Light triggers molecular shuttling in rotaxanes: control over proximity and charge recombination

We present the synthesis of novel rotaxanes based on mechanically interlocked porphyrins and fullerene and their advanced investigations by means of photophysical measurements. To this end, a fullerene-capped dumbbell-type axle containing a central triazole was threaded through strapped (metallo)porphyrins—either a free-base or a zinc porphyrin. Femtosecond-resolved transient absorption measurements revealed charge-separation between the porphyrin and fullerene upon light excitation. Solvent polarity and solvent coordination effects induced molecular motion of the rotaxanes upon charge separation and enabled, for the first time, subtle control over the charge recombination by enabling and controlling the directionality of shuttling

Reactivity of Myoglobin Reconstituted with Cobalt Corrole toward Hydrogen Peroxide

The protein matrix of natural metalloenzymes regulates the reactivity of metal complexes to establish unique catalysts. We describe the incorporation of a cobalt complex of corrole (CoCor), a trianionic porphyrinoid metal ligand, into an apo-form of myoglobin to provide a reconstituted protein (rMb(CoCor)). This protein was characterized by UV-vis, EPR, and mass spectroscopic measurements. The reaction of rMb(CoCor) with hydrogen peroxide promotes an irreversible oxidation of the CoCor cofactor, whereas the same reaction in the presence of a phenol derivative yields the cation radical form of CoCor. Detailed kinetic investigations indicate the formation of a transient hydroperoxo complex of rMb(CoCor) which promotes the oxidation of the phenol derivatives. This mechanism is significantly different for native heme-dependent peroxidases, which generate a metal-oxo species as an active intermediate in a reaction with hydrogen peroxide. The present findings of unique reactivity will contribute to further design of artificial metalloenzymes

Iron-Strapped Porphyrins with Carboxylic Acid Groups Hanging over the Coordination Site: Synthesis, X-ray Characterization, and Dioxygen Binding

International audienceA series of myoglobin active site analogues were synthesized and characterized to investigate the dioxygen binding effects of a flexible distal strap over the coordination site. These four synthetic models differ mostly by the shape and polarity of their cavities and also possibly by motion of the distal strap attached to two of the meso carbon atoms. Each of the four models has an intramolecular nitrogen base that axially binds the iron(II) cation inside the porphyrin, but they differ either by the nature of the distal strap or by its mobility. The overhanging distal group is either a generally apolar ethyl malonate group or a polar malonic acid group which is also a strong H-bond donor. It is shown that, in the ferrous complex 2b bearing such an overhung malonic acid group in close proximity to the iron atom, the equilibrium rate for dioxygen binding is significantly enhanced in comparison to that of its ester precursor. In the case of the analogous complex 1b bearing a more mobile distal strap, one of the carboxylic acid groups binds the iron(II) cation, leading to a six-coordinate ferrous complex. Unexpectedly, this complex proved to be high-spin (S = 2) as shown by solid-state magnetic measurements. Whereas this unprecedented complex still binds dioxygen, the formation of the intramolecular six-coordinate complex precluded the measurement of its dioxygen affinity through direct quantitative gas titration monitored by UV-vis spectroscopy. However, in this case, the determination of the kinetic rate constants for dioxygen binding and dissociation by laser flash photolysis allowed the evaluation of the equilibrium rate. Together with three previous X-ray structures of iron complexes in the ααββ conformation, the structure of the cavity and the shape of the relaxed distal strap are also discussed with the consideration of the resolution of X-ray structures of two different free-base ligands in the ααββ conformation, with one bearing the ethyl malonate group and the second one bearing the malonic acid group. A third X-ray structure of the analogous ligand with the overhanging ethyl malonate group in the αβαβ series allows a direct comparison of the distal strap in both geometries. This work reveals that the compound with the overhanging carboxylic acid group which cannot directly interact with the ferrous heme exhibits an increased dioxygen affinity by 2 orders of magnitude versus its ester precursor

Chemically Programmed Supramolecular Assembly of Hemoprotein and Streptavidin with Alternating Alignment

It is shown that an intramolecularly linked cofactor dyad enforces the assocn. of two different proteins according to a predefined program.  The present study demonstrates that the heme-bis(biotin) conjugate contains the information to produce a stable 2:1 Mb-SAv complex.  Propagation of this moiety into a polymer results from using a dimericMb building block.  The supramol. composite fiber obsd. by AFM techniques is the first example of a chem. programmed heterotropic protein copolymer with alternating alignment.  The heme cofactor's dioxygen binding function is maintained upon incorporation within the fiber, suggesting that this approach is well-suited for the creation of functional nanobiomaterials.  Immobilization of the first building block will allow to: (1) study the programmed assembly by quartz microbalance anal., (2) increase the complexity by varying the capping groups on the dyad and/or (3) introduce addnl. protein building blocks to afford protein polymers with a desired sequence and function.  Potential applications include multifunctional catalysis, artificial photosynthesis, and smart nanomaterials for medical applications

Dynamic protease activation on a multimeric synthetic protein scaffold via adaptable DNA-based recruitment domains

Hexameric hemoprotein (HTHP) is employed as a scaffold protein for the supramolecular assembly and activation of the apoptotic signalling enzyme caspase-9, using short DNA elements as modular recruitment domains. Caspase-9 assembly and activation on the HTHP platform due to enhanced proximity is followed by combinatorial inhibition at high scaffold concentrations. The DNA recruitment domains allow for reversible switching of the caspase-9 assembly and activity state using short modulatory DNA strands. Tuning of the recruitment domain affinity allows for generating kinetically trapped active enzyme complexes, as well as for dynamic repositioning of caspases over scaffold populations and inhibition using monovalent sink platforms. The conceptual combination of a highly structured multivalent protein platform with modular DNA recruitment domains provides emergent biomimicry properties with advanced levels of control over protein assembly

C(sp³)–H Bond Hydroxylation Catalyzed by Myoglobin Reconstituted with Manganese Porphycene

Myoglobin reconstituted with manganese porphycene was prepared in an effort to generate a new biocatalyst and was characterized by spectroscopic techniques. The X-ray crystal structure of the reconstituted protein reveals that the artificial cofactor is located in the intrinsic heme-binding site with weak ligation by His93. Interestingly, the reconstituted protein catalyzes the H₂O₂-dependent hydroxylation of ethylbenzene to yield 1-phenylethanol as a single product with a turnover number of 13 at 25 °C and pH 8.5. Native myoglobin and other modified myoglobins do not catalyze C–H hydroxylation of alkanes. Isotope effect experiments yield KIE values of 2.4 and 6.1 for ethylbenzene and toluene, respectively. Kinetic data, log <i>k</i>_obs versus BDE­(C­(sp³)–H) for ethylbenzene, toluene, and cyclohexane, indicate a linear relationship with a negative slope. These findings clearly indicate that the reaction occurs via a rate-determining step that involves hydrogen-atom abstraction by a Mn­(O) species and a subsequent rebound hydroxylation process which is similar to the reaction mechanism of cytochrome P450

Photoinduced Electron Transfer of ZnS–AgInS₂ Solid-Solution Semiconductor Nanoparticles: Emission Quenching and Photocatalytic Reactions Controlled by Electrostatic Forces

The electron transfer between the fluorescent ZnS–AgInS₂ solid-solution (ZAIS) nanoparticles (NPs) and redox species is investigated in an aqueous solution in terms of chemosensors and photocatalysts. In a case where the ZAIS NPs and quenchers are oppositely charged, very efficient photoluminescence (PL) quenching (<i>K</i>_S ≈ 10⁶ M^–1) is observed because of the specific adsorption of quenchers on particles. The quenching magnitude increases with the potential gap between the photoexcited NPs and quenchers. Methyl viologen (MV²⁺) shows the largest quenching behavior; i.e., it is revealed to quench most of PL by the adsorption of only a single molecule. In such a case, the linearity of quenching magnitude is lost, and distribution of the quencher adsorption on NPs should be considered to clarify the quenching mechanism. In the opposite case where both the ZAIS NPs and quenchers are negatively charged, although the quenching efficiency is two to three orders of magnitude lower than that in the aforementioned adsorption case, electron transfer occurs from the NPs to redox species. The efficiency of photoreduction is investigated in association with the charge repulsion between both species

Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging COOH That Increases O(2)/CO Binding Selectivity in Aqueous Solution

International audienceA water-soluble strapped iron(III)tetraarylporphyrin () bearing two propylpyridinium groups at the side chains and a carboxylic acid group at the overhanging position of the strap was synthesized to mimic the function of myoglobin with the distal polar functionality in aqueous solution. forms a stable 1:1 inclusion complex with a per--methylated β-cyclodextrin dimer having a pyridine linker (), providing a hydrophobic environment and a proximal fifth ligand to stabilize the O-complex. The ferrous complex () binds both O and CO in aqueous solution. The O and CO binding affinities ( and ) and half-life time () of the O complex of are 6.3 and 0.021 Torr, and 7 h, respectively, at pH 7 and 25 °C. The control compound without the strap structure () has similar oxygen binding characteristics ( = 8.0 Torr), but much higher CO binding affinity ( = 3.8 × 10 Torr), and longer (30 h). The O and CO kinetics indicate that the strapped structure in inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower and values. Interestingly, the CO complex of is significantly destabilized (relatively larger ), while the value is much smaller than that of , resulting in significantly increased O/CO selectivity (reduced value, where = / = 320) in compared to ( = 21000)