Targeting cellular mitophagy as a strategy for human cancers.

Mitophagy is the cellular process to selectively eliminate dysfunctional mitochondria, governing the number and quality of mitochondria. Dysregulation of mitophagy may lead to the accumulation of damaged mitochondria, which plays an important role in the initiation and development of tumors. Mitophagy includes ubiquitin-dependent pathways mediated by PINK1/Parkin and non-ubiquitin dependent pathways mediated by mitochondrial autophagic receptors including NIX, BNIP3, and FUNDC1. Cellular mitophagy widely participates in multiple cellular process including metabolic reprogramming, anti-tumor immunity, ferroptosis, as well as the interaction between tumor cells and tumor-microenvironment. And cellular mitophagy also regulates tumor proliferation and metastasis, stemness, chemoresistance, resistance to targeted therapy and radiotherapy. In this review, we summarized the underlying molecular mechanisms of mitophagy and discussed the complex role of mitophagy in diverse contexts of tumors, indicating it as a promising target in the mitophagy-related anti-tumor therapy.

Efficient CO2 electroreduction to ethanol enabled by tip-curvature-induced local electric fields.

Electrocatalytic reduction of CO2 into multicarbon (C2+) products offers a promising pathway for CO2 utilization. However, achieving high selectivity towards multicarbon alcohols, such as ethanol, remains a challenge. In this work, we present a novel CuO nanoflower catalyst with engineered tip curvature, achieving remarkable selectivity and efficiency in the electroreduction of CO2 to ethanol. This catalyst exhibits an ethanol faradaic efficiency (FEethanol) of 47% and a formation rate of 320 µmol h-1 cm-2, with an overall C2+ product faradaic efficiency (FEC2+) reaching ∼77.8%. We attribute this performance to the catalyst's sharp tip, which generates a strong local electric field, thereby accelerating CO2 activation and facilitating C-C coupling for deep CO2 reduction. In situ Raman spectroscopy reveals an increased *OH coverage under operating conditions, where the enhanced *OH adsorption facilitates the stabilization of *CHCOH intermediates through hydrogen bonding interaction, thus improving ethanol selectivity. Our findings demonstrate the pivotal role of local electric fields in altering reaction kinetics for CO2 electroreduction, presenting a new avenue for catalyst design aiming at converting CO2 to ethanol.

A photocatalytic redox cycle over a polyimide catalyst drives efficient solar-to-H2O2 conversion.

Circumventing the conventional two-electron oxygen reduction pathway remains a great problem in enhancing the efficiency of H2O2 photosynthesis. A promising approach to achieve outstanding photocatalytic activity involves the utilization of redox intermediates. Here, we engineer a polyimide aerogel photocatalyst with photoreductive carbonyl groups for non-sacrificial H2O2 production. Under photoexcitation, carbonyl groups on the photocatalyst surface are reduced, forming an anion radical intermediate. The produced intermediate is oxidized by O2 to produce H2O2 and subsequently restores the carbonyl group. The high catalytic efficiency is ascribed to a photocatalytic redox cycle mediated by the radical anion, which not only promotes oxygen adsorption but also lowers the energy barrier of O2 reduction reaction for H2O2 generation. An apparent quantum yield of 14.28% at 420 ± 10 nm with a solar-to-chemical conversion efficiency of 0.92% is achieved. Moreover, we demonstrate that a mere 0.5 m2 self-supported polyimide aerogel exposed to natural sunlight for 6 h yields significant H2O2 production of 34.3 mmol m-2.

Activating Inert Perovskite Oxides for CO2 Electroreduction via Slight Cu2+ Doping in B-Sites.

Perovskite oxides are proven as a striking platform for developing high-performance electrocatalysts. Nonetheless, a significant portion of them show CO2 electroreduction (CO2RR) inertness. Here a simple but effective strategy is reported to activate inert perovskite oxides (e.g., SrTiO3) for CO2RR through slight Cu2+ doping in B-sites. For the proof-of-concept catalysts of SrTi1-xCuxO3 (x = 0.025, 0.05, and 0.1), Cu2+ doping (even in trace amount, e.g., x = 0.025) can not only create active, stable CuO6 octahedra, increase electrochemical active surface area, and accelerate charge transfer, but also significantly regulate the electronic structure (e.g., up-shifted band center) to promote activation/adsorption of reaction intermediates. Benefiting from these merits, the stable SrTi1-xCuxO3 catalysts feature great improvements (at least an order of magnitude) in CO2RR activity and selectivity for high-order products (i.e., CH4 and C2+), compared to the SrTiO3 parent. This work provides a new avenue for the conversion of inert perovskite oxides into high-performance electrocatalysts toward CO2RR.

High Tartronic Acid Content Germplasms Screening of Cucumber and Its Response to Exogenous Agents.

Tartronic acid is known for its potential to inhibit sugar-to-lipid conversion in the human body, leading to weight loss and fat reduction. This compound is predominantly found in cucumbers and other cucurbit crops. Therefore, cultivating cucumbers with high tartronic acid content holds significant health implications. In this study, we assessed the tartronic acid content in 52 cucumber germplasms with favorable overall traits and identified 8 cucumber germplasms with elevated tartronic acid levels. Our investigation into factors influencing cucumber tartronic acid revealed a decrease in content with fruit development from the day of flowering. Furthermore, tartronic acid content was higher in early-harvested fruits compared to late-harvested ones, with the rear part of the fruit exhibiting significantly higher content than other parts. Foliar spraying of microbial agents increased tartronic acid content by 84.4%. This study provides valuable resources for breeding high tartronic acid cucumbers and offers practical insights for optimizing cucumber production practices.

The CsTM alters multicellular trichome morphology and enhances resistance against aphid by interacting with CsTIP1;1 in cucumber.

The multicellular trichomes of cucumber (Cucumis sativus L.) serve as the primary defense barrier against external factors, whose impact extends beyond plant growth and development to include commercial characteristics of fruits. The aphid (Aphis gossypii Glover) is one of prominent pests in cucumber cultivation. However, the relationship between physical properties of trichomes and the aphid resistance at molecular level remains largely unexplored. Here, a spontaneous mutant trichome morphology (tm) was characterized by increased susceptibility towards aphid. Further observations showed the tm exhibited a higher and narrower trichome base, which was significantly distinguishable from that in wild-type (WT). We conducted map-based cloning and identified the candidate, CsTM, encoding a C-lectin receptor-like kinase. The knockout mutant demonstrated the role of CsTM in trichome morphogenesis. The presence of SNP does not regulate the relative expression of CsTM, but diminishes the CsTM abundance of membrane proteins in tm. Interestingly, CsTM was found to interact with CsTIP1;1, which encodes an aquaporin with extensive reports in plant resistance and growth development. The subsequent aphid resistance experiments revealed that both CsTM and CsTIP1;1 regulated the development of trichomes and conferred resistance against aphid by affecting cytoplasmic H2O2 contents. Transcriptome analysis revealed a significant enrichment of genes associated with pathogenesis, calcium binding and cellulose synthase. Overall, our study elucidates an unidentified mechanism that CsTM-CsTIP1;1 alters multicellular trichome morphology and enhances resistance against aphid, thus providing a wholly new perspective for trichome morphogenesis in cucumber.

Zinc Finger-Homeodomain Transcriptional Factors (ZHDs) in Cucumber (Cucumis sativus L.): Identification, Evolution, Expression Profiles, and Function under Abiotic Stresses.

Cucumber (Cucumis sativus L.) is a globally prevalent and extensively cultivated vegetable whose yield is significantly influenced by various abiotic stresses, including drought, heat, and salinity. Transcription factors, such as zinc finger-homeodomain proteins (ZHDs), a plant-specific subgroup of Homeobox, play a crucial regulatory role in stress resistance. In this study, we identified 13 CsZHDs distributed across all six cucumber chromosomes except chromosome 7. Phylogenetic analysis classified these genes into five clades (ZHDI-IV and MIF) with different gene structures but similar conserved motifs. Collinearity analysis revealed that members of clades ZHD III, IV, and MIF experienced amplification through segmental duplication events. Additionally, a closer evolutionary relationship was observed between the ZHDs in Cucumis sativus (C. sativus) and Arabidopsis thaliana (A. thaliana) compared to Oryza sativa (O. sativa). Quantitative real-time PCR (qRT-PCR) analysis demonstrated the general expression of CsZHD genes across all tissues, with notable expression in leaf and flower buds. Moreover, most of the CsZHDs, particularly CsZHD9-11, exhibited varying responses to drought, heat, and salt stresses. Virus-induced gene silencing (VIGS) experiments highlighted the potential functions of CsZHD9 and CsZHD10, suggesting their positive regulation of stomatal movement and responsiveness to drought stress. In summary, these findings provide a valuable resource for future analysis of potential mechanisms underlying CsZHD genes in response to stresses.

Target mediated bioreaction to engineer surface vacancy effect on Bi2O2S nanosheets for photoelectrochemical detection of FEN1.

Photoelectrochemistry represents a promising technique for bioanalysis, though its application for the detection of Flap endonuclease 1 (FEN1) has not been tapped. Herein, this work reports the exploration of creating oxygen vacancies (Ov) in situ onto the surface of Bi2O2S nanosheets via the attachment of dopamine (DA), which underlies a new anodic PEC sensing strategy for FEN1 detection in label-free, immobilization-free and high-throughput modes. In connection to the target-mediated rolling circle amplification (RCA) reaction for modulating the release of the DA aptamer to capture DA, the detection system showed good performance toward FEN1 analysis with a linear detection range of 0.001-10 U/mL and a detection limit of 1.4 × 10-4 U/mL (S/N = 3). This work features the bioreaction engineered surface vacancy effect of Bi2O2S nanosheets as a PEC sensing strategy, which allows a simple, easy to perform, sensitive and selective method for the detection of FEN1. This sensing strategy might have wide applications in versatile bioasssays, considering the diversity of a variety of biological reactions may produce the DA aptamer.

Thioredoxin Reductase-Mediated Reaction Evokes In Situ Surface Polarization Effect on BiOIO3: Toward a New Sensing Strategy for Cathodic Photoelectrochemistry.

We have witnessed the fast progress of cathodic photoelectrochemistry over the past decades, though its signal transduction tactic still lacks diversity. Exploring new sensing strategies for cathodic photoelectrochemistry is extremely demanding yet hugely challenging. This article puts forward a unique idea to incorporate an enzymatic reaction-invoked surface polarization effect (SPE) on the surface of BiOIO3 to implement an innovative cathodic photoelectrochemical (PEC) bioanalysis. Specifically, the thioredoxin reductase (TrxR)-mediated reaction produced the polar glutathione (GSH), which spontaneously coordinated to the surface of BiOIO3 and induced SPE by forming a polarized electric field, resulting in improved electron (e-) and hole (h+) pair separation efficiency and an enhanced photocurrent output. Correlating this phenomenon with the detection of TrxR exhibited a high performance in terms of sensitivity and selectivity, achieving a linear range of 0.007-0.5 µM and a low detection limit of 2.0 nM (S/N = 3). This study brings refreshing inspiration for the cathodic PEC signal transduction tactic through enzyme-mediated in situ reaction to introduce SPE, which enriches the diversity of available signaling molecules. Moreover, this study unveils the potential of in situ generated SPE for extended and futuristic applications.

Simulation and experimental study on the stability and comfortability of the wheelchair human system under uneven pavement.

With the improvement in the level of science and technology and the improvement of people's living standards, the functions of traditional manual wheelchairs have been unable to meet people's living needs. Therefore, traditional wheelchairs have been gradually replaced by smart wheelchairs. Compared with traditional wheelchairs, smart wheelchairs have the characteristics of light operation and faster speed. However, when driving on some complex road surfaces, the vibration generated by the bumps of the motorcycle will cause damage to the human body, so wheelchairs with good electric power and stability can better meet the needs of people and make up for their travel needs. Based on the traditional vehicle stability analysis method, the mathematical theory of roll stability and pitch stability of the wheelchair-human system was established. We built a multi-body dynamics model with human skeleton and joint stiffness based on the multi-body dynamics method. The functioning of the wheelchair-human system was simulated and analyzed on the ditch, step, and combined road. The acceleration and Euler angle changes of the human head, chest, and wheelchair truss position were obtained, and the data results were analyzed to evaluate the stability and comfort of the system. Finally, a wheelchair test platform was built, and the road driving test was carried out according to the simulation conditions to obtain the system acceleration and angle data during the driving process. The simulation analysis was compared to verify the accuracy of the multi-body dynamics method, and the stability and comfort of the system were evaluated.

Advances in understanding epigenetic regulation of plant trichome development: a comprehensive review.

Plant growth and development are controlled by a complex gene regulatory network, which is currently a focal point of research. It has been established that epigenetic factors play a crucial role in plant growth. Trichomes, specialized appendages that arise from epidermal cells, are of great significance in plant growth and development. As a model system for studying plant development, trichomes possess both commercial and research value. Epigenetic regulation has only recently been implicated in the development of trichomes in a limited number of studies, and microRNA-mediated post-transcriptional regulation appears to dominate in this context. In light of this, we have conducted a review that explores the interplay between epigenetic regulations and the formation of plant trichomes, building upon existing knowledge of hormones and transcription factors in trichome development. Through this review, we aim to deepen our understanding of the regulatory mechanisms underlying trichome formation and shed light on future avenues of research in the field of epigenetics as it pertains to epidermal hair growth.

Cation-Radius-Controlled Sn-O Bond Length Boosting CO2 Electroreduction over Sn-Based Perovskite Oxides.

Despite the intriguing potential shown by Sn-based perovskite oxides in CO2 electroreduction (CO2 RR), the rational optimization of their CO2 RR properties is still lacking. Here we report an effective strategy to promote CO2 -to-HCOOH conversion of Sn-based perovskite oxides by A-site-radius-controlled Sn-O bond lengths. For the proof-of-concept examples of Ba1-x Srx SnO3 , as the A-site cation average radii decrease from 1.61 to 1.44 Å, their Sn-O bonds are precisely shortened from 2.06 to 2.02 Å. Our CO2 RR measurements show that the activity and selectivity of these samples for HCOOH production exhibit volcano-type trends with the Sn-O bond lengths. Among these samples, the Ba0.5 Sr0.5 SnO3 features the optimal activity (753.6 mA ⋅ cm-2 ) and selectivity (90.9 %) for HCOOH, better than those of the reported Sn-based oxides. Such optimized CO2 RR properties could be attributed to favorable merits conferred by the precisely controlled Sn-O bond lengths, e.g., the regulated band center, modulated adsorption/activation of intermediates, and reduced energy barrier for *OCHO formation. This work brings a new avenue for rational design of advanced Sn-based perovskite oxides toward CO2 RR.

Z-scheme cobalt-iron oxide/perylene diimide supermolecule heterojunction for high-efficiency ciprofloxacin removal in a photocatalysis-self-Fenton system.

Fenton technology has been famous on antibiotics removal, but seriously restricted by the extra addition of H2O2 and low mineralization efficiency. Herein, we develop a novel cobalt-iron oxide/perylene diimide organic supermolecule (CoFeO/PDIsm) Z-scheme heterojunction under photocatalysis-self-Fenton system, in which the holes (h+) of photocatalyst can mineralize organic pollutants and the photo-generated electrons (e-) are used to in-situ H2O2 production with high efficiency. The CoFeO/PDIsm exhibits superior in-situ H2O2 production at a rate of 281.7 µmol g-1 h-1 in contaminating solution, correspondingly of total organic carbon (TOC) removal rate of ciprofloxacin (CIP) is 63.7 %, far exceeding current photocatalysts. The high H2O2 production rate and remarkable mineralization ability are ascribed to great charge separation in Z-scheme heterojunction. This work provides a novel Z-scheme heterojunction with photocatalysis-self-Fenton system for environmental-friendly removing the organic containment.

Catechol exerts an in situ surface oxygen vacancy effect on BiOI: an innovative signal transduction mode for cathodic photoelectrochemistry.

Cathodic photoelectrochemistry, a research hotspot in state-of-art bioassays, is generally circumscribed by its monotonous signal transduction tactic of photoinduced electron transfer (PET) mechanism, which significantly narrows the scope of its applications. In this study, we reveal the surface oxygen vacancy (VO) formation elicited by the spontaneous coordination of catechol (CA) onto the surface of BiOI nanoplates for the innovative operation of the cathodic PEC signal transduction tactic. The in situ-generated VO functions as a carrier separation center to efficiently promote photocurrent generation. Taking tyrosinase (TYR) and Escherichia coli O157:H7 (E. coli O157:H7) as model targets, the established signal transduction tactic was validated as efficient and sensitive for the detection of the two targets with linear ranges from 1.0 × 10-4 to 1.0 U mL-1 and 5.0 to 1.0 × 106 CFU mL-1, respectively. Low-detection limits of 1.0 × 10-4 U mL-1 and 3.0 CFU mL-1 were achieved for TYR and E. coli O157:H7, respectively. This study opens up a new perspective of in situ generated surface VO on semiconductors, which underlies an innovative PEC signal transduction mechanism with convincing analytical performance. Hopefully, it might encourage more explorations of new methodologies for introducing surface vacancies with exquisite applications.

Flexible Ultrasound Transducer With Embedded Optical Shape Sensing Fiber for Biomedical Imaging Applications.

Flexible ultrasound transducers (FUTs), capable of conforming to irregular surfaces, have become a research hotspot in the field of medical imaging. With these transducers, high-quality ultrasound images can be obtained only if strict design criteria are fulfilled. Moreover, the relative positions of array elements must be determined, which are important for ultrasound beamforming and image reconstruction. These two major characteristics present great challenges to the design and fabrication of FUTs compared to that for traditional rigid probes. In this study, an optical shape-sensing fiber was embedded into a 128-element flexible linear array transducer to acquire the real-time relative positions of array elements to produce high-quality ultrasound images. Minimum concave and convex bend diameters of approximately 20 and 25 mm, respectively, were achieved. The transducer was flexed 2000 times, and yet no obvious damage was observed. Stable electrical and acoustic responses confirmed its mechanical integrity. The developed FUT exhibited an average center frequency of 6.35 MHz, and average -6-dB bandwidth of 69.2%. The array profile and element positions measured by the optic shape-sensing system were instantly transferred to the imaging system. Phantom experiments for both spatial resolution and contrast-to-noise ratio proved that FUTs can maintain satisfactory imaging capability despite bending to sophisticated geometries. Finally, color Doppler images and Doppler spectra of the peripheral arteries of healthy volunteers were obtained in real time.

2,3-Dihydroxynaphthalene invoked surface oxygen vacancy effect on Fe2O3 nanorods for photoanodic signal transduction tactic.

The state-of-art signal transduction mechanism of anodic photoelectrochemistry is constrained to the hole oxidation reaction, which greatly hinders its application for prospective biosensing applications. Herein, we present an innovative strategy for signal transduction by exploiting the in situ formation of surface oxygen vacancies (VOs) on Fe2O3 nanorods (NRs) through the self-coordination of 2,3-dihydroxynaphthalene (2,3-DHN) on their surfaces. The 2,3-DHN was connected with Fe(Ⅲ) on the surface of Fe2O3 NRs vis the formation of the five-membered ring structures accompanied by the generation of VOs. And the generated VOs introduced a new defect energy level for trapping the photogenerated holes, which enhanced the charge separation and realized the enhancement of photocurrent signal. The developed signal transduction strategy was validated by the first photoelectrochemical (PEC) sensing platform for ß-glucoside (ß-Glu) and lipase (LPS), which can catalyze the hydrolysis of 3-hydroxy-2-naphthalenyl-ß-D-glucoside and naphthalene-2,3-diol diacetate, respectively, to produce 2,3-DHN for signal stimuli. The ß-Glu and LPS were detected with linear ranges of 0.01-10.0 U/mL and 0.001-5.0 mg/mL, respectively. Detection limits of 3.3 × 10-3 U/mL and 0.32 µg/mL (S/N = 3) were achieved, for ß-Glu and LPS, respectively. The present study not only provides a new strategy for spontaneous induction of VOs in situ for n-type semiconductors, but also innovates the anodic PEC signal transduction strategy with broadened biosensing applications.

A Novel Catheter Distal Contact Force Sensing for Cardiac Ablation Based on Fiber Bragg Grating with Temperature Compensation.

OBJECTIVE: To accurately achieve distal contact force, a novel temperature-compensated sensor is developed and integrated into an atrial fibrillation (AF) ablation catheter. METHODS: A dual elastomer-based dual FBGs structure is used to differentiate the strain on the two FBGs to achieve temperature compensation, and the design is optimized and validated by finite element simulation. RESULTS: The designed sensor has a sensitivity of 90.5 pm/N, resolution of 0.01 N, and root-mean-square error (RMSE) of 0.02 N and 0.04 N for dynamic force loading and temperature compensation, respectively, and can stably measure distal contact forces with temperature disturbances. CONCLUSION: Due to the advantages, i.e., simple structure, easy assembly, low cost, and good robustness, the proposed sensor is suitable for industrial mass production.

Trap remediation of CuBi2O4 nanopolyhedra via surface self-coordination by H2O2: an innovative signaling mode for cathodic photoelectrochemical bioassay.

This work conveys a new philosophy of surface self-coordination mediated trap remediation for innovative cathodic photoelectrochemical (PEC) signal transduction. Initially, the surface trap states of CuBi2O4 nanopolyhedra resulting from dangling bonds can function as charge carrier recombination centers, which suppress the carrier separation efficiency and result in a low photocurrent output. Particularly, hydrogen peroxide (H2O2) spontaneously interacts with the uncoordinated Cu(II) on the surface of CuBi2O4, enabling efficient elimination of dangling bonds and remedy of trap states, thereby outputting intensified photocurrent readout. Exemplified by Flap endonuclease 1 (FEN1) as a model target, a tetrahedron DNA (THD)-based strand displacement amplification (SDA) was introduced to manipulate the formation of hemin impregnated G-quadruplex (G-quadruplex/hemin) DNAzyme and the resultant catalytic reduction for H2O2. In addition, a highly efficient and ultra-sensitive PEC sensing platform was achieved for FEN1 detection with a wide linear range from 1.0 fM to 100.0 pM and a detection limit of 0.3 fM (S/N = 3). This work not only establishes a new idea of cathodic PEC signal transduction, but also offers an efficient biosensing platform for FEN1.

Coral-like B-doped g-C3N4 with enhanced molecular dipole to boost photocatalysis-self-Fenton removal of persistent organic pollutants.

Fenton process is a popular advanced oxidation process for water purification. However, it requires an external addition of H2O2, thus raising safety threats and economic costs and encountering the problems of slow cycling of Fe2+/Fe3+ and low mineralization efficiency. Herein, we developed a novel photocatalysis-self-Fenton system based on coral-like B-doped g-C3N4 (Coral-B-CN) photocatalyst for 4-chlorophenol (4-CP) removal where H2O2 can be in situ generated by photocatalysis over Coral-B-CN, the cycling of Fe2+/Fe3+ was accelerated by photoelectrons, and the photoholes promoted 4-CP mineralization. Coral-B-CN was innovatively synthesized by hydrogen bond self-assembly followed by calcination. B heteroatom doping produced enhanced molecular dipole, while the morphological engineering exposed more active sites and optimized band structure. The effective combination of the two enhances charge separation and mass transfer between the phases, resulting in efficient in-situ H2O2 production, faster Fe2+/Fe3+ valence cycling and enhanced hole oxidation. Accordingly, nearly all 4-CP can be degraded during 50 min under the combined action of more ·OH and holes with stronger oxidation capacity. The mineralization rate of this system reached 70.3%, which is 2.6 and 4.9 times higher than that of Fenton process and photocatalysis, respectively. Besides, this system maintained excellent stability and can be applied in a broad range of pHs. The study would provide important insights into developing improved Fenton process with high performance for the removal of persistent organic pollutants.

Selectively Permeable FeOOH Amorphous Layer Coating CdS for Enhancing Photocatalytic Conversion of Benzyl Alcohol and Selectivity to Benzaldehyde.

The development of new strategies to improve reaction efficiency and light utilization is one of the biggest challenges in photosynthetic chemistry. Dynamics control, particularly tuning the adsorption/desorption of reactants and products, is an ideal way to improve the conversion and selectivity in catalytic reactions, but it is rarely studied for photocatalytic organic synthesis. This study concerns the design of an amorphous FeOOH coating to decorate CdS photocatalyst to control the adsorption and desorption of reactants and products to improve reaction efficiency for the photocatalytic conversion of benzyl alcohol (BA) into benzaldehyde (BAD). The best conversion of the core-shell photocatalyst is 74.1 % in 2 h, together with >99.9 % selectivity to BAD, and the photocatalyst exhibits response above 600 nm, which is the longest active wavelength reported for the reaction. Further data illustrate that the amorphous FeOOH coating enables selective sorption of BA/BAD molecules by H-bonding interactions, which may result in the excellent performance. Construction of amorphous coating layers and understanding the selective permeability may provide a new strategy for the design of more efficient photocatalytic systems for organic synthesis.