Electronic structure and interfacial features of triphenylamine- and phenothiazine-based hole transport materials for methylammonium lead iodide perovskite solar cells.

Recently, great research efforts have been devoted to perovskite solar cells (PSCs) leading to sunlight-to-power conversion efficiencies above 25%. However, several barriers still hinder the full deployment of these devices. Critical issues are related to PCE stability and device lifetimes, which could be improved by targeted engineering of the hole transport material (HTM). Indeed, the HTM is not only responsible for transporting holes and preventing direct contact between the photo-active perovskite and the charge collector layer, but it plays important structural and protective roles too. As alternatives to the widely used yet expensive and unstable Spiro-OMeTAD, organic HTMs based on triphenylamine (TPA) and phenothiazine (PTZ) moieties have been proposed. However, their performances in PSC devices, and in particular their interfacial properties with the most popular methylammonium lead iodide perovskite (MAPI) still need investigations to be fully determined. In this framework, here we report a first-principles study on the structural and the electronic properties of a recently designed TPA and PTZ-based HTM (HTM1) and its interface with the MAPI (001) surface, considering both the PbI2- and the MAI-terminations. We also addressed already known HTM molecular systems to allow for a direct comparison with the recently proposed HTM1: we characterized the molecular parameters and the MAPI/HTM interfacial properties for Spiro-OMeTAD, PTZ1, and PTZ2. Our results suggest that good adhesion properties do not ensure effective and efficient MAPI-HTM hole injection. Despite the theoretical good alignment between HTM1 HOMO and MAPI valence band edge, our results for the mutually polarized interface point out the lack of a sufficient driving force for hole transport. While the hole mobility of HTM1 outperforms those of the other HTM molecules, for this HTM molecule, our findings suggest the application of lead halide perovskite compositions other than MAPI, with substituents that lower its valence band maximum potential value.

Spectroscopic Characterisation of the Naphthalene Dioxygenase from Rhodococcus sp. Strain NCIMB12038.

Polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, are potential health risks due to their carcinogenic and mutagenic effects. Bacteria from the genus Rhodococcus are able to metabolise a wide variety of pollutants such as alkanes, aromatic compounds and halogenated hydrocarbons. A naphthalene dioxygenase from Rhodococcus sp. strain NCIMB12038 has been characterised for the first time, using electron paramagnetic resonance (EPR) spectroscopy and UV-Vis spectrophotometry. In the native state, the EPR spectrum of naphthalene 1,2-dioxygenase (NDO) is formed of the mononuclear high spin Fe(III) state contribution and the oxidised Rieske cluster is not visible as EPR-silent. In the presence of the reducing agent dithionite a signal derived from the reduction of the [2Fe-2S] unit is visible. The oxidation of the reduced NDO in the presence of O2-saturated naphthalene increased the intensity of the mononuclear contribution. A study of the "peroxide shunt", an alternative mechanism for the oxidation of substrate in the presence of H2O2, showed catalysis via the oxidation of mononuclear centre while the Rieske-type cluster is not involved in the process. Therefore, the ability of these enzymes to degrade recalcitrant aromatic compounds makes them suitable for bioremediative applications and synthetic purposes.

Spectroscopic Characterization of Natural Melanin from a Streptomyces cyaneofuscatus Strain and Comparison with Melanin Enzymatically Synthesized by Tyrosinase and Laccase.

An actinobacteria strain was isolated from Algerian Sahara soil and assigned to Streptomyces cyaneofuscatus Pridham et al. 1958 species. This strain was selected for its ability to produce melanin exopigments in liquid and solid media. Melanin synthesis was associated with tyrosinase activity and the enzyme from this strain was isolated and biochemically characterized. Synthetic melanin was then enzymatically produced using the S. cyaneofuscatus Pridham et al. 1958 tyrosinase. As this enzyme showed a higher diphenolase activity, a synthetic melanin from the enzymic oxidation of 3,4-dihydroxyphenylalanine (dopa) was obtained by the use of a Trametes versicolor (L.) Lloyd laccase for comparison. The natural and synthetic pigments were physico-chemically characterized by the use of ultraviolet (UV)-Visible, and Fourier transform infrared (FT-IR) and multifrequency electron paramagnetic resonance (EPR) spectroscopies. All the melanin samples displayed a stable free radical when analyzed by X-band EPR spectroscopy. Once the samples were recorded at Q-band EPR, a copolymer derived from a mixture of different constituents was evident in the natural melanin. All radical species were analyzed and discussed. The use of water-soluble melanin naturally produced by S. cyaneofuscatus Pridham et al. 1958 represents a new biotechnological alternative to commercial insoluble pigments.

Photoinduced excitation and charge transfer processes of organic dyes with siloxane anchoring groups: a combined spectroscopic and computational study.

Dye-sensitized solar cells (DSSCs) have attracted significant interest in the last few years as effective low-cost devices for solar energy conversion. We have analyzed the excited state dynamics of several organic dyes bearing both cyanoacrylic acid and siloxane anchoring groups. The spectroscopic properties of the dyes have been studied both in solution and when adsorbed on a TiO2 film using stationary and time-resolved techniques, probing the sub-picosecond to nanosecond time interval. The comparison between the spectra registered in solution and on the solid substrate evidences different pathways for energy and electron relaxation. The transient spectra of the TiO2-adsorbed dyes show the appearance of a long wavelength excited state absorption band, attributed to the cationic dye species, which is absent in the spectra measured in solution. Furthermore, the kinetic traces of the samples adsorbed on the TiO2 film show a long decay component not present in solution which constitutes indirect evidence of electron transfer between the dye and the semiconductor. The interpretation of the experimental results has been supported by theoretical DFT calculations of the excited state energies and by the analysis of molecular orbitals of the analyzed dye molecules.

Tyrosine or Tryptophan? Modifying a Metalloradical Catalytic Site by Removal of the Cys-Tyr Cross-Link in the Galactose 6-Oxidase Homologue GlxA.

The concerted redox action of a metal ion and an organic cofactor is a unique way to maximize the catalytic power of an enzyme. An example of such synergy is the fungal galactose 6-oxidase, which has inspired the creation of biomimetic copper oxidation catalysts. Galactose 6-oxidase and its bacterial homologue, GlxA, possess a metalloradical catalytic site that contains a free radical on a covalently linked Cys-Tyr and a copper atom. Such a catalytic site enables for the two-electron oxidation of alcohols to aldehydes. When the ability to form the Cys-Tyr in GlxA is disrupted, a radical can still be formed. Surprisingly, the radical species is not the Tyr residue but rather a copper second-coordination sphere Trp residue. This is demonstrated through the introduction of a new algorithm for Trp-radical EPR spectra simulation. Our findings suggest a new mechanism of free-radical transfer between aromatic residues and that the Cys-Tyr cross-link prevents radical migration away from the catalytic site.

Spectroscopic and computational characterization of laccases and their substrate radical intermediates.

Laccases are multicopper oxidases which oxidize a wide variety of aromatic compounds with the concomitant reduction of oxygen to water as by-product. Due to their high stability and biochemical versatility, laccases are key enzymes to be used as eco-friendly biocatalyst in biotechnological applications. The presence of copper paramagnetic species in the catalytic site paired with the substrate radical species produced in the catalytic cycle makes laccases particularly attractive to be studied by spectroscopic approaches. In this review, the potentiality of a combined multifrequency electron paramagnetic spectroscopy /computational approach to gain information on the nature of the catalytic site and radical species is presented. The knowledge at molecular level of the enzyme oxidative process can be of great help to model new enzymes with increased efficiency and robustness.

On the mechanism of ion transport through lipid membranes mediated by PEGylated cyclic oligosaccharides (CyPLOS): an ESR study.

The mechanism underlying the ionophoric activity of CyPLOS (cyclic phosphate-linked oligosaccharide, 2), a carbohydrate-based synthetic ion transporter decorated with four tetraethylene glycol (TEG) chains, has been investigated by an integrated electron spin resonance (ESR) approach. The mode of interaction of the ionophore with lipid bilayers has been studied by quantitatively analyzing the perturbations in the ESR spectrum of an ad hoc synthesized spin-labeled CyPLOS analog (6), and, in parallel, in the spectra of spin-labeled lipids mixed with 2. The results point to a positioning of the cyclic saccharide backbone close to the lipid headgroups, largely exposed to the aqueous medium. The TEG chains, carrying a terminal benzyl group, are deeply inserted among the lipid acyl chains, showing good mobility and flexibility. As a consequence, the order of the acyl chain packing is significantly reduced, and water penetration in the bilayer is enhanced. The resulting asymmetric perturbation of the bilayer leads to its local destabilization, thus facilitating, through a non-specific mechanism, the ion transport through the membrane.

A Mn(II)-Mn(II) center in human prolidase.

Human prolidase, the enzyme responsible for the hydrolysis of the Xaa-Pro/Hyp peptide bonds, is a key player in the recycling of imino acids during the final stage of protein catabolism and extracellular matrix remodeling. Its metal active site composition corresponding to the maximal catalytic activity is still unknown, although prolidase function is of increasing interest due to the link with carcinogenesis and mutations in prolidase gene cause a severe connective tissue disorder. Here, using EPR and ICP-MS on human recombinant prolidase produced in Escherichia coli (hRecProl), the Mn(II) ion organized in a dinuclear Mn(II)-Mn(II) center was identified as the protein cofactor. Furthermore, thermal denaturation, CD/fluorescence spectroscopy and limited proteolysis revealed that the Mn(II) is required for the proper protein folding and that a protein conformational modification is needed in the transition from apo- to Mn(II)loaded-enzyme. The collected data provided a better knowledge of the human holo-prolidase and, although limited to the recombinant enzyme, the exact identity and organization of the metal cofactor as well as the conformational change required for activity were proven.

Effective mutations in a high redox potential laccase from Pleurotus ostreatus.

Since the first report on a laccase, there has been a notable development in the interest towards this class of enzymes, highlighted from the number of scientific papers and patents about them. At the same time, interest in exploiting laccases-mainly high redox potential-for various functions has been growing exponentially over the last 10 years. Despite decades of work, the molecular determinants of the redox potential are far to be fully understood. For this reason, interest in tuning laccase redox potential to provide more efficient catalysts has been growing since the last years. The work herein described takes advantage of the filamentous fungus Aspergillus niger as host for the heterologous production of the high redox potential laccase POXA1b from Pleurotus ostreatus and of one of its in vitro selected variants (1H6C). The system herein developed allowed to obtain a production level of 35,000 U/L (583.3 µkat/L) for POXA1b and 60,000 U/L (1,000 µkat/L) for 1H6C, corresponding to 13 and 20 mg/L for POXA1b and 1H6C, respectively. The characterised proteins exhibit very similar characteristics, with some exceptions regarding catalytic behaviour, stability and spectro-electrochemical properties. Remarkably, the 1H6C variant shows a higher redox potential with respect to POXA1b. Furthermore, the spectro-electrochemical results obtained for 1H6C make it tempting to claim that we spectro-electrochemically determined the redox potential of the 1H6C T2 site, which has not been studied in any detail by spectro-electrochemistry yet.

Formation of a tyrosine adduct involved in lignin degradation by Trametopsis cervina lignin peroxidase: a novel peroxidase activation mechanism.

LiP (lignin peroxidase) from Trametopsis cervina has an exposed catalytic tyrosine residue (Tyr181) instead of the tryptophan conserved in other lignin-degrading peroxidases. Pristine LiP showed a lag period in VA (veratryl alcohol) oxidation. However, VA-LiP (LiP after treatment with H2O2 and VA) lacked this lag, and H2O2-LiP (H2O2-treated LiP) was inactive. MS analyses revealed that VA-LiP includes one VA molecule covalently bound to the side chain of Tyr181, whereas H2O2-LiP contains a hydroxylated Tyr181. No adduct is formed in the Y171N variant. Molecular docking showed that VA binding is favoured by sandwich π stacking with Tyr181 and Phe89. EPR spectroscopy after peroxide activation of the pre-treated LiPs showed protein radicals other than the tyrosine radical found in pristine LiP, which were assigned to a tyrosine-VA adduct radical in VA-LiP and a dihydroxyphenyalanine radical in H2O2-LiP. Both radicals are able to oxidize large low-redox-potential substrates, but H2O2-LiP is unable to oxidize high-redox-potential substrates. Transient-state kinetics showed that the tyrosine-VA adduct strongly promotes (>100-fold) substrate oxidation by compound II, the rate-limiting step in catalysis. The novel activation mechanism is involved in ligninolysis, as demonstrated using lignin model substrates. The present paper is the first report on autocatalytic modification, resulting in functional alteration, among class II peroxidases.

Effects of the protein environment on the spectral properties of tryptophan radicals in Pseudomonas aeruginosa azurin.

Many biological electron-transfer reactions involve short-lived tryptophan radicals as key reactive intermediates. While these species are difficult to investigate, the recent photogeneration of a long-lived neutral tryptophan radical in two Pseudomonas aeruginosa azurin mutants (Az48W and ReAz108W) made it possible to characterize the electronic, vibrational, and magnetic properties of such species and their sensitivity to the molecular environment. Indeed, in Az48W the radical is embedded in the hydrophobic core while, in ReAz108W it is solvent-exposed. Here we use density functional theory and multiconfigurational perturbation theory to construct quantum-mechanics/molecular-mechanics models of Az48W(•) and ReAz108W(•) capable of reproducing specific features of their observed UV-vis, resonance Raman, and electron paramagnetic resonance spectra. The results show that the models can correctly replicate the spectral changes imposed by the two contrasting hydrophobic and hydrophilic environments. Most importantly, the same models can be employed to disentangle the molecular-level interactions responsible for such changes. It is found that the control of the hydrogen bonding between the tryptophan radical and a single specific surface water molecule in ReAz108W(•) represents an effective means of spectral modulation. Similarly, a specific electrostatic interaction between the radical moiety and a Val residue is found to control the Az48W(•) excitation energy. These modulations appear to be mediated by the increase in nitrogen negative charge (and consequent increase in hydrogen bonding) of the spectroscopic D2 state with respect to the D0 state of the chromophore. Finally, the same protein models are used to predict the relaxed Az48W(•) and ReAz108W(•) D2 structures, showing that the effect of the environment on the corresponding fluorescence maxima must parallel that of D0 absorption spectra.

Crystallographic, kinetic, and spectroscopic study of the first ligninolytic peroxidase presenting a catalytic tyrosine.

Trametes cervina lignin peroxidase (LiP) is a unique enzyme lacking the catalytic tryptophan strictly conserved in all other LiPs and versatile peroxidases (more than 30 sequences available). Recombinant T. cervina LiP and site-directed variants were investigated by crystallographic, kinetic, and spectroscopic techniques. The crystal structure shows three substrate oxidation site candidates involving His-170, Asp-146, and Tyr-181. Steady-state kinetics for oxidation of veratryl alcohol (the typical LiP substrate) by variants at the above three residues reveals a crucial role of Tyr-181 in LiP activity. Moreover, assays with ferrocytochrome c show that its ability to oxidize large molecules (a requisite property for oxidation of the lignin polymer) originates in Tyr-181. This residue is also involved in the oxidation of 1,4-dimethoxybenzene, a reaction initiated by the one-electron abstraction with formation of substrate cation radical, as described for the well known Phanerochaete chrysosporium LiP. Detailed spectroscopic and kinetic investigations, including low temperature EPR, show that the porphyrin radical in the two-electron activated T. cervina LiP is unstable and rapidly receives one electron from Tyr-181, forming a catalytic protein radical, which is identified as an H-bonded neutral tyrosyl radical. The crystal structure reveals a partially exposed location of Tyr-181, compatible with its catalytic role, and several neighbor residues probably contributing to catalysis: (i) by enabling substrate recognition by aromatic interactions; (ii) by acting as proton acceptor/donor from Tyr-181 or H-bonding the radical form; and (iii) by providing the acidic environment that would facilitate oxidation. This is the first structure-function study of the only ligninolytic peroxidase described to date that has a catalytic tyrosine.

The nature of tryptophan radicals involved in the long-range electron transfer of lignin peroxidase and lignin peroxidase-like systems: Insights from quantum mechanical/molecular mechanics simulations.

A catalytically active tryptophan radical has been demonstrated to be involved in the long-range electron transfer to the heme cofactor of lignin peroxidase (LiP) from Phanerochaete chrysosporium although no direct detection by EPR spectroscopy of the tryptophan radical intermediate has been reported to date. An engineering-based approach has been used to manipulate the microenvironment of the redox-active tryptophan site in LiP and Coprinus cinereus Peroxidase (CiP), allowing the direct evidence of the tryptophan radical species. In light of the newly available EPR experimental data, we performed a quantum mechanical/molecular mechanics computational study to characterize the tryptophan radicals in the above protein matrices as well as in pristine LiP. The nature of the tryptophan radicals is discussed together with the analysis of their environment with the aim of understanding the different behavior of pristine LiP in comparison with that of LiP and CiP variants.

Enzymatic decolorization of spent textile dyeing baths composed by mixtures of synthetic dyes and additives.

The effects of different components of real dyeing bath formulations, such as the equalizing and fixing additives-acids, salts, and surfactants-on the decolorization catalyzed by Funalia trogii enzymatic extracts, were investigated to understand their influence on the recalcitrance to biodegradation of this type of wastewater. The decolorization of selected dyes and dye mixtures after tissue dyeing was performed in the presence/absence of auxiliary compounds. All spent dyeing baths were enzymatically decolorized to different extents, by the addition of extracts containing laccase only or laccase plus cellobiose dehydrogenase. Whereas surfactant auxiliaries, in some instances, inhibit the decolorization of spent dyeing baths, in several occurrences the acid/salt additives favor the enzymatic process. In general, the complete spent dyeing formulations are better degraded than those containing the dyes only. The comparison of extracellular extracts obtained from spent straws from the commercial growth of Pleurotus sp. mushrooms with those from F. trogii reveals similar decolorization extents thus allowing to further reduce the costs of bioremediation.

EPR parameters of amino acid radicals in P. eryngii versatile peroxidase and its W164Y variant computed at the QM/MM level.

Quantum mechanics/molecular mechanics (QM/MM) methods, employing density functional theory (DFT), have been used to compute the electron paramagnetic resonance (EPR) parameters of tryptophan and tyrosyl radical intermediates involved in the catalytic cycle of Pleurotus eryngii versatile peroxidase (VP) and its W164Y variant, respectively. These radicals have been previously experimentally detected and characterized both in the two-electron and one-electron activated forms of the enzymes. In this work, the well-studied W164 radical in VP has been chosen for calibration purposes because its spectroscopic properties have been extensively studied by multifrequency EPR and ENDOR spectroscopies. Using a B3LYP/CHARMM procedure, appropriately accounting for electrostatic, such as hydrogen bonding, and steric environmental interactions, a good agreement between the calculated and measured EPR parameters for both radicals has been achieved; g-tensors, hyperfine coupling constants (hfcc) and Mulliken spin densities have been correlated to changes in geometries, hydrogen bond networks and electrostatic environment, with the aim of understanding the influence of the protein surroundings on EPR properties. In addition, the present calculations demonstrate, for VP, the formation of a neutral tryptophan radical, hydrogen bonded to the nearby E243, via a stepwise electron and proton transfer with earlier involvement of a short-lived tryptophan cationic species. Instead, for W164Y, the QM/MM dynamics simulation shows that the tyrosine oxidation proceeds via a concerted electron and proton transfer and is accompanied by a significant reorganization of residues and water molecules surrounding the tyrosyl radical.

An artificial molecular switch that mimics the visual pigment and completes its photocycle in picoseconds.

Single molecules that act as light-energy transducers (e.g., converting the energy of a photon into atomic-level mechanical motion) are examples of minimal molecular devices. Here, we focus on a molecular switch designed by merging a conformationally locked diarylidene skeleton with a retinal-like Schiff base and capable of mimicking, in solution, different aspects of the transduction of the visual pigment Rhodopsin. Complementary ab initio multiconfigurational quantum chemistry-based computations and time-resolved spectroscopy are used to follow the light-induced isomerization of the switch in methanol. The results show that, similar to rhodopsin, the isomerization occurs on a 0.3-ps time scale and is followed by <10-ps cooling and solvation. The entire (2-photon-powered) switch cycle was traced by following the evolution of its infrared spectrum. These measurements indicate that a full cycle can be completed within 20 ps.

A novel environmental indicator for monitoring of pesticides.

The identification of an indicator for monitoring pesticides is a very effective analytical approach because it allows one to schedule and simplify the analytical routine. In this study, a new indicator has been designed, which has to be able to define a scale of priorities in the pesticides monitoring. A starting equation was developed involving the escaping tendency of a given substance from a phase (based on the Mackay model I level). The reliability of the indicator was tested by application to a model system, consisting of a defined and limited area, choosing water as matrix over a period of 6 years. A group of marker compounds was also defined to implement the predictive efficacy of the indicator. The results obtained by modeling were compared to those obtained by experimentation of the same model system. The indicator was subsequently and appropriately modified creating a new equation, including a kinetic factor, which considers the environmental degradation processes. The effect of the rectified indicator was consistent with the sales data list of compounds, when applied to the markers. The indicator developed in this study, tested as a model on specific area-phase-period (Province of Siena, water phase, 2000-2006), is applicable to any other area-phase-period, adjusting the partition value of the Mackay model for the case under study.

The critical issue of using lead for sustainable massive production of perovskite solar cells: a review of relevant literature.

This work aims to review the most significant studies dealing with the environmental issues of the use of lead in perovskite solar cells (PSCs). A careful discussion and rationalization of the environmental and human health toxicity impacts, evaluated by life cycle assessment and risk assessment studies, is presented. The results of this analysis are prospectively related to the possible future massive production of PSC technology.

Modeling, preparation, and characterization of a dipole moment switch driven by Z/E photoisomerization.

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Environmental and economic optima of solar home systems design: A combined LCA and LCC approach.

This paper compares the economic and environmental optimal design of Solar Home Systems (SHSs) and explores the role of economic incentives (such as tariffs and technology costs) in approximating the two optima. To achieve that, we present a methodology for the environmental and economic evaluation of grid-connected SHSs: user-scale electric systems involving a photovoltaic (PV) power system and a battery energy storage system. The proposed methodology is based on a mixed integer linear programming (MILP) optimization, life cycle assessment and life cycle costing. This methodological framework is applied to a case study involving a typical SHS installation in Italy. The results of the environmental optimal design brought to the evaluation of a 3.25 kW PV assisted by 8.66 kWh of nickel cobalt manganese batteries, whereas the costs of the SHS are minimized by a small PV system (less than 1 kW). Results underline that the environmental optimal configurations rely on battery technologies, which entails a significant cost compared to the grid connection. In contrast, the economic optimal design solutions is less impactful than the grid mix both from an environmental and economic points of view. Thanks to a reduction of batteries and PV costs, the environmental impact of the economic optimal design is expected to decrease in the future.