Enrique García-España

PROTON COORDINATION BY POLYAMINE COMPOUNDS IN AQUEOUS SOLUTION

Abstract The present article is concerned with proton transfer reactions in aqueous solution of open-chain, macrocyclic and macropolycyclic or cage compounds having nitrogen atoms as protonation sites in the molecular framework, although several compounds with additional different donors will be considered. The main purpose of this review is to collect some significant examples of proton transfer processes in order to show how the electronic properties and molecular topology of polyamines affect the thermodynamic parameters of their protonation equilibria.

Fluorescent Chemosensors Containing Polyamine Receptors

Chemosensors have attracted interest in many different scientific fields, such as environmental chemistry, medicine, and the processing and storage of information. These molecular-scale devices have the advantage of working on the same spatial scale as the chemical structures that are responsible for macroscopic behaviour observed in the environment or those associated with health problems. Moreover, they allow the construction of molecular-scale devices for information storage. In this review, we describe a family of chemosensors based on a polyamine receptor and a fluorescent signalling unit. Polyamine receptors are water-soluble ambidentate receptors; they are able to coordinate either metal ions, when sufficient deprotonated amino groups are available, or anionic species, when there are sufficient protonated amino groups. On the other hand, the use of fluorescent signalling units confers the advantage of an immediate visual response/signal.

Supramolecular complexation for environmental control

Supramolecular complexes offer a new and efficient way for the monitoring and removal of many substances emanating from technical processes, fertilization, plant and animal protection, or e.g. chemotherapy. Such pollutants range from toxic or radioactive metal ions and anions to chemical side products, herbicides, pesticides to drugs including steroids, and include degradation products from natural sources. The applications involve usually fast and reversible complex formation, due to prevailing non-covalent interactions. This is of importance for sensing as well as for separation techniques, where the often expensive host compounds can then be reused almost indefinitely. Immobilization of host compounds, e.g. on exchange resins or on membranes, and their implementation in smart new materials hold particular promise. The review illustrates how the design of suitable host compounds in combination with modern sensing and separation methods can contribute to solve some of the biggest problems facing chemistry, which arise from the everyday increasing pollution of the environment.

Modulation of DNA binding by reversible metal-controlled molecular reorganizations of scorpiand-like ligands.

DNA interaction with scorpiand azamacrocycles has been achieved through modulation of their binding affinities. Studies performed with different experimental techniques provided evidence that pH or metal-driven molecular reorganizations of these ligands regulate their ability to interact with calf thymus DNA (ctDNA) through an intercalative mode. Interestingly enough, metal-driven molecular reorganizations serve to increase or decrease the biological activities of these compounds significantly.

Copper complexes of polyaza[n]cyclophanes and their interaction with DNA and RNA ☆

Abstract The complexation properties of Cu 2+ ions with the cyclophane receptors 2,6,9,13-tetraaza[14]metacyclophane ( L1 ), 2,6,10,13,17,21-hexaaza[22]metacyclophane ( L2 ) and 2,6,10,13,17,21-hexaaza[22]paracyclophane ( L3 ) are presented. Formation of mononuclear complexes in the case of L1 and of mono- and binuclear complexes in the case of the hexaaazcyclophane ligands L2 and L3 is observed. The coordination numbers around each Cu 2+ in the binuclear complexes involve at most three nitrogen donors for each metal ion. Cyclophanes L1 – L3 and several acyclic polyamine ligands are tested for their affinity towards double-stranded nucleic acid models of RNA and DNA. The binding affinity of the acyclic and macrocyclic polyamines towards DNA and RNA models, measured by changes in their melting temperature Δ T M , increases progressively with the average number of charges present on the amine, with higher stabilisation for RNA. Unusually large differences of up to Δ(Δ T M )=30°C were observed with tripropylenetetraamine ( L15 ) and with the macrocyclic amines ( L2 and L3 ). Introduction of copper in the ligands leads to considerable affinity variations. One macrocyclic copper complex (with L3 ) shows a strong discrimination between the RNA and DNA polymers with a record value of ΔΔ T M =41°C, and a small destabilisation of the DNA. The copper complexes exhibit nuclease activity; with plasmid DNA nicking was increased by one complex by a factor of k / k un =10 7 . Experiments with hydroxyl radical quenchers indicate a predominant redox cleavage mechanism.

Multifunctional molecular recognition of ATP, ADP and AMP nucleotides by the novel receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane

The novel cyclophane receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane L presents a molecular architecture which enables recognition in aqueous solution of ATP, ADP and AMP through electrostatic, hydrogen bonding and π-stacking interactions; electrostatic interactions occur between the polyammonium sites of L and the phosphate chain of the nucleosides, and π-stacking interactions occur between the m-phenylene subunit incorporated in the receptor as a non-pendant integral part of the macrocyclic framework and the adenine ring of the nucleotides.

Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C-H Bonds with Stereoretention.

An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex [Fe(II)(CF3SO3)2(PyNMe3)] (1) at cryogenic temperatures, and characterized spectroscopically. Compound 2 is kinetically competent for breaking strong C-H bonds of alkanes (BDE ≈ 100 kcal·mol(-1)) through a hydrogen-atom transfer mechanism, and the transformations proceed with stereoretention and regioselectively, responding to bond strength, as well as to steric and polar effects. Bimolecular reaction rates are at least an order of magnitude faster than those of the most reactive synthetic high-valent nonheme oxoiron species described to date. EPR studies in tandem with kinetic analysis show that the 490 nm chromophore of 2 is associated with two S = 1/2 species in rapid equilibrium. The minor component 2a (∼5% iron) has g-values at 2.20, 2.19, and 1.99 characteristic of a low-spin iron(III) center, and it is assigned as [Fe(III)(OOAc)(PyNMe3)](2+), also by comparison with the EPR parameters of the structurally characterized hydroxamate analogue [Fe(III)(tBuCON(H)O)(PyNMe3)](2+) (4). The major component 2b (∼40% iron, g-values = 2.07, 2.01, 1.95) has unusual EPR parameters, and it is proposed to be [Fe(V)(O)(OAc)(PyNMe3)](2+), where the O-O bond in 2a has been broken. Consistent with this assignment, 2b undergoes exchange of its acetate ligand with CD3CO2D and very rapidly reacts with olefins to produce the corresponding cis-1,2-hydroxoacetate product. Therefore, this work constitutes the first example where a synthetic nonheme iron species responsible for stereospecific and site selective C-H hydroxylation is spectroscopically trapped, and its catalytic reactivity against C-H bonds can be directly interrogated by kinetic methods. The accumulated evidence indicates that 2 consists mainly of an extraordinarily reactive [Fe(V)(O)(OAc)(PyNMe3)](2+) (2b) species capable of hydroxylating unactivated alkyl C-H bonds with stereoretention in a rapid and site-selective manner, and that exists in fast equilibrium with its [Fe(III)(OOAc)(PyNMe3)](2+) precursor.

Squaramide-based reagent for selective chromogenic sensing of Cu(II) through a zwitterion radical.

A minimalist squaramide-based chemodosimeter for Cu(2+) is described. Upon selective chelation to 2, Cu(2+) induces the formation of a highly colored zwitterionic radical, which is kinetically stable for hours. The presence of a radical is confirmed by EPR and ESI-MS. It is then possible to use reagent 2 for visual and selective sensing of Cu(2+) at neutral pH.

A Ferromagnetic [Cu3(OH)2]4+ Cluster Formed inside a Tritopic Nonaazapyridinophane: Crystal Structure and Solution Studies†

Chemists working in coordination and/or supramolecular chemistry find a continuous source of inspiration in biomolecules and enzyme active sites. In this respect, trinuclear Cu centers have attracted a lot of interest due to their resemblance to multicenter blue copper oxidases like ascorbate oxidase, ceruloplasmin, lacase oxidase, and particulate methane monooxygenase. These enzymes contain at least four copper centers, which are necessary for four-electron reduction of molecular oxygen to water. For instance, the high-resolution structure of ascorbate oxidase shows that the mononuclear electron-transfer copper site of type 1 (T1) is connected to the trinuclear site through a patch formed by a cysteine residue from which two histidine residues diverge to bind the T3 copper ions. In the oxidized form of the enzyme, the T3 Cu atoms, apart from the imidazole moiety of the histidine residues of the patch, are coordinated by another two imidazole units and by an OH group that bridges the two atoms, which are 3.71 far apart from each other. The geometry around each metal ion can be best described as a trigonal bipyramid with a vacant equatorial position oriented towards the T2 copper atom, which completes the trinuclear center. In the T2 site the Cu center is coordinated in a very particular T-shaped geometry by two histidine residues and a monodentate hydroxo group or a water molecule. The mechanism by which these centers catalyze the fourelectron reduction of molecular dioxygen to water and the magnetic properties of these particular arrangements of copper ions have attracted great interest from both the biochemical and magnetochemical points of view. Therefore, small-molecule studies aimed at mimicking their properties are of great relevance in this respect.

Potential ATPase mimics by polyammonium macrocycles: Criteria for catalytic activity

Abstract A series of polyammonium macrocycles, ranging in size from the 18-membered ring [18]aneN6 to the 36-membered [36]aneN12 were examined as potential ATPase mimics. The rates of hydrolysis of ATP were followed at pH 3.0 and 7.0 using 31P NMR and HPLC techniques. Stability constants as a function of degree of protonation, distribution curves for the ligands as a function of pH, and distribution curves for the mixed species of nucleotides, inorganic phosphate, and macrocycle were also determined. All of the macrocycles catalyzed the hydrolysis of ATP to some extent compared to noncatalyzed hydrolysis. A critical dependence on macrocyclic ring size was observed, with [21]aneN7 being the best catalyst at both pHs. Stability constants of the complexes formed between the phosphate species and macrocycle increase with increasing degree of protonation and decreasing ring size. The trend in stability constants for phosphate species was found to be PO43− > P2O74− > ATP4− > ADP3− > AMP2− for a given degree of protonation. The crystal structure of tetraprotonated [21]aneN7 was determined. The compound N7C14Cl4H41O crystallizes in the monoclinic space group P21 (#4) with unit cell dimensions a = 7.472(1), b = 19.480(2), c = 8.3638(9) A , β = 100.38(1)o, and V = 1197.4(3) A 3 . The structure was solved by direct methods and refined using full-matrix least-squares techniques to give a final R = 0.041 and Rw = 0.055.