Autocatalytic, bistable, oscillatory networks of biologically relevant organic reactions.

Networks of organic chemical reactions are important in life and probably played a central part in its origin. Network dynamics regulate cell division, circadian rhythms, nerve impulses and chemotaxis, and guide the development of organisms. Although out-of-equilibrium networks of chemical reactions have the potential to display emergent network dynamics such as spontaneous pattern formation, bistability and periodic oscillations, the principles that enable networks of organic reactions to develop complex behaviours are incompletely understood. Here we describe a network of biologically relevant organic reactions (amide formation, thiolate-thioester exchange, thiolate-disulfide interchange and conjugate addition) that displays bistability and oscillations in the concentrations of organic thiols and amides. Oscillations arise from the interaction between three subcomponents of the network: an autocatalytic cycle that generates thiols and amides from thioesters and dialkyl disulfides; a trigger that controls autocatalytic growth; and inhibitory processes that remove activating thiol species that are produced during the autocatalytic cycle. In contrast to previous studies that have demonstrated oscillations and bistability using highly evolved biomolecules (enzymes and DNA) or inorganic molecules of questionable biochemical relevance (for example, those used in Belousov-Zhabotinskii-type reactions), the organic molecules we use are relevant to metabolism and similar to those that might have existed on the early Earth. By using small organic molecules to build a network of organic reactions with autocatalytic, bistable and oscillatory behaviour, we identify principles that explain the ways in which dynamic networks relevant to life could have developed. Modifications of this network will clarify the influence of molecular structure on the dynamics of reaction networks, and may enable the design of biomimetic networks and of synthetic self-regulating and evolving chemical systems.

Structural Dependence of the Ising-type Magnetic Anisotropy and of the Relaxation Time in Mononuclear Trigonal Bipyramidal Co(II) Single Molecule Magnets.

This paper describes the correlation between Ising-type magnetic anisotropy and structure in trigonal bipyramidal Co(II) complexes. Three sulfur-containing trigonal bipyramidal Co(II) complexes were synthesized and characterized. It was shown that we can engineer the magnitude of the Ising anisotropy using ligand field theory arguments in conjunction with structural parameters. To prepare this series of compounds, we used, on the one hand, a tetradentate ligand containing three sulfur atoms and one amine (NS3tBu) and on the other hand three different axial ligands, namely, Cl-, Br-, and NCS-. The organic ligand imposes a trigonal bipyramidal arrangement with the three sulfur atoms lying in the trigonal plane with long Co-S bond distances. The magnetic properties of the compounds were measured, and ab initio calculations were used to analyze the anisotropy parameters and perform magneto-structural correlations. We demonstrate that a smaller axial zero-field splitting parameter leads to slower relaxation time when the symmetry is strictly axial, while the presence of very weak rhombicity decreases the energy barrier and speeds the relaxation of the magnetization.

Structural and electronic dependence of the single-molecule-magnet behavior of dysprosium(III) complexes.

We investigate and compare the magnetic properties of two isostructural Dy(III)-containing complexes. The Dy(III) ions are chelated by hexadentate ligands and possess two apical bidendate nitrate anions. In dysprosium(III) N,N'-bis(imine-2-yl)methylene-1,8-diamino-3,6-dioxaoctane (1), the ligand's donor atoms are two alkoxo, two pyridine, and two imine nitrogen atoms. Dysprosium(III) N,N'-bis(amine-2-yl)methylene-1,8-diamino-3,6-dioxaoctane (2) is identical with 1 except for one modification: the two imine groups have been replaced by amine groups. This change has a minute effect on the structure and a larger effect the magnetic behavior. The two complexes possess slow relaxation of the magnetization in the presence of an applied field of 1000 Oe but with a larger barrier for reorientation of the magnetization for 1 (Ueff/kB = 50 K) than for 2 (Ueff/kB = 34 K). First-principles calculations using the spin-orbit complete active-space self-consistent-field method were performed and allowed to fit the experimental magnetization data. The calculations gave the energy spectrum of the 2J + 1 sublevels issued from the J = 15/2 free-ion ground state. The lowest-lying sublevels were found to have a large contribution of MJ = ±15/2 for 1, while for 2, MJ = ±13/2 was dominant. The observed differences were attributed to a synergistic effect between the electron density of the ligand and the small structural changes provoked by a slight alteration of the coordination environment. It was observed that the stronger ligand field (imine) resulted in complex 1 with a larger energy barrier for reorientation of the magnetization than 2.

Subcomponent self-assembly of rare-earth single-molecule magnets.

A family of lanthanide complexes has been synthesized by the subcomponent self-assembly methodology. Molecular architectures, which were stable in solution and under ambient conditions, were designed by the in situ formation of ligands around lanthanide ion templates. Magnetic studies indicated that, despite the low C2 symmetry, 1 and 2 display single molecule magnet (SMM) behavior, with 1 exhibiting an effective energy barrier of the relaxation of the magnetization U(eff)/k(B) = 50 K and the pre-exponential factor τ(o) = 6.80 × 10(-7) s. Step-like features in the hysteresis loops indicate the presence of quantum tunneling of the magnetization (QTM).

Consumer-perceived risks and choices about pharmaceuticals in the environment: a cross-sectional study.

BACKGROUND: There is increasing concern that pollution from pharmaceuticals used in human medicine and agriculture can be a threat to the environment. Little is known, however, if people are aware that pharmaceuticals may have a detrimental influence on the environment. The present study examines people's risk perception and choices in regard to environmental risks of pharmaceuticals used in human medicine and for agricultural purposes. METHODS: A representative sample of the U.S. population (N = 640) was surveyed. Respondents completed a hypothetical choice task that involved tradeoffs between human and environmental health. In addition, it was examined how much people would support an environment policy related to drug regulation. RESULTS: For agricultural pharmaceuticals, respondents reported a high level of satisfaction for a policy requiring farms to limit their use of antibiotics. In the domain of pharmaceuticals used in human medicine, we found that people were willing to consider environmental consequences when choosing a drug, but only when choices were made about treatment options for a rather harmless disease. In contrast, when decisions were made about treatment options for a severe disease, the drug's effectiveness was the most important criterion. CONCLUSIONS: It can be concluded that the environmental impact of a drug will be hardly considered in decisions about pharmaceuticals for severe diseases like cancer, and this may be due to the fact that these decisions are predominantly affective in nature. However, for less severe health risks, people are willing to balance health and environmental considerations.

Interplay of interactions governing the dynamic conversions of acyclic and macrocyclic helicates.

A rigid, helical macrocycle that contains two copper(I) ions has been synthesized through subcomponent self-assembly. Although it does not obey the "rule of coordinative saturation", this macrocycle could be prepared through subcomponent substitution starting from a tri(copper(I)) helicate, in a reaction in which copper(I) was ejected. The macrocycle was observed to readily participate in a sequence of transformations between helical structures mediated by the electronic effects of substituents, entropic effects, the conformational preferences of organic building blocks, and the coordinative preferences of the metal ion. The thermodynamic parameters governing the interconversion of an "open" helicate and the "closed" macrocycle were determined through van 't Hoff analysis, allowing quantification of the entropic driving force for macrocyclization.

Engineering the magnetic coupling and anisotropy at the molecule-magnetic surface interface in molecular spintronic devices.

A challenge in molecular spintronics is to control the magnetic coupling between magnetic molecules and magnetic electrodes to build efficient devices. Here we show that the nature of the magnetic ion of anchored metal complexes highly impacts the exchange coupling of the molecules with magnetic substrates. Surface anchoring alters the magnetic anisotropy of the cobalt(II)-containing complex (Co(Pyipa)2), and results in blocking of its magnetization due to the presence of a magnetic hysteresis loop. In contrast, no hysteresis loop is observed in the isostructural nickel(II)-containing complex (Ni(Pyipa)2). Through XMCD experiments and theoretical calculations we find that Co(Pyipa)2 is strongly ferromagnetically coupled to the surface, while Ni(Pyipa)2 is either not coupled or weakly antiferromagnetically coupled to the substrate. These results highlight the importance of the synergistic effect that the electronic structure of a metal ion and the organic ligands has on the exchange interaction and anisotropy occurring at the molecule-electrode interface.

Tuning the Ising-type anisotropy in trigonal bipyramidal Co(II) complexes.

This paper demonstrates the engineering and tuning of Ising-type magnetic anisotropy in trigonal bipyramidal Co(II) complexes. Here, we predict that employing a ligand that forces a trigonal bipyramidal arrangement and has weak equatorial σ-donating atoms, increases (in absolute value) the negative zero field splitting parameter D. With these considerations in mind, we used a sulfur containing ligand (NS3(iPr)), which imposes a trigonal bipyramidal geometry to the central Co(II) ion with long equatorial Co-S bonds. The resulting complex exhibits a larger anisotropy barrier and a longer relaxation time in comparison to the complex prepared with a nitrogen containing ligand (Me6tren).

Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative.

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga4Ln2(shi3-)4(Hshi2-)2(H2shi-)2(C5H5N)4(CH3OH) x (H2O) x ]·xC5H5N·xCH3OH·xH2O (where H3shi = salicylhydroxamic acid and Ln = GdIII1; TbIII2; DyIII3; ErIII4; YIII5; YIII0.9DyIII0.16). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled DyIII ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy2 than for the Er2 complex.

Correction: Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative.

[This corrects the article DOI: 10.1039/C5SC01029B.].

Chemical tuning of the magnetic relaxation in dysprosium(iii) mononuclear complexes.

A dysprosium(iii) complex, exhibiting slow relaxation of magnetization, was prepared. Crystallographic studies showed a perturbation of local symmetry upon deprotonation of the ligand, with concomitant faster relaxation of magnetization. This was attributed to a large shift in the direction of the main magnetic axis, as indicated by ab initio calculations.

Cascading transformations within a dynamic self-assembled system.

Molecular subcomponents such as phosphate groups are often passed between biomolecules during complex signalling cascades, the flow of which define the motion of the machinery of life. Here, we show how an abiological system consisting of organic subcomponents knitted together by metal-ion coordination can respond to simple signals in complex ways. A Cu(I)(3) helicate transformed into its Zn(II)(2)Cu(I) analogue following the addition of zinc(II), and the ejected copper(I) went on to induce the self-assembly of a Cu(I)(2) helicate from other free subcomponents present in solution. The addition of an additional subcomponent, 8-aminoquinoline, resulted in the formation of a third, more stable Cu(I)(3) helicate, requiring the destruction of both the Zn(II)(2)Cu(I) and Cu(I)(2) helicates to scavenge sufficient Cu(I) for the new structure. This system thus demonstrates two examples in which the application of one signal provokes two distinct responses involving the creation or destruction of complex assemblies as the system seeks thermodynamic equilibrium following perturbation.