. However, gelation would occur in some aprotic solvents after the concentration reached a definite value. More interesting, the ribbon-like structure was able to change to G-quartets in the presence of K⁺, which led to the transformation from a gel to a sol. Upon the addition of the cryptand [2.2.2], which can efficiently complex with K⁺, G-quartets reverted to the original ribbon-like structure and the gel recovered. Subsequently, upon the addition of acids, K⁺ was released from the cryptate with the transformation of gel-to-sol simultaneously. Finally, upon the addition of bases which deprotonated [H⁺ ? 2.2.2], the liberated cryptand [2.2.2] recaptured K⁺ and the gel was regenerated again. This process of interconversion between G-ribbon 1_n and octamer 1₈·K⁺ was well monitored by circular dichroism spectra.     

Reversible, electrochemically controlled binding of phosphine to iron and cobalt bis(dithiolene) complexes

The homoleptic bis(dithiolene) complexes [M(S(2)C(2)R(2))(2)](2) (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S(2)C(2)R(2))(2)](2)(2-) <--> 2[M(S(2)C(2)R(2))(2)](1-) solution equilibria. The neutral dimers react with Ph3P to form square pyramidal [M(Ph(3)P)(S(2)C(2)R(2))(2)](0). Voltammetric measurements upon [M(Ph(3)P)(S(2)C(2)R(2))(2)](0) in CH(2)Cl(2) reveal only irreversible features at negative potentials, consistent with Ph(3)P dissociation upon reduction. Dissociation and reassociation of Ph(3)P from and to [Fe(Ph(3)P)(S(2)C(2)R(2))(2)](0) is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph(3)P to [M(S(2)C(2)R(2))(2)](2) (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S(2)C(2)R(2))(2)](2)(0), [M(S(2)C(2)R(2))(2)](2)(1-), [MM(S(2)C(2)R(2))(2)](2)(2-), [M(S(2)C(2)R(2))(2)](1-), [M(Ph(3)P)(S(2)C(2)R(2))(2)]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S(2)C(2)R(2))(2)](1-) is the first such iron dithiolene species to be structurally identified and reveals Fe-S bond distances of 2.172(1) and 2.179(1) Angstrom, which are appreciably shorter than those in corresponding square planar dianions.