Redox reactivity and reorganization energy of zinc cytochrome c cation radical

Photoactivation studies of zinc porphyrin-myoglobin system and its application for light-chemical energy conversion

An artificial zinc porphyrin-myoglobin-based photo-chemical energy conversion system, consisting of ZnPP-Mb or ZnPE₁-Mb as a photosensitizer, NADP⁺ as an electron acceptor, and triethanolamine as an electron donor, has been constructed to mimic photosystem I. The photoirradiated product is able to reduce a single-electron acceptor protein cytochrome c, but cannot catalyze the two-electron reduction of acetaldehyde by alcohol dehydrogenase, thus demonstrating a single electron transfer mechanism. Furthermore, the artificial system can bifunctionally promote oxidoredox reactions, depending on the presence or absence of a sacrificial electron donor, thus suggesting its potential application in electrochemical regeneration steps involved in chemical transformation and/or energy conversion.

Dynamic Quenching of Porphyrin Triplet States by Two-Photon Absorbing Dyes: Towards Two-Photon-Enhanced Oxygen Nanosensors

Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [1]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern-Volmer quenching constants were correlated with the corresponding ET driving forces (DeltaG(ET)), calculated using the Rehm-Weller equation. FRET-pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.

Ligand-Field Dependence of the Excited State Dynamics of Hangman Bisporphyrin Dyad Complexes

A new Hangman porphyrin architecture has been developed to interrogate the ligand-field dependence of photoinduced PCET versus excitation energy transfer and intersystem crossing in PZn(II)-PFe(III)-OH dyads (P = porphyrin). In this design, a hanging carboxylic acid group establishes a hydrogen-bonding network to anchor the weak-field OH- ligand in the distal site of the PFe(III)-OH acceptor, whereas the proximal site is left available to accept strong-field imidazole ligands. Thus, controlling the tertiary coordination environment gives access to the first synthetic example of a porphyrin dyad with a biologically relevant weak-field/strong-field configuration of axial ligands at the heme. Transient absorption spectroscopy has been employed to probe the fate of the initial PZn(II)-based S1 excited state, revealing rapid S1 quenching for all dyads in the presence and absence of strong-field imidazole ligands (tau = 6-50 ps). The absence of a (P*+)Zn(II) signal that would complement photoinduced PCET at the PFe(III)-OH subunit (i.e., PFe(III)-OH --> PFe(II)-OH2) shows that excitation energy transfer and intersystem crossing channels dominate the quenching, regardless of whether proximal strong field ligands are present. Moreover, this photophysical assignment is independent of the solvent dielectric constant and whether a phenylene or biphenylene spacer is used to span the two porphyrin subunits. Electronic structure calculations suggest that the structural reorganization attendant to reductive PCET at the high-spin Fe(III)-OH center imposes a severe kinetic cost that can only be alleviated by inducing a low-spin electronic configuration with two strong-field axial ligands.

A Photoelectrochemical Device with a Nanostructured SnO2Electrode Modified with Composite Clusters of Porphyrin-Modified Silica Nanoparticle and Fullerene

A silica nanoparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C60 molecules on a nanostructured SnO2 electrode. The quenching of the porphyrin excited singlet state on the silica nanoparticle is suppressed significantly, showing that silica nanoparticles are promising scaffolds for organizing photoactive molecules three-dimensionally in nanometer scale. Marked enhancement of the photocurrent generation was achieved in the present system compared with the reference system, where a gold core was employed as a scaffold of porphyrins instead of a silica nanoparticle. The rather small incident photon-to-current efficiency relative to a similar photoelectrochemical device using a silica microparticle may result from poor electron and hole mobility in the composite film due to poor connection between the composite clusters of a porphyrin-modified silica nanoparticle and C60 in micrometer scale.

The mechanism of GroEL/GroES folding/refolding of protein substrates revisited

The thermodynamics and kinetics of zinc-cytochrome c (ZnCyt c) interactions with Escherichia coli molecular chaperone GroEL (Chaperonin 60; Cpn60) are described. Zinc(II)-porphyrin represents a flexible fluorescent probe for thermodynamic complex formation between GroEL and ZnCyt c, as well as for stopped-flow fluorescence kinetic experiments. Data suggests that GroEL and GroEL/GroES-assisted refolding of unfolded ZnCyt c takes place by a mechanism that is quite close to the Anfinsen Cage hypothesis for molecular chaperone activity. However, even in the presence of ATP, GroEL/GroES-assisted refolding of ZnCyt c takes place at approximately half the rate of refolding of ZnCyt c alone. On the other hand, there is little evidence for refolding behaviour consistent with the Iterative Annealing hypothesis. This includes a complete lack of GroEL or GroEL/GroES-assisted enhancement of refolding rate constant k(2) associated with the unfolding of a putative misfolded state I (Zn) on the pathway to the native state. Reviewing our data in the light of data from other laboratories, we observe that all forward rate enhancements or reductions could be accounted for in terms of thermodynamic coupling (adjusting positions of refolding equilibria) due to binding interactions between GroEL and unfolded protein substrates, driven by thermodynamic considerations. Therefore, we propose that passive kinetic partitioning should be considered the core mechanism of the GroEL/GroES molecular chaperone machinery, wherein the core function is to bind unfolded protein substrates leading to a blockade of aggregation pathways and to increases in molecular flux through productive folding pathway(s).

Photovoltaic Cells Using Composite Nanoclusters of Porphyrins and Fullerenes with Gold Nanoparticles

Novel organic solar cells have been prepared using quaternary self-organization of porphyrin (donor) and fullerene (acceptor) units by clusterization with gold nanoparticles on nanostructured SnO2 electrodes. First, porphyrin-alkanethiolate monolayer-protected gold nanoparticles (H2PCnMPC: n is the number of methylene groups in the spacer) are prepared (secondary organization) starting from the primary component (porphyrin-alkanethiol). These porphyrin-modified gold nanoparticles form complexes with fullerene molecules (tertiary organization), and they are clusterized in acetonitrile/toluene mixed solvent (quaternary organization). The highly colored composite clusters can then be assembled as three-dimensional arrays onto nanostructured SnO2 films to afford the OTE/SnO2/(H2PCnMPC+C60)m electrode using an electrophoretic deposition method. The film of the composite clusters with gold nanoparticle exhibits an incident photon-to-photocurrent efficiency (IPCE) as high as 54% and broad photocurrent action spectra (up to 1000 nm). The power conversion efficiency of the OTE/SnO2/(H2PC15MPC+C60)m composite electrode reaches as high as 1.5%, which is 45 times higher than that of the reference system consisting of the both single components of porphyrin and fullerene.

Mimicking biological electron transport in sol-gel glass: photoinduced electron transfer from zinc cytochrome C to plastocyanin or cytochrome C mediated by mobile inorganic complexes

Biomimetic studies of electron-transport chains are important for establishing the molecular mechanisms of long-range communications between proteins. We mimic these biological assemblies by encapsulating metalloproteins in sol-gel silica glass and letting mobile inorganic complexes shuttle electrons between the immobilized proteins. We present two examples of such rudimentary electron-transport chains. In both of them the immobilized electron donor is the zinc-substituted cytochrome c, Zncyt; the immobilized electron acceptor is either cupriplastocyanin, pc(II), or ferricytochrome c, cyt(III); and the mobile charge carrier Q/Q(-) is the redox couple FeEDTA(-)(/2)(-) or Ru(NH(3))(6)(3+/2+). The redox processes are photoinduced: Zncyt is excited by the laser pulse and converted to the triplet state, (3)Zncyt, which is a strong reducing agent. Visible absorption, circular dichroism, and electron paramagnetic resonance spectra of the metalloproteins show that encapsulation in sol-gel glass does not affect their intrinsic redox properties. The rigid silica glass spatially separates the proteins from each other. In this matrix, the electron-transfer reactions between (3)Zncyt and pc(II) and between (3)Zncyt and cyt(III), which occur fast in solution, are completely suppressed in the absence of a charge carrier Q/Q(-). The reactivity of FeEDTA(-) and Ru(NH(3))(6)(3+) (as quenchers Q of (3)Zncyt) is minimally affected by the interior of the sol-gel glass. In the glass, the second-order rate constants for the excited-state electron transfer, from (3)Zncyt to Q, are (8.9 +/- 0.6) x 10(6) and (8.0 +/- 2.4) x 10(6) M(-)(1) s(-)(1) for FeEDTA(-) and Ru(NH(3))(6)(3+), respectively. This reaction is followed by the ground-state back electron transfer, from Q(-) to Zncyt(+). In the "monoprotein" glasses Zncyt/Q, the respective second-order rate constants for this back electron-transfer reaction are (4.9 +/- 0.2) x 10(7) and (7.8 +/- 2.7) x 10(7) M(-)(1) s(-)(1). In the "diprotein" glasses Zncyt/Q/pc(II) and Zncyt/Q/cyt(III), containing also the acceptor protein pc(II) or cyt(III), Zncyt(+) decays on two time scales. The faster and major component of this decay is analogous to the only mode of the decay in the Zncyt/Q glasses and is a second-order process. Between 25 and 40% of the initially formed Zncyt(+), however, lives longer (k(slow) =1.1 +/- 0.2 s(-)(1)) and decays by a first-order process. We attribute the lengthening of the Zncyt(+) lifetime to a partial escape of the photogenerated Q(-) into the glass pores, where it reacts with the immobilized pc(II) or cyt(III). Indeed, the visible absorption spectra show the photoinduced reduction of pc(II) and cyt(III). Evidently, the small inorganic complexes, FeEDTA(-)(/2)(-) and Ru(NH(3))(6)(3+/2+), move through the glass pores, react with the encapsulated metalloproteins, and establish the interprotein electron transfer. Each interprotein reaction now occurs in two steps: a mobile charge carrier Q receives an electron from (3)Zncyt, and Q(-) then delivers an electron to pc(II) or cyt(III). Ultimately, the energy of visible light is converted to reducing equivalents for plastocyanin and cytochrome c. The sequential electron transfer described here resembles the events in a rudimentary electron-transport chain. Our findings demonstrate the promise of integrating proteins, with their optimally adjusted redox sites, in photocatalytic materials.

Molten-globule and other conformational forms of zinc cytochrome C. Effect of partial and complete unfolding of the protein on its electron-transfer reactivity

To test the effect of protein conformation on reactivity, we use laser flash photolysis to compare the electron-transfer properties of the triplet state of zinc-substituted cytochrome c, designated (3)Zncyt, in the folded forms at low (F(low)) and high (F(high)) ionic strength, molten-globule (MG) form, and the forms unfolded by acid (U(acid)) and urea (U(urea)) toward the following four oxidative quenchers: Fe(CN)(6)(3-), Co(acac)(3), Co(phen)(3)(3+), and iron(III) cytochrome c. We characterize the conformational forms of Zncyt on the basis of the far-UV circular dichroism, Soret absorption, and rate constant for natural decay of the triplet state. This rate constant in the absence of quencher increases in the order F(high) < F(low) < MG < U(acid) < U(urea) because the exposure of porphyrin to solvent increases as Zncyt unfolds. Bimolecular rate constants for the reaction of (3)Zncyt with the four quenchers show significant effects on reactivity of electrostatic interactions and porphyrin exposure to solvent. This rate constant at the ionic strength of 20 mM increases upon unfolding by urea and acid, respectively, as follows: 1340-fold and 466-fold when the quencher is Co(phen)(3)(3+) and 168-fold and 36-fold when the quencher is cyt(III). To compare reactivity of (3)Zncyt in the F(low), F(high), MG, U(acid), and U(urea) forms without complicating effects of electrostatic interactions, we used the electroneutral quencher Co(acac)(3). Indeed, reactivity of folded (3)Zncyt with Co(acac)(3) was independent of ionic strength. Reactivity of (3)Zncyt with Co(acac)(3) upon partial and complete unfolding increases 10-fold, 54-fold, and 64-fold in the molten-globule, urea-unfolded, and acid-unfolded forms.