PPi Complex Research Articles

A Structure-based Data Set of Protein-peptide Affinities and its Nonredundant Benchmark: Potential Applications in Computational Peptidology.

Peptides play crucial roles in diverse cellular functions and participate in many biological processes by interacting with a variety of proteins, which have also been exploited as a promising class of therapeutic agents to target druggable proteins over the past decades. Understanding the intrinsic association between the structure and affinity of protein-peptide interactions (PpIs) should be considerably valuable for the computational peptidology area, such as guiding protein-peptide docking calculations, developing protein-peptide affinity scoring functions, and designing peptide ligands for specific protein receptors. We attempted to create a data source for relating PpI structure to affinity. By exhaustively surveying the whole protein data bank (PDB) database as well as the ontologically enriched literature information, we manually curated a structure- based data set of protein-peptide affinities, PpI[S/A]DS, which assembled over 350 PpI complex samples with both the experimentally measured structure and affinity data. The data set was further reduced to a nonredundant benchmark consisting of 102 culled samples, PpI[S/A]BM, which only selected those of structurally reliable, functionally diverse and evolutionarily nonhomologous. The collected structures were resolved at a high-resolution level with either Xray crystallography or solution NMR, while the deposited affinities were characterized by dissociation constant, i.e. Kd value, which is a direct biophysical measure of the intermolecular interaction strength between protein and peptide, ranging from subnanomolar to millimolar levels. The PpI samples in the set/benchmark were arbitrarily classified into α-helix, partial α-helix, β-sheet formed through binding, β-strand formed through selffolding, mixed, and other irregular ones, totally resulting in six classes according to the secondary structure of their peptide ligands. In addition, we also categorized these PpIs in terms of their biological function and binding behavior. The PpI[S/A]DS set and PpI[S/A]BM benchmark can be considered a valuable data source in the computational peptidology community, aiming to relate the affinity to structure for PpIs.

Al3+ induced hydrolysis of anthraquinone-based Schiff base fluorescent probe for determination PPi ions and bioimaging

Herein, a novel anthraquinone derivative appended fluorescent probe (HAD), has been developed as a reversible probe for detection of aluminium ions (Al3+) and pyrophosphate (PPi) in buffer medium. The probe displays fluorescence “turn-on” response towards Al3+ with high specificity. The activation of the chelation-enhanced fluorescence (CHEF) effect occurs as a consequence of the formation of a complex via coordination with Al3+, which was due to the imine bond is dynamic and reversible and further causing subsequent cleavage of the -C=N bond in the presence of Al3+. This complex significantly amplifies the fluorescence response. Additionally, according to the Job's plot and high-resolution mass spectrometry (HRMS) measurements, the binding stoichiometry of HAD and Al3+ ions is 1:2. Moreover, Density functional theory (DFT) investigations further elucidate the binding mechanism. The original spectrum is restored after the addition of PPi, which is due to the formation of more stable Al3+-PPi complex. The specific determination of Al3+ and PPi with the detecting limit of 47.6 nM and 73.7 nM (S/N = 3), respectively. Due to its exceptional biocompatibility, the probe has facilitated the development of Al3+ and PPi imaging in vitro and in vivo, thereby showcasing its remarkable capabilities in the realm of environmental and biological analysis.

Integrated unsupervised-supervised modeling and prediction of protein-peptide affinities at structural level.

Cell signal networks are orchestrated directly or indirectly by various peptide-mediated protein-protein interactions, which are normally weak and transient and thus ideal for biological regulation and medicinal intervention. Here, we develop a general-purpose method for modeling and predicting the binding affinities of protein-peptide interactions (PpIs) at the structural level. The method is a hybrid strategy that employs an unsupervised approach to derive a layered PpI atom-residue interaction (ulPpI[a-r]) potential between different protein atom types and peptide residue types from thousands of solved PpI complex structures and then statistically correlates the potential descriptors with experimental affinities (KD values) over hundreds of known PpI samples in a supervised manner to create an integrated unsupervised-supervised PpI affinity (usPpIA) predictor. Although both the ulPpI[a-r] potential and usPpIA predictor can be used to calculate PpI affinities from their complex structures, the latter seems to perform much better than the former, suggesting that the unsupervised potential can be improved substantially with a further correction by supervised statistical learning. We examine the robustness and fault-tolerance of usPpIA predictor when applied to treat the coarse-grained PpI complex structures modeled computationally by sophisticated peptide docking and dynamics simulation. It is revealed that, despite developed solely based on solved structures, the integrated unsupervised-supervised method is also applicable for locally docked structures to reach a quantitative prediction but can only give a qualitative prediction on globally docked structures. The dynamics refinement seems not to change (or improve) the predictive results essentially, although it is computationally expensive and time-consuming relative to peptide docking. We also perform extrapolation of usPpIA predictor to the indirect affinity quantities of HLA-A*0201 binding epitope peptides and NHERF PDZ binding scaffold peptides, consequently resulting in a good and moderate correlation of the predicted KD with experimental IC50 and BLU on the two peptide sets, with Pearson's correlation coefficients Rp= 0.635 and 0.406, respectively.

DNAzyme-Amplified Label-Free Biosensor for the Simple and Sensitive Detection of Pyrophosphatase.

The level of pyrophosphatase (PPase) expression has been suggested as a potential biomarker of various cancers, and its prognostic value has been evaluated in patients suffering from lung cancer, colorectal cancer, and hyperthyroidism. However, the detection of PPase usually needs specific materials that require complicated, time-consuming reactions with restricted linear range and sensitivity, limiting their application in early clinical diagnosis. Herein, we developed a DNAzyme-based biosensor for the detection of PPase. In the presence of PPase, pyrophosphate (PPi) and Cu2+ ions released from the PPi–Cu2+–PPi complex induce the cleavage of the DNAzyme and the corresponding substrate. An apurinic/apyrimidinic (AP) site was elaborately designed within substrates that could encase the fluorophore 2-amino-5,6,7-trimethyl-1,8-naphthyridine (ATMND). The fluorescence of ATMND was initially quenched but restored when the DNAzyme/substrate complex was hydrolyzed with the release of ATMND. In this way, the PPase activity can be estimated by detecting the increased fluorescence of the released ATMND. Under optimized conditions, the activity of PPase could be analyzed at concentrations from 0.5 to 1000 mU, with the lowest detectable concentration being 0.5 mU. This work lays a foundation for developing a DNAzyme-amplified fluorescent biosensor with a high sensitivity, a wide linear range, and single-step operation for use as an easy diagnostic for PPase analysis.

Portable, quantitative, and sequential monitoring of copper ions and pyrophosphate based on a DNAzyme-Fe3O4 nanosystem and glucometer readout.

In this report, portable, quantitative, and sequential monitoring of copper ions and pyrophosphate (PPi) with a single sensor based on a DNAzyme-Fe3O4 system and glucometer readout was performed. Initially, streptavidin was functionalized on the surface of magnetic Fe3O4 spheres through glutaraldehyde. Then, an invertase-modified DNA Cu substrate was connected to the magnetic Fe3O4 spheres by a specific reaction between streptavidin and biotin. The sensing system was formed by a hybridization reaction between the Cu substrate and Cu enzyme. In the presence of Cu2+, Cu2+ will recognize the Cu DNA substrate and form an “off-on” signal switch, thereby resulting in the separation of invertase from the Fe3O4 nanospheres. PPi recognizes Cu2+ to form a Cu2+-PPi complex, resulting in an “on-off” signal switch. Under optimized conditions, linear detection ranges for Cu2+ and PPi of 0.01–5 and 0.5–10 μM, and detection limits for Cu2+ and PPi of 10 nM and 500 nM, respectively, were obtained. Good selectivity was achieved for the analysis of Cu2+ and PPi. Satisfactory results were achieved for this biosensor during the determination of Cu2+ in real tap samples and PPi in human urine samples. This verified that the sensor is portable and low cost, and can be applied to the sequential monitoring of multiple analytes with a single point-of-care biosensor.Graphical abstract

Highly Sensitive Homogeneous Electrochemiluminescence Biosensor for Alkaline Phosphatase Detection Based on Click Chemistry-Triggered Branched Hybridization Chain Reaction.

Alkaline phosphatase (ALP) has been used as a diagnostic index of clinical diseases since its expression level is closely related to many pathological processes. In this work, a highly sensitive electrochemiluminescence (ECL) method for the determination of ALP based on a click chemistry-induced branched hybridization chain reaction (BHCR) for signal amplification and ultrafiltration technology for the separation of homogeneous amplification products is introduced. ALP can release copper ions from a Cu2+/PPi complex by hydrolyzing pyrophosphoric acid, which initiates click chemistry in the system. A BHCR amplification is triggered afterward by the long single-stranded DNA (ssDNA) generated by click chemistry, resulting in a three-dimensional double-stranded DNA (dsDNA) with a large molecular weight. Based on the characteristic that Ru(phen)32+ can stably insert into the groove of dsDNA, a large amount of Ru(phen)32+ is retained together with the amplified product after ultrafiltration, and therefore a significantly enhanced ECL signal can be obtained. The test results show that this method can be used for the quantitative determination of ALP ranging from 0.002 to 50 U/L, with a detection limit of 0.7 mU/L. This method has also been confirmed to have good selectivity and anti-interference, and the results of the analysis of the ALP content in the diluted serum samples are satisfactory, showing great application potential in clinical diagnosis.

Luminescent Neutral Cyclometalated Iridium(III) Complexes Featuring a Cubic Polyhedral Oligomeric Silsesquioxane for Lipid Droplet Imaging and Photocytotoxic Applications

New neutral iridium(III) complexes featuring a cubic polyhedral oligomeric silsesquioxane (POSS) unit, [Ir(N∧C)2(L1-POSS)] [HN∧C = 2-phenylpyridine (Hppy; 1), 2-phenylbenzothioazole (Hbt; 2), and 2-(1-naphthyl)benzothiazole (Hbsn; 3); L1-POSS = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-heptakis(isobutyl)POSS-propyl carbamate], were designed and synthesized. Their POSS-free counterparts, [Ir(N∧C)2(L1)] [L1 = (E)-N-(4-hydroxymethylphenyl)-1-(2-hydroxyphenyl)methanimine; HN∧C = Hppy (1a), Hbt (2a), and Hbsn (3a)], and the poly(ethylene glycol) (PEG) derivatives [Ir(N∧C)2(L1-PEG)] [L1-PEG = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-[2-[ω-methoxypoly(1-oxapropyl)]ethyl]carbamate; HN∧C = Hppy (1b), Hbt (2b), and Hbsn (3b)] were also prepared. The photophysical, photochemical, and biological properties of the POSS complexes were compared with those of their POSS-free and PEG-modified counterparts. Upon irradiation, all of these complexes displayed orange-to-red emission and long emission lifetimes under ambient conditions. The bsn complexes 3, 3a, and 3b exhibited the highest singlet oxygen (1O2) generation quantum yields (ΦΔ = 0.85-0.86) in aerated CH3CN. Laser-scanning confocal microscopy images revealed that complexes 1-3 and 1a-3a showed exclusive lipid-droplet staining upon cellular uptake, while the PEG derivatives 1b-3b displayed lysosomal localization. Complex 3 was utilized to study various lipid-droplet-related biological events including lipid-droplet accumulation under oleic acid stimulation, the movement of lipid droplets, and preadipocyte differentiation. Notably, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays indicated that the ppy complexes 1 and 1b and the bt complexes 2 and 2b were noncytotoxic both in the dark and upon irradiation at 450 nm for 5 min (IC50 > 200 μM), while the bsn complexes 3, 3a, and 3b showed low dark cytotoxicity (IC50 = 52.9 to >200 μM) and high photocytotoxicity (IC50 = 1.1-5.3 μM). The cellular uptake, internalization mechanisms, and cell death pathways of these complexes were also investigated. This work not only offers promising luminescent probes for lipid droplets through the structural modification of iridium(III) complexes but also paves the way to the construction of new reagents for theranostics.

Spectroelectrochemical investigation of electrodeposited polypyrrole complexes with sulfonated polyelectrolytes

Films of polypyrrole(PPy)-polyelectrolyte(PE) complexes have high potential as active material in bio-sensing and biomedical applications due to high hydrophilicity and biocompatibility, as well as, as an electroactive polymer matrix in electrode materials for rechargeable lithium batteries and supercapacitors. Comparative study is presented on the influence of PE structure on the properties of PPy-PE complexes. PPy films were prepared by pyrrole electrochemical polymerization in cyclic voltammetry, potentio- and galvanostatic regimes in aqueous solutions of acid or salt forms of sulfonated polyelectrolytes (PEs) with different flexibility of the polymer backbone. Electrochemical polymerization of pyrrole in these conditions results in obtaining high-quality polymer films efficiently rechargeable in aqueous and nonaqueous media. The films obtained were characterized by cyclic voltammetry, UV-Vis-NIR spectroelectrochemistry in aqueous and nonaqueous media, as well as, by Raman spectroelectrochemistry in aqueous medium. Gaussian deconvolution of the Raman spectra in the range of 850–1150 cm−1 allowed one to trace peculiarities of polaron-bipolaron transitions depending on the PE structure. It was shown that the films of PPy complexes with the salt forms of rigid-chain PEs had redox numbers comparable to those of the complexes with the flexible-chain PEs, but they were more stable during CV cycling in nonaqueous medium.

Electrodeposition of thin films of polypyrrole-polyelectrolyte complexes and their ammonia-sensing properties

Polypyrrole (PPy) films were electrodeposited in a galvanostatic mode in the presence of salt and acid forms of sulfonated polyelectrolytes distinguished by different rigidity of the polymer backbone. The process of PPy electrosynthesis was studied by in situ spectroscopy in the UV-visible-NIR spectral regions with simultaneous monitoring of the electrochemical parameters of the synthesis. The PPy films were characterized by Raman spectroscopy. The morphology of the PPy-polyelectrolyte films was investigated by atomic force microscopy. PPy-polyelectrolyte films were successfully tested as sensitive layers in optical ammonia sensors. The behavior of the polymer-based gas sensors, including response time and response amplitude, to various ammonia gas concentrations ranging from 25 to 529 ppm was investigated. The experimental results showed that these parameters are dependent on the polyelectrolytes structure and form (acid or salt). The optical sensors based on the films of PPy complexes with the salt forms of all polyelectrolytes and the acid form of the rigid-chain polyelectrolytes can reliably detect 25-ppm ammonia, the complexes of the rigid-chain polyacids responding significantly faster.

Gold nanozyme as an excellent co-catalyst for enhancing the performance of a colorimetric and photothermal bioassay

Advanced oxidation processes (AOPs) have recently proposed for advancing colorimetric sensing applications, owing to their excellent performance of sensitive color readout that generated from the oxidation of chromogenic substrates like 3,3′,5,5′-tetramethylbenzidine (TMB) by reactive oxygen species (ROS) of AOPs such as ·OH and ·O2− radicals. However, the efficiency of ROS generation and the related H2O2 decomposition in most AOPs is quite low especially at neutral pH, which greatly hampered the practical sensing applications of the AOPs. We herein communicated that β-cyclodextrin (β-CD)-capped gold nanoparticles (β-CD@AuNPs) can promote catalysis at neutral pH for AOP as an excellent co-catalyst. In this strategy, inorganic pyrophosphate (PPi) ions was first used to coordinate with Cu2+ and form Cu2+-PPi complex. In the presence of hydrogen peroxide, target inorganic pyrophosphatase (PPase) can hydrolyze PPi into inorganic phosphate (Pi) and release free Cu2+ simultaneously, resulting in a Cu2+-triggered Fenton-like AOP reaction. The introduced β-CD@AuNPs acts as a co-catalyst, analogous to mediators in the most co-catalyzed system, to enhance the rate-limiting step of Cu2+/Cu+ conversion in Cu2+/H2O2 Fenton-like AOP and resulting in an efficient generation of ·OH and ·O2− radicals, which further producing an intense blue color by oxidizing TMB into its oxidation product (TMBox) within a short time. Finally, this reaction system was used to simply detecting target PPase with the colorimetric and photothermal readout based on the in-situ generated TMBox indicator. More significantly, we successfully demonstrated nanozyme can serve as a co-catalyst to promote the AOP catalysis at neutral pH, and inspire other strategies to overcome the pH limitation in the AOP catalysis and expand its colorimetric and photothermometric application.

Quantum Dot Doping-Induced Photoluminescence for Facile, Label-Free, and Sensitive Pyrophosphatase Activity Assay and Inhibitor Screening.

Development of simple, convenient, and sensitive assay methods for pyrophosphatase (PPase) activity is of importance, for disease diagnosis and drug discovery. Herein, a simple, rapid, label-free, and sensitive fluorescence sensor for PPase activity assay is developed, using Cu2+ doping-induced quantum dot (QD) photoluminescence as a signal reporter. The Cu2+ doping of ZnSe QD can induce a dopant-dependent emission response, which will be inhibited after the premixing of Cu2+ with pyrophosphate (PPi), to form a Cu2+-PPi complex. Then, the hydrolysis of PPi into phosphate (Pi), specifically catalyzed by PPase, liberates the free Cu2+ to regain the QD doping for the fluorescence response, which is highly dependent on the PPase activity. The PPase can be sensitively and selectively assayed, with a detection limit of 0.1 mU/mL. The developed sensing strategy can be also employed for the PPase inhibitor screening. Thus, the current QD doping-based sensing strategy offers an efficient and promising avenue for Cu2+, PPi, or PPase-related target analysis, and might hold great potential for the further applications in the clinical disease diagnosis.

Mechanism of Sulfate Activation Catalyzed by ATP Sulfurylase - Magnesium Inhibits the Activity

ATPS Sulfurylase (ATPS) is the first of three enzymes in the sulfate reduction pathway - one of the oldest metabolic pathways on Earth, utilized by Sulfate Reducing Bacteria (SRB). Due to the low redox potential of the sulfate ion, its reduction requires activation via formation of adenosine 5′-phosphosulfate (APS), which is catalyzed by ATPS. Dispersion-corrected hybrid density functional theory (DFT/B3LYP-D3) was used to test three reaction mechanisms proposed for conversion of ATP to APS: two-step SN-1 reaction running through AMP anhydride intermediate, two-step reaction involving cyclic AMP intermediate and direct SN-2 conversion of ATP to APS molecule. The study employed five different cluster models of the ATPS active site: one containing magnesium cation and four without it, constructed based on the crystal structure (PDB code: 1G8H) solved for ATPS from Saccharomyces cerevisiae in complex with APS and pyrophosphate (PPi), where Mg2+ was not detected. The model with magnesium ion was constructed based on the representative structure obtained from trajectory analysis of the molecular dynamics simulations (MD) performed for the hexameric ATPS-APS-Mg2+-PPi complex. The results obtained for all considered models suggest that ATPS-AMP anhydride intermediate is a highly energetic and unstable complex, while formation of cyclic AMP molecule requires formation of unfavorable hypervalent geometry at the transition state. Among all tested mechanism, the energetically most feasible mechanism of the ATPS reaction is SN-2 one-step conversion of ATP to APS occurring via a pentavalent transition state. Interestingly, such a reaction is inhibited by the presence of Mg2+ in the ATPS active site. Magnesium cation forces unfavorable geometry of reactants for SN-2 mechanism and formation of pentavalent transition state. Such a reaction requires rearrangement of Mg2+ ligands, which raises the barrier from 11-14 kcal/mol for the models without Mg2+ to 48 kcal/mol for model with magnesium ion included.

A pyrophosphatase biosensor with photocurrent analysis

A light-induced photochemical biosensor was developed for the analysis of inorganic pyrophosphatase (PPase). PPase hydrolyzes pyrophosphate (PPi) into two independent o-phosphate ions. Two PPi units can chelate a copper ion (Cu2+), forming a PPi–Cu2+–PPi complex and preventing the Cu2+ triggers other reactions. A transparent indium tin oxide (ITO) electrode was coated with a layer of CdS quantum dots (QDs), and then 3,4-diaminobenzoic acid (DBA) was deposited as the anchor. A solution of the PPi–Cu2+–PPi complex and o-phenylenediamine (OPD) was mixed with the analytical sample and then a drop of the mixture was placed on the modified ITO electrode. In the absence of PPase, no reaction occurred between OPD and DBA. A photocurrent was obtained upon excitation of the CdS QDs under light. In the presence of PPase, Cu2+ was released from the complex, triggering the reaction of OPD with DBA on the electrode surface, thereby shielding the CdS QDs from excitation by the light. The observed photocurrent decreased. The difference in the two measured photocurrents corresponded to the activity of PPase. This photochemical biosensor had excellent sensitivity for PPase in the range from 0.8 to 5000 mU, with a limit of detection of 0.41 mU.

A novel fluorometric method for inorganic pyrophosphatase detection based on G-quadruplex-thioflavin T

In this paper, we propose a fluorometric approach for the highly sensitive detection of inorganic pyrophosphatase (PPase) based on G-quadruplex-thioflavin T (ThT). In the absence of PPase, Cu2+ can coordinate with pyrophosphate (PPi) to generate a Cu2+/PPi complex. Then the G-rich sequence folds into the G-quadruplex structure, which can combine with ThT to generate a remarkable fluorescent signal. In the presence of PPase, the coordinated compound can be destroyed by the PPase catalyzed hydrolysis of PPi into inorganic phosphate (Pi). The subsequent release of Cu2+ can compete with ThT to induce a tighter G-quadruplex structure, causing the release of ThT and a sharp fluorescence decrease. Based on this mechanism, a facile and quantitative strategy for PPase detection was developed. The fluorescence intensity of the system shows a linear relationship with the PPase activities in the range of 0.5–30 U/L with a detection limit as low as 0.48 U/L. The proposed strategy for fluorescence spectrometric PPase detection is convenient, cost effective, and sensitive. This can be utilized to evaluate the inhibition effect of NaF on PPase as well as diagnose PPase-related diseases.

A new copper mediated on-off assay for alkaline phosphatase detection based on MoOx quantum dots

We present a new on-off assay for alkaline phosphatase (ALP) detection using molybdenum oxide quantum dots (MoOx QDs) based on three favorable properties: the quenching ability of Cu2+ on MoOx QDs, the strong binding affinity between Cu2+ and PPi, and the catalysis ability of ALP to hydrolyze PPi to orthophosphate (Pi). Benefiting from the strong affinity of Cu2+ and PPi, the effective quenching of MoOx QDs by Cu2+ was hampered when PPi was added into the system containing MoOx QDs and Cu2+, leading to a significant photoluminescence enhancement (“ON”). Further addition of ALP, PPi was catalytically hydrolyzed into Pi, which would disable the formation of Cu2+-PPi complex and release free Cu2+, resulting in photoluminescence quenching (“OFF”). The on-off assay allows the analysis of PPi and ALP by measuring the photoluminescence intensity, and quantitative detection of ALP in a linear range from 0.1 to 5.0 U/L with the detection limit of 0.02 U/L (3σ/K) is realized. Moreover, the present assay was successfully applied in ALP inhibitor screening and ALP detection in diluted human serum samples, and satisfactory recoveries between 99.6% and 103.0% were obtained. The on-off photoluminescence assay shows many merits, including convenient operation, cost-saving, high sensitivity and superior selectivity, which allow the assay for ALP detection in real human serum samples.

A luminescent europium-dipicolinic acid nanohybrid for the rapid and selective sensing of pyrophosphate and alkaline phosphatase activity.

As a ubiquitous hydrolysis enzyme in phosphate metabolism, alkaline phosphatase (ALP) is a significant biomarker in laboratory research and clinic diagnosis. Herein, we report a highly water-soluble Eu(DPA)3@Lap nanohybrid material for the rapid and selective assay of PPi and ALP through a luminescence off-on recognition process. Eu(DPA)3@Lap was successfully prepared in an aqueous solution, and it exhibited strong luminescence emission, high photostability, and long lifetime. More interestingly, the strong luminescence of Eu(DPA)3@Lap can be remarkably quenched by Cu2+ due to the high impetus of coordination between the DPA ligand and Cu2+ ion. Using Cu2+ as a signal transducer, the luminescence could be recovered upon the addition of PPi ion owing to the formation of a Cu2+-PPi complex; thus, a luminescence turn-on assay for PPi ions was realized. Utilizing the ability of Cu2+ to differentiate between PPi and Pi, a convenient and straightforward luminescence assay for ALP activity was accomplished based on the specific dephosphorylation of PPi to Pi. To the best of our knowledge, this elaborate luminescence sensing system constitutes the first luminescent nanohybrid material based on a europium organic complex for ALP activity assay. Furthermore, the recognition process of PPi and ALP was completed in a convenient and facile mix-and-readout manner, and it revealed significant potential in point of care testing.

Ir(tpy)(bpy)Cl] as a Photocatalyst for CO2 Reduction under Visible‐Light Irradiation

AbstractMononuclear iridium(III) terpyridine (tpy) 2,2′‐bipyridine (bpy) [Ir(tpy)(bpy)Cl]2+ photocatalysts (denoted [Ir(bpy)]) were developed for selective CO2 reduction to HCOOH under visible light. The CO2 reduction product could be changed dramatically by substituting a 2‐phenylpyridine (ppy) ligand with bpy, with the mononuclear Ir ppy complex ([Ir(tpy)(ppy)Cl]+) acting as a photocatalyst for selective CO2 reduction to CO. A mechanistic study showed a structural change in [Ir(bpy)] during the photocatalytic reaction. The [Ir(bpy)] complex was transformed into an iridium–hydride complex ([Ir(tpy)(bpy)H]2+) during an early stage of the photocatalytic reaction. However, [Ir(tpy)(bpy)H]2+ did not function as a key intermediate in the photochemical CO2 reduction because an additional structural change occurred. The tpy ligand of the Ir complex was reduced to piperidine‐2,6‐di‐2‐pyridine ligand during the photocatalytic reaction, resulting in the production of [Ir(piperidine‐2,6‐di‐2‐pyridine)(bpy)H]2+, which was the actual photocatalyst for HCOOH production.

Label-free upconversion nanoparticles-based fluorescent probes for sequential sensing of Cu2+, pyrophosphate and alkaline phosphatase activity

An efficient near-infrared fluorescence probe has been developed for the sequential detection of Cu2+, pyrophosphate (P2O74−, PPi), and alkaline phosphatase (ALP), which is based on the "off−on−off" fluorescence switch of branched polyethyleneimine (PEI)-capped NaGdF4:Yb/Tm upconversion nanoparticles (UCNPs). The fluorescence is quenched via energy transfer from UCNPs to Cu2+ for the coordination of PEI with Cu2+. The strong affinity between Cu2+ and PPi leads to the formation of Cu2+-PPi complex and results in the detachment of Cu2+ from the surface of UCNPs, thus the fluorescence is triggered on. ALP-directed hydrolysis of PPi causes the disassembly of Cu2+-PPi complex and re-conjugation between Cu2+ with PEI, which leads to the switch-off fluorescence of UCNPs. The system allows sequential analysis of Cu2+, PPi, and ALP by modulating the switch of the fluorescence of UCNPs with detection limits of 57.8nM, 184nM, and 0.019U/mL for Cu2+, PPi, and ALP, respectively. By virtue of the NIR feature and excellent biocompatibility, the UCNPs-based probes are suitable for bioimaging. Taking Cu2+ visualization as a model, the nanoprobes have been successfully applied for intracellular imaging of Cu2+ in living cells.

Colorimetric Logic Gate for Pyrophosphate and Pyrophosphatase via Regulating the Catalytic Capability of Horseradish Peroxidase.

By regulating the catalytic capability of horseradish peroxidase (HRP), an artful colorimetric assay platform for pyrophosphate (PPi) and pyrophosphatase (PPase) was unprecedentedly designed. In this work, Cu(I), generated by reducing Cu(II) in the presence of ascorbate, could inhibit HRP's catalytic capability of transforming colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). The robust coordination between PPi and Cu(II) is able to discourage the reduction of Cu(II) to Cu(I) effectively, thus restoring the original catalytic capability of HRP and regenerating blue-colored oxTMB. Upon PPase introduction, PPi would be hydrolyzed into orthophosphate, which could release Cu(II) free from the Cu(II)-PPi complex, and thus in turn allows the catalytic capability of HRP to be inhibited by Cu(I). HRP was activated or deactivated to different degrees depending on PPi or PPase levels, which could be indicated by using HRP-triggered catalytic system as a signal amplifier, thus paving a way for PPi and PPase sensing. Based on the colorimetric sensor for PPi and PPase, an "INH" logic gate was rationally constructed. With the merits of high sensitivity and selectivity, cost-effectiveness, and simplification, our proposed analytical system has also been verified to have potential to be utilized for enzyme inhibitor screening and diagnosis of PPase-related diseases.

A Ruthenium(II) Polypyridyl Nucleoside as a Potential Photodynamic Therapy Agent

A new ruthenium(II) nucleoside Ru(II)-PPY has been synthesized and characterized by NMR and high resolution mass spectrometry. This complex couples a ruthenium(II) polypyridyl light absorber to a nucleoside through a pyridyl group. The absorption spectroscopy of the complex shows a Ru(II) to bipyridyl metal-to-ligand charge transfer (MLCT) transition in the visible region of the spectrum. Cyclic voltammetry of the nucleoside and the ruthenium(II) nucleoside confirms coordination of the ruthenium moiety to the nucleoside with a quasi-reversible Ru(III/II) redox couple of E1/2 = 0.82 V vs. Ag/AgCl. Irradiation of buffered solutions of plasmid DNA and the Ru(II)-PPY complex using a long band pass filter to block out ultraviolet light indicate that the complex is capable of photo-nicking the dsDNA. Additional irradiation experiments in the presence of singlet oxygen and superoxide inhibitors suggest that the Ru(II)-PPY complex generates both singlet oxygen and oxygen radicals when photo-excited.