Three ligands L1, L2, and L3 with 2, 4, and 6 1,4,7,10-tetraazacyclododecane (cyclen) moieties attached to a cyclotriphosphazene core, respectively, were synthesized, and oxidation activities of their CuII complexes were investigated. Aerobic oxidation of catechol by these complexes follows an intramolecular dinuclear pathway with significant cooperativity (i.e., θ ≈ 1.5 out of a maximum of 2 for two potential substrate binding sites) and kinetic constants (i.e., kcat = 17.5 × 10–3 s–1, Km = 2.8 mm, and quite remarkable catalytic specificity kcat/Km 12.5 m–1 s–1 per di-Cu center), while that by untethered CuII–cyclen follows a bimolecular dinuclear pathway without noticeable cooperativity (θ = 0.96) and fourfold lower kcat, despite their similar dinuclear mechanisms. The proximity of CuII centers is suggested by EPR spectra and relaxations, showing a broad spectral component particularly in Cu6L3. Thermodynamic parameters also indicate the significance of multi-CuII sites in the oxidative catalysis. Air is a more specific oxidation agent for the representative complex Cu2L1, showing 3.2-fold higher catalytic specificity kcat/Km than H2O2 toward a catechol substrate. The research provides further molecular basis for future design of O2/H2O2-specific oxidation of multi-domain Cu complexes.

Three ligands L1, L2, and L3 with 2, 4, and 6 1,4,7,10-tetraazacyclododecane (cyclen) moieties attached to a cyclotriphosphazene core, respectively, were synthesized, and oxidation activities of their CuII complexes were investigated. Aerobic oxidation of catechol by these complexes follows an intramolecular dinuclear pathway with significant cooperativity (i.e., θ ≈ 1.5 out of a maximum of 2 for two potential substrate binding sites) and kinetic constants (i.e., kcat = 17.5 × 10–3 s–1, Km = 2.8 mm, and quite remarkable catalytic specificity kcat/Km 12.5 m–1 s–1 per di-Cu center), while that by untethered CuII–cyclen follows a bimolecular dinuclear pathway without noticeable cooperativity (θ = 0.96) and fourfold lower kcat, despite their similar dinuclear mechanisms. The proximity of CuII centers is suggested by EPR spectra and relaxations, showing a broad spectral component particularly in Cu6L3. Thermodynamic parameters also indicate the significance of multi-CuII sites in the oxidative catalysis. Air is a more specific oxidation agent for the representative complex Cu2L1, showing 3.2-fold higher catalytic specificity kcat/Km than H2O2 toward a catechol substrate. The research provides further molecular basis for future design of O2/H2O2-specific oxidation of multi-domain Cu complexes.

The Front Cover shows the pathways for catechol oxidation (foreground) by the copper complexes of cyclen-containing derivatives of cyclotriphosphazene (background) in aqueous media. Despite the mononuclear nature of the cyclen ligand, the reaction follows a dinuclear pathway determined by means of a “mechanistic Job plot” to reveal the stoichiometry of the transition state by monitoring the reaction rate. The dinuclear transition states imitate “The Creation” by Michelangelo to add some artistic flavor to the chemistry described herein. More information can be found in the Full Paper by Y. Ye, A. Angerhofer, Y. Zhao, L.-J. Ming et al. For more on the story behind the cover research, see the Cover Profile.

AbstractInvited for the cover of this issue are the groups of Y. Ye from Zhengzhou University, China, A. Angerhofer from the University of Florida, USA, Y. Zhao from Xiamen University, China, and L.-J. Ming from the University of South Florida, USA. The cover image shows the pathways for dinuclear catechol oxidation by CuII complexes of cyclotriphosphazene derivatives with up to six metal-binding sites.

•The detection limit of probe for Hg2+ was 2.11 × 10−8 M (the ppb scale).•High selectivity and sensitivity detection of in aqueous solution with a wide pH span (1.0–8.0).•The significant changes in color could be used for naked-eye detection.•The response time to Hg2+ is less than 2.5 min.•The fluorescence imaging experiment demonstrated its value of practical application.On the basis of fluorescent resonance energy transfer, a new fluorophore dyad (L) bearing rhodamine B and naphthalimide was developed as fluorescent ratiometric chemosensor for Hg2+ in aqueous solution. L exhibited high selectivity and excellent sensitivity towards Hg2+ with a broad pH span (1.0–8.0) and the detection limit of L was 2.11 × 10−8 M. Sensor L for the detection of Hg2+ was rapid and the recognizing event could complete in 2.5 min. A significant change in the color could be used for naked-eye detection. The selective fluorescence response of L to Hg2+ is due to the Hg2+-promoted ring opening of spirolactam of rhodamine moiety, leading to a cyclization reaction of thiourea moiety. In addition, fluorescence imaging experiments of Hg2+ in living EC 109 cells demonstrated its value of practical applications in biological systems.On the basis of fluorescent resonance energy transfer, a new fluorophore dyad (L) bearing rhodamine B and naphthalimide was developed as fluorescent ratiometric chemosensor for Hg2+ in aqueous solution. L exhibited high selectivity and excellent sensitivity towards Hg2+ with a broad pH span (1.0–8.0) and the detection limit of L was 2.11 × 10−8 M. Sensor L for the detection of Hg2+ was rapid and the recognizing event could complete in 2.5 min. A significant change in the fluorescence color could be used for naked-eye detection. The selective fluorescence response of L to Hg2+ is due to the Hg2+-promoted ring opening of spirolactam of rhodamine moiety, leading to a cyclization reaction of thiourea moiety. In addition, fluorescence imaging experiments of Hg2+ in living EC 109 cells demonstrated its value of practical applications in biological systems.

•A new rhodamine-based chemodosimeter(L2) was synthesized and characterized.•L2 exhibited high selectivity and sensitivity detection of Hg2+ in aqueous solution.•The significant changes in color could be used for naked-eye detection.•The fluorescence imaging experiment demonstrated its value of practical application.A new rhodamine-based chemodosimeter(L2) was synthesized and characterized, which contained a tosyl group acted as a strong electron acceptor. As expected, L2 exhibited high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Hg2+ in aqueous solution. The coordination of L2 with Hg2+ was chemically nonreversible and it was based on the fact that Hg2+ exhibited a strong thiophilic affinity. Addition of Hg2+ to an ethanol aqueous solution of L2 resulted in a color change from color-less to obvious pink color, these significant changes in color could be used for naked-eye detection. Furthermore, fluorescence imaging experiments of Hg2+ ions in living MGC803 cells demonstrated its value of practical applications in biological systems.

Three new fluorescent chemosensors (2a, 2b, 2c) bearing rhodamine B and sugar groups have been designed and synthesized. The chemosensors (2a, 2b, 2c) exhibit high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Cu2+ in aqueous solution. The significant changes in the fluorescence color could be used for naked-eye detection. Furthermore, all the three chemosensors (2a, 2b, 2c) exhibited selective absorbance enhancement to Cu2+ over other metal ions and a detection limit of 1 ppm Cu2+.

Five multinuclear cyclotriphosphazene ligands were synthesized and tested for their cleavage activities to plasmid DNA. All of these new compounds were confirmed by MS, 1H NMR, 31P NMR, 13C NMR and IR. Preliminary studies on the cleavage of pUC19 DNA in the presence of metal complexes were performed. The results revealed that these complexes could act as powerful catalysts under physiological conditions. The complexes 3b + Cu can effectively cleave DNA to nicked form, giving hydrolysis rate constant of 0.08/h under physiological conditions. An acid–base catalyzed DNA phosphate-diester hydrolysis mechanism was also proposed.

Three new fluorescent chemosensors (2a, 2b, 2c) bearing rhodamine B and sugar groups have been designed and synthesized. The chemosensors (2a, 2b, 2c) exhibit high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Cu2+ in aqueous solution. The significant changes in the fluorescence color could be used for naked-eye detection. Furthermore, all the three chemosensors (2a, 2b, 2c) exhibited selective absorbance enhancement to Cu2+ over other metal ions and a detection limit of 1 ppm Cu2+.