Highly Efficient Cyclic Dinucleotide Based Artificial Metalloribozymes for Enantioselective Friedel–Crafts Reactions in Water

The diverse secondary structures of nucleic acids are emerging as attractive chiral scaffolds to construct artificial metalloenzymes (ArMs) for enantioselective catalysis. DNA-based ArMs containing duplex and G-quadruplex scaffolds have been widely investigated, yet RNA-based ArMs are scarce. Here we report that a cyclic dinucleotide of c-di-AMP and Cu²⁺ ions assemble into an artificial metalloribozyme (c-di-AMP⋅Cu²⁺) that enables catalysis of enantioselective Friedel–Crafts reactions in aqueous media with high reactivity and excellent enantioselectivity of up to 97 % ee. The assembly of c-di-AMP⋅Cu²⁺ gives rise to a 20-fold rate acceleration compared to Cu²⁺ ions. Based on various biophysical techniques and density function theory (DFT) calculations, a fine coordination structure of c-di-AMP⋅Cu²⁺ metalloribozyme is suggested in which two c-di-AMP form a dimer scaffold and the Cu²⁺ ion is located in the center of an adenine-adenine plane through binding to two N7 nitrogen atoms and one phosphate oxygen atom.     

Conformationally Constrained Cyclic Peptides: Powerful Scaffolds for Asymmetric Catalysis

Cyclic peptides containing a disulfide bridge were identified as a simple and versatile coordination sphere for asymmetric catalysis. Upon complexation with Cu²⁺ ions they catalyze Diels–Alder and Friedel–Crafts reactions with high enantioselectivities of up to 99 % ee and 86 % ee, respectively. Moreover, the peptides ligands were systematically optimized with the assistance of “Alanine Scanning”. This biomolecular design could greatly expand the choice of peptide scaffolds for artificial metallopeptide catalysts.     

Harnessing the Flexibility of Peptidic Scaffolds to Control their Copper(II)‐Coordination Properties: A Potentiometric and Spectroscopic Study

Designing small peptides that are capable of binding Cu²⁺ ions mainly through the side‐chain functionalities is a hard task because the amide nitrogen atoms strongly compete for Cu²⁺ ion coordination. However, the design of such peptides is important for obtaining biomimetic small systems of metalloenyzmes as well as for the development of artificial systems. With this in mind, a cyclic decapeptide, C‐Asp, which contained three His residues and one Asp residue, and its linear derivative, O‐Asp, were synthesized. The C‐Asp peptide has two Pro? Gly β‐turn‐inducer units and, as a result of cyclization, and as shown by CD spectroscopy, its backbone is constrained into a more defined conformation than O‐Asp, which is linear and contains a single Pro? Gly unit. A detailed potentiometric, mass spectrometric, and spectroscopic study (UV/Vis, CD, and EPR spectroscopy) showed that at a 1:1 Cu²⁺/peptide ratio, both peptides formed a major [CuHL]²⁺ species in the pH range 5.0–7.5 (C‐Asp) and 5.5–7.0 (O‐Asp). The corrected stability constants of the protonated species (log K*_CuH(O?Asp)=9.28 and log K*_CuH(C?Asp)=10.79) indicate that the cyclic peptide binds Cu²⁺ ions with higher affinity. In addition, the calculated value of K_eff shows that this higher affinity for Cu²⁺ ions prevails at all pH values, not only for a 1:1 ratio but even for a 2:1 ratio. The spectroscopic data of both [CuHL]²⁺ species are consistent with the exclusive coordination of Cu²⁺ ions by the side‐chain functionalities of the three His residues and the Asp residue in a square‐planar or square‐pyramidal geometry. Nonetheless, although these data show that, upon metal coordination, both peptides adopt a similar fold, the larger conformational constraints that are present in the cyclic scaffold results in different behaviour for both [CuHL]²⁺ species. CD and NMR analysis revealed the formation of a more rigid structure and a slower Cu²⁺‐exchange rate for [CuH(C‐Asp)]²⁺ compared to [CuH(O‐Asp]²⁺. This detailed comparative study shows that cyclization has a remarkable effect on the Cu²⁺‐coordination properties of the C‐Asp peptide, which binds Cu²⁺ ions with higher affinity at all pH values, stabilizes the [CuHL]²⁺ species in a wider pH range, and has a slower Cu²⁺‐exchange rate compared to O‐Asp.     

A Highly Copper‐Selective Ratiometric Fluorescent Sensor Based on BODIPY

A new ratiometric fluorescent sensor ( 1 ) for Cu²⁺ based on 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) with di(2‐picolyl)amine (DPA) as ion recognition subunit has been synthesized and investigated in this work. The binding abilities of 1 towards different metal ions such as alkali and alkaline earth metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺) and other metal ions ( Ba²⁺, Zn²⁺, Cd²⁺, Fe²⁺, Fe³⁺, Pb²⁺, Ni²⁺, Co²⁺, Hg²⁺, Ag⁺) have been examined by UV‐vis and fluorescence spectroscopies. 1 displays high selectivity for Cu²⁺ among all test metal ions and a ～10‐fold fluorescence enhancement in I₅₈₂/I₅₅₈ upon excitation at visible excitation wavelength. The binding mode of 1 and Cu²⁺ is a 1:1 stoichiometry determined via studies of Job plot, the nonlinear fitting of the fluorometric titration and ESI mass.     

A New Luminescent Ruthenium(II) Polypyridine‐derived Dipicolylamine Complex as a Sensor for Cu2+ Ions

A chemo‐sensor [Ru(bpy)₂(bpy‐DPF)](PF₆)₂ ( 1 ) (bpy=2,2′‐bipyridine, bpy‐DPF=2,2′‐bipyridyl‐4,4′‐bis(N,N‐di(2‐picolyl))formylamide) for Cu²⁺ using di(2‐picolyl)amine (DPA) as the recognition group and a ruthenium(II) complex as the reporting group was synthesized and characterized successfully. It demonstrates a high selectivity and efficient signaling behavior only for Cu²⁺ with obvious red‐shifted MLCT (metal‐to‐ligand charge transfer transitions) absorptions and dramatic fluorescence quenching compared with Zn²⁺ and other metal ions.     

Asymmetric Alkynylation/Lactamization Cascade: An Expeditious Entry to Enantiomerically Enriched Isoindolinones

An unprecedented Cu^I–pybox‐diPh‐catalyzed highly enantioselective (up to >99 % ee) alkynylation/lactamization cascade has been developed as a general catalytic system for the synthesis of diversely substituted isoindolinones of immense biological importance. The cascade effects one C? C and two C? N bond‐forming events in one reaction vessel under operationally simple, additive‐free reaction conditions in good to excellent yields. The methodology was further extended to the synthesis of tetrahydroisoquinoline scaffolds common to several biologically active natural products in a two‐step sequence with remarkable selectivity (up to 94 % ee).     

Highly Selective All‐Plastic,Disposable, Cu2+‐Selective Electrodes

Conducting polymers (CP) remain a promising material to construct stable potential all‐solid‐state ion‐selective potentiometric electrodes. The unique properties of poly(3,4‐ethylenedioxythiophene) doped with poly(4‐styrenesulfonate) ions, PEDOT‐PSS: high CP stability and affinity of doping anions towards Cu²⁺ ions, make it highly attractive for construction of all‐solid‐state copper(II)‐selective electrodes with outstanding selectivity. The additional benefits can arise from solution processability of commercially available PEDOT‐PSS system. This material was highly promising for a new sensor arrangement, i.e. to obtain disposable, planar and flexible all‐plastic Cu²⁺‐selective electrodes. These sensors can be obtained by casting a commercially available dispersion of PEDOT‐PSS (Baytron P) on a plastic, non‐conducting support material. The CP being both electrical lead and ion‐to‐electron transducer, was covered with plastic, solvent polymeric Cu²⁺ selective membrane. This extremely simple arrangement, after conditioning in dilute Cu²⁺ solution, was characterized with linear Nernstian responses within the activities range from: 0.1 to 10^?4 M, followed by super‐Nernstian responses for lower activities. The latter result points to effective elimination of primary ions leakage from the plastic membrane / transducer phase and has resulted in significantly improved selectivities. Obtained log K values were equal to ?7.6 for Co²⁺, ?7.4 for Zn²⁺, ?7.2 for Ca²⁺ and ?6.8 for Na⁺, respectively.     

Structural studies on Helicobacter pylori 3‐deoxy‐D‐manno‐2‐octulosonate‐8‐phosphate synthase using electrospray ionization mass spectrometry: a tetrameric complex composed of dimeric dimers

Helicobacter pylori 3‐deoxy‐D ‐manno‐2‐octulosonate‐8‐phosphate (KDO8P) synthase catalyzes the conversion of D ‐arabinose‐5‐phosphate (A5P) and phosphoenolpyruvate (PEP) to produce KDO8P and inorganic phosphate. Since this protein is absent in mammals, it might therefore be an attractive target for the development of new antibiotics. Unlike E. coli KDO8P synthase (class I), the H. pylori counterpart is a class II enzyme, where it requires a divalent transition metal ion for catalysis. Although the metal ions have been shown to be important for catalysis, their role in the structure is not understood. Using electrospray ionization mass spectrometry (ESI‐MS), the role of the metal ions in H. pylori KDO8P synthase has been investigated. This protein is found to be a tetramer in the gas phase but dissociates into the dimer with increasing declustering potential (DP2) suggesting an existence of a ‘structurally specific’ tetramer. An examination of mass spectra revealed that the tetrameric state of the Cd²⁺‐reconstituted enzyme is less stable than those of the Zn²⁺‐, Co²⁺‐ and Cu²⁺‐enzymes. The stoichiometry of metal binding to the protein depends on the nature of the metal ion. Taken together, our data suggest that divalent metal ions play an important role in the quaternary structure of the protein and the tetrameric state may be primarily responsible for catalysis. This study demonstrates the first structural characterization and stoichiometry of metal binding in class II KDO8P synthase using electrospray ionization quadrupole time‐of‐flight mass spectrometry under nondenaturing conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of Metal Ions (Cu2+, Hg2+) and Cocaine by Using Ligation DNAzyme Machinery

The Cu²⁺‐dependent ligation DNAzyme is implemented as a biocatalyst for the colorimetric or chemiluminescence detection of Cu²⁺ ions, Hg²⁺ ions, or cocaine. These sensing platforms are based on the structural tailoring of the sequence of the Cu²⁺‐dependent ligation DNAzyme for specific analytes. The tethering of a subunit of the hemin/G‐quadruplex DNAzyme to the ligation DNAzyme sequence, and the incorporation of an imidazole‐functionalized nucleic‐acid sequence, which acts as a co‐substrate for the ligation DNAzyme that is tethered to the complementary hemin/G‐quadruplex subunit. In the presence of different analytes, Cu²⁺ ions, Hg²⁺ ions, or cocaine, the pretailored Cu²⁺‐dependent ligation DNAzyme sequence stimulates the respective ligation process by combining the imidazole‐functionalized co‐substrate with the ligation DNAzyme sequence. These reactions lead to the self‐assembly of stable hemin/G‐quadruplex DNAzyme nanostructures that enable the colorimetric analysis of the substrate through the DNAzyme‐catalyzed oxidation of 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid), ABTS^2?, by H₂O₂ into the colored product ABTS^.?, or the chemiluminescence detection of the substrate through the DNAzyme‐catalyzed oxidation of luminol by H₂O₂. The detection limits for the sensing of Cu²⁺ ions, Hg²⁺ ions, and cocaine correspond to 1 nM , 10 nM and 2.5 μM , respectively. These different sensing platforms also reveal impressive selectivities.     

Amphiphilic Schiff Base Organogels: Metal‐Ion‐Mediated Chiral Twists and Chiral Recognition

New amphiphilic gelators that contained both Schiff base and L ‐glutamide moieties, abbreviated as o‐SLG and p‐SLG, were synthesized and their self‐assembly in various organic solvents in the absence and presence of metal ions was investigated. Gelation test revealed that o‐SLG formed a thermotropic gel in many organic solvents, whilst p‐SLG did not. When metal ions, such as Cu²⁺, Zn²⁺, Mg²⁺, Ni²⁺, were added, different behaviors were observed. The addition of Cu²⁺ induced p‐SLG to from an organogel. In the case of o‐SLG, the addition of Cu²⁺ and Mg²⁺ ions maintained the gelating ability of the compound, whilst Zn²⁺ and Ni²⁺ ions destroyed the gel. In addition, the introduction of Cu²⁺ ions caused the nanofiber gel to perform a chiral twist, whilst the Mg²⁺ ions enhanced the fluorescence of the gel. More interestingly, the Mg²⁺‐ion‐mediated organogel showed differences in the fluorescence quenching by D ‐ and L ‐tartaric acid, thus showing a chiral recognition ability.     

Diastereochemical differentiation of bicyclic diols using metal complexation and collision‐induced dissociation mass spectrometry

Metal complex formation was investigated for di‐exo‐, di‐endo‐ and trans‐2,3‐ and 2,5‐disubstituted trinorbornanediols, and di‐exo‐ and di‐endo‐ 2,3‐disubstituted camphanediols using different divalent transition metals (Co²⁺, Ni²⁺, Cu²⁺) and electrospray ionization quadrupole ion trap mass spectrometry. Many metal‐coordinated complex ions were formed for cobalt and nickel: [2M+Met]²⁺, [3M+Met]²⁺, [M–H+Met]⁺, [2M–H+Met]⁺, [M+MetX]⁺, [2M+MetX]⁺ and [3M–H+Co]⁺, where M is the diol, Met is the metal used and X is the counter ion (acetate, chloride, nitrate). Copper showed the weakest formation of metal complexes with di‐exo‐2,3‐disubstituted trinorbornanediol yielding only the minor singly charged ions [M–H+Cu]⁺, [2M–H+Cu]⁺ and [2M+CuX]⁺. No clear differences were noted for cobalt complex formation, especially for cis‐2,3‐disubstituted isomers. However, 2,5‐disubstituted trinorbornanediols showed moderate diastereomeric differentiation because of the unidentate nature of the sterically more hindered exo‐isomer. trans‐Isomers gave rise to abundant [3M–H+Co]⁺ ion products, which may be considered a characteristic ion for bicyclo[221]heptane trans‐2,3‐ and trans‐2,5‐diols. To differentiate cis‐2,3‐isomers, the collision‐induced dissociation (CID) products for [3M+Co]²⁺, [M+CoOAc]⁺, [2M–H+Co]⁺ and [2M+CoOAc]⁺ cobalt complexes were investigated. The results of the CID of the monomeric and dimeric metal adduct complexes [M+CoOAc]⁺ and [2M–H+Co]⁺ were stereochemically controlled and could be used for stereochemical differentiation of the compounds investigated. In addition, the structures and relative energies of some complex ions were studied using hybrid density functional theory calculations. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and Structures of the Novel Biferrocenamine‐Based Receptor and its Voltammetric Responses to Transition Metal Ions

Reactions of formylferrocene and 1,2‐di‐(o‐aminophenoxy)ethane yield the novel bis(ferrocenyl) receptor (FcL). This compound has been characterized by IR, ¹H NMR and elemental analysis. In addition, the electrochemical behavior of FcL was investigated in detail in 0.1 M tetra‐n‐butylammonium perchlorate (TBAP) + CH₃CN by cyclic voltammetry (CV) and chronoamperometry. Its co‐ordination properties with metal ions in acetonitrile were also studied. The FcL shows a two‐wave behavior for H⁺, Cu²⁺, Zn²⁺ and Ni²⁺, but was unresponsive to Mg²⁺ and Ca²⁺. The maximum oxidation peak shift of about 250 mV was found for FcL in the presence of Cu²⁺, Zn²⁺ or Ni²⁺.     

Reversible Electrochemical Modulation of a Catalytic Nanosystem

A catalytic system based on monolayer‐functionalized gold nanoparticles (Au NPs) that can be electrochemically modulated and reversibly activated is reported. The catalytic activity relies on the presence of metal ions (Cd²⁺ and Cu²⁺), which can be complexed by the nanoparticle‐bound monolayer. This activates the system towards the catalytic cleavage of 2‐hydroxypropyl‐p‐nitrophenyl phosphate (HPNPP), which can be monitored by UV/Vis spectroscopy. It is shown that Cu²⁺ metal ions can be delivered to the system by applying an oxidative potential to an electrode on which Cu⁰ was deposited. By exploiting the different affinity of Cd²⁺ and Cu²⁺ ions for the monolayer, it was also possible to upregulate the catalytic activity after releasing Cu²⁺ from an electrode into a solution containing Cd²⁺. Finally, it is shown that the activity of this supramolecular nanosystem can be reversibly switched on or off by oxidizing/reducing Cu/Cu²⁺ ions under controlled conditions.     

Selective and Sensitive Fluorescent Chemosensors for Cu2+ Ion Based upon Bis‐1,8‐naphthalimide Dyads

A series of new fluorescent chemosensors 5a – 5e , composed of two aminonaphthalimide fluorophores and 2,6‐bis((N‐aminoalkyl)aminocarboxy)pyridines, were prepared, characterized, and their fluorescent properties towards heavy and transition metal (HTM) ions were investigated. Chemosensors 5c – 5e exhibited high selectivity and sensitivity for Cu²⁺ ion over other HTM ions with fluorescent quenching (green to colourless). It clearly demonstrated that the length of the linkers (diamines) between the aminonaphthalimides and 2,6‐dicarboxypyridine of 5a – 5e was very important for their sensitivity and selectivity for Cu²⁺ ion over other HTM ions.     

Controlling the optical and catalytic properties of artificial metalloenzyme photocatalysts using chemogenetic engineering

Visible light photocatalysis enables a broad range of organic transformations that proceed via single electron or energy transfer. Metal polypyridyl complexes are among the most commonly employed visible light photocatalysts. The photophysical properties of these complexes have been extensively studied and can be tuned by modifying the substituents on the pyridine ligands. On the other hand, ligand modifications that enable substrate binding to control reaction selectivity remain rare. Given the exquisite control that enzymes exert over electron and energy transfer processes in nature, we envisioned that artificial metalloenzymes (ArMs) created by incorporating Ru(ii) polypyridyl complexes into a suitable protein scaffold could provide a means to control photocatalyst properties. This study describes approaches to create covalent and non-covalent ArMs from a variety of Ru(ii) polypyridyl cofactors and a prolyl oligopeptidase scaffold. A panel of ArMs with enhanced photophysical properties were engineered, and the nature of the scaffold/cofactor interactions in these systems was investigated. These ArMs provided higher yields and rates than Ru(Bpy)₃²⁺ for the reductive cyclization of dienones and the [2 + 2] photocycloaddition between C-cinnamoyl imidazole and 4-methoxystyrene, suggesting that protein scaffolds could provide a means to improve the efficiency of visible light photocatalysts.

Artificial metalloenzyme visible light photocatalysts possess enhanced optical properties and are competent towards single electron and energy transfer organic transformations.     

A Copper(II) Ion‐Selective Potentiometric Sensor Based on N,N′,N″,N′′′‐Tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane in PVC Matrix

The simple PVC‐based membrane containing N,N′,N″,N′′′‐tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane (tpmc) as an ionophore and dibutyl phthalate as a plasticizer, directly coated on a glassy carbon electrode was examined as a new sensor for Cu²⁺ ions. The potential response was linear within the concentration range of 1.0×10^?1–1.0×10^?6 M with a Nernstian slope of 28.8 mV/decade and detection limit of 7.0×10^?7 M. The electrode was used in aqueous solutions over a wide pH range (1.3–6). The sensor exhibited excellent selectivity for Cu²⁺ ion over a number of cations and was successfully used in its determination in real samples.     

Hairpin DNA‐Dependent Click Conjugation of Oligonucleotides for Electrochemical Monitoring of Copper(II)

A new electrochemical sensing platform was designed for sensitive detection of copper(II) (Cu²⁺) based on click conjugation of two short oligonucleotides by using methylene blue‐functionalized hairpin DNA as the template. The analyte (Cu²⁺) was in situ reduced to Cu⁺ by sodium ascorbate, which catalyzed the click conjugation between two single‐stranded oligonucleotides one was labelled with a 5′‐alkyne and the other with 3′‐azide group via the Cu⁺‐catalyzed azide‐alkyne cycloaddition. The newly formed long‐chain oligonucleotide induced the conformational change of hairpin DNA to open the hairpin, resulting in methylene blue far away from the electrode for the decrease of redox current. Under optimal conditions, the decrease in the electronic signal was directly proportional to target Cu²⁺ concentration, and allowed the detection of Cu²⁺ at a concentration as low as 1.23 nM. Our strategy also displayed high selectivity for Cu²⁺ against other metal ions owing to the highly specific Cu⁺‐catalyzed click chemistry reaction, and was applicable for monitoring of Cu²⁺ in drinking water with satisfactory results.     

Determination of Copper and Zinc Ions by Flame-AAS After Preconcentraction Using Sodium Dodecyl Sulfate Coated Alumina Modified with 3-((1H-Indol-3-yl)-3,4,5-trimethyl)-1H-indole

A sensitive and simple method for the simultaneous preconcentration of trace amount Cu²⁺ and Zn²⁺ ions in some real samples has been established, which is based on the sorption of Cu²⁺ and Zn²⁺ on 3‐((1H‐indol‐3‐yl)‐3,4,5‐trimethyl)‐1H‐indole (ITMI) loaded on sodium dodecyl sulfate (SDS) coated alumina. The metal absorbed on the complexes was eluted using 3 mol/L nitric acid. The influences of the analytical parameters including pH and sample volume were investigated. The effects of matrix ions on the retentions of the analytes were also examined. The recoveries of analytes were generally higher than 95% with a low RSD. The method has been successfully applied to content evaluation of these metals in real samples.     

Bipyridinophane‐containing conjugated polymers modulated with an intramolecular aromatic C?H/π interaction

Bipyridinophane–fluorene conjugated copolymers have been synthesized via Suzuki and Heck coupling reactions from 5,8‐dibromo‐2,11‐dithia[3]paracyclo[3](4,4′)‐2,2′‐bipyridinophane and suitable fluorene precursors. Poly[2,7‐(9,9‐dihexylfluorene)‐co‐alt‐5,8‐(2,11‐dithia[3]paracyclo[3](4,4′)‐2,2′‐bipyridinophane)] ( P7 ) exhibits large absorption and emission redshifts of 20 and 34 nm, respectively, with respect to its planar reference polymer Poly[2,7‐(9,9‐dihexylfluorene)‐co‐alt‐1,4‐(2,5‐dimethylbenzene)] ( P11 ), which bears the same polymer backbone as P7 . These spectral shifts originate from intramolecular aromatic C? H/π interactions, which are evidenced by ultraviolet–visible and ¹H NMR spectra as well as X‐ray single‐crystal structural analysis. However, the effect of the intramolecular aromatic C? H/π interactions on the spectral shift in poly[9,9‐dihexylfluorene‐2,7‐yleneethynylene‐co‐alt‐5,8‐(2,11‐dithia[3]paracyclo[3](4,4′)‐2,2′‐bipyridinophane)] ( P10 ) is much weaker. Most interestingly, the quenching behaviors of these two conjugated polymers are largely dependent on the polymer backbone. For example, the fluorescence of P7 is efficiently quenched by Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Mn²⁺, and Ag⁺ ions. In contrast, only Cu²⁺, Co²⁺, and Ni²⁺ ions can partially quench the fluorescence of P10 , but much less efficiently than the fluorescence of P7 . The static Stern–Volmer quenching constants of Cu²⁺, Co²⁺, and Ni²⁺ ions toward P7 are of the order of 10⁶ M^?1, being 1300, 2500, and 37,300 times larger than those of P10 , respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4154–4164, 2006