Wadih Ghattas

Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.

Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion.

Binding of 2-Hydroxypyridine-N-oxide on Dicopper(II) Centers: Insights into Tyrosinase Inhibition Mechanism by Transition-State Analogs

2-Hydroxypyridine-N-oxide (HOPNO) is described as a new and efficient transition-state analog (TS-analog) inhibitor for the mushroom tyrosinase with an IC(50) = 1.16 microM and a K(I) = 1.8 microM. Using the binuclear copper(II) complex [Cu(2)(BPMP)(mu-OH)](ClO(4))(2) (2) known as a functional model for the tyrosinase catecholase activity, we isolated and fully characterized a 1:1 (2)/OPNO adduct in which the HOPNO is deprotonated and chelates only one Cu-atom of the binuclear site in a bidentate mode. On the basis of these results, a structural model for the tyrosinase inhibition by HOPNO is proposed.

Artificial Metalloenzymes with the Neocarzinostatin Scaffold: Toward a Biocatalyst for the Diels-Alder Reaction.

A copper(II) cofactor coupled to a testosterone anchor, copper(II)‐(5‐(Piperazin‐1‐yl)‐1,10‐phenanthroline)testosterone‐17‐hemisuccinamide (10) was synthesized and associated with a neocarzinostatin variant, NCS‐3.24 (KD=3 μm), thus generating a new artificial metalloenzyme by following a “Trojan horse” strategy. Interestingly, the artificial enzyme was able to efficiently catalyze the Diels–Alder cyclization reaction of cyclopentadiene (1) with 2‐azachalcone (2). In comparison with what was observed with cofactor 10 alone, the artificial enzymes favored formation of the exo products (endo/exo ratios of 84:16 and 62:38, respectively, after 12 h). Molecular modeling studies assigned the synergy between the copper complex and the testosterone (KD=13 μm) moieties in the binding of 10 to good van der Waals complementarity. Moreover, by pushing the modeling exercise to its limits, we hypothesize on the molecular grounds that are responsible for the observed selectivity.

Synthesis and characterization of a binuclear iron(III) complex bridged by 1-aminocyclopropane-1-carboxylic acid. Ethylene production in the presence of hydrogen peroxide.

A mu-oxo-diiron(III) complex bridged by two molecules of 1-aminocyclopropane-1-carboxylic acid (ACCH) was prepared with the ligand 1,4,7-triazacyclononane (TACN): [(TACN)Fe(2)(mu-O)(mu-ACCH)(2)](ClO(4))(4) x 2 H(2)O (1). This complex was characterized, and its crystal structure was solved. The bridging amino acid moieties were found in their zwitterionic forms (noted as ACCH). Reactivity assays were performed in the presence of hydrogen peroxide, and 1 turned out to be the first example of a well-characterized iron-ACCH complex able to produce ethylene from the bound ACCH moiety. The reaction requires the presence of a few equivalents of base, probably involved in the deprotonation of the amine groups of the ACCH bridges.

Probing the Peptidylglycine α-Hydroxylating Monooxygenase Active Site with Novel 4-Phenyl-3-butenoic Acid Based Inhibitors

Specific inhibition of the copper‐containing peptidylglycine α‐hydroxylating monooxygenase (PHM), which catalyzes the post‐translational modification of peptides involved in carcinogenesis and tumor progression, constitutes a new approach for combating cancer. We carried out a structure–activity study of new compounds derived from a well‐known PHM substrate analogue, the olefinic compound 4‐phenyl‐3‐butenoic acid (PBA). We designed, synthesized, and tested various PBA derivatives both in vitro and in silico. We show that it is possible to increase PBA affinity for PHM by appropriate functionalization of its aromatic nucleus. Compound 2 d, for example, bears a meta‐benzyloxy substituent, and exhibits better inhibition features (Ki=3.9 μM, kinact/Ki=427 M−1 s−1) than the parent PBA (Ki=19 μM, kinact/Ki=82 M−1 s−1). Docking calculations also suggest two different binding modes for PBA derivatives; these results will aid in the development of further PHM inhibitors with improved features.

2-Amino[1,2,4]triazolo[1,5-c]quinazolines and Derived Novel Heterocycles: Syntheses and Structure-Activity Relationships of Potent Adenosine Receptor Antagonists.

2‐Amino[1,2,4]triazolo[1,5‐c]quinazolines were identified as potent adenosine receptor (AR) antagonists. Synthetic strategies were devised to gain access to a broad range of derivatives including novel polyheterocyclic compounds. Potent and selective A3AR antagonists were discovered, including 3,5‐diphenyl[1,2,4]triazolo[4,3‐c]quinazoline (17, Ki human A3AR 1.16 nm) and 5′‐phenyl‐1,2‐dihydro‐3′H‐spiro[indole‐3,2′‐[1,2,4]triazolo[1,5‐c]quinazolin]‐2‐one (20, Ki human A3AR 6.94 nm). In addition, multitarget antagonists were obtained, such as the dual A1/A3 antagonist 2,5‐diphenyl[1,2,4]triazolo[1,5‐c]quinazoline (13 b, Ki human A1AR 51.6 nm, human A3AR 11.1 nm), and the balanced pan‐AR antagonists 5‐(2‐thienyl)[1,2,4]triazolo[1,5‐c]quinazolin‐2‐amine (11 c, Ki human A1AR 131 nm, A2AAR 32.7 nm, A2BAR 150 nm, A3AR 47.5 nm) and 9‐bromo‐5‐phenyl[1,2,4]triazolo[1,5‐c]quinazolin‐2‐amine (11 q, Ki human A1AR 67.7 nm, A2AAR 13.6 nm, A2BAR 75.0 nm, A3AR 703 nm). In many cases, significantly different affinities for human and rat receptors were observed, which emphasizes the need for caution in extrapolating conclusions between different species.

Characterization of Cu(II)-ACC Complexes and Conversion of the Bound ACC into Ethylene in the Presence of Hydrogen Peroxide. Detection of a Brown Intermediate at Low Temperature

Two copper(II)-ACC complexes were prepared and characterized: [Cu(bpy)(ACC)(H2O)]⋅ CO4 (1) and [Cu(ACC)2]3 ⋅ 4H2O (2). Their crystallographic structures are described and analyzed. Spectroscopic characterizations (UV-visible and EPR) confirm that the structure is maintained in solution. These complexes are able to produce ethylene in the presence of hydrogen peroxide in an “ACC Oxidase-like” reaction in water and in methanol. The conversion of ACC into ethylene depends on the amount of base, and, in methanol, 3 equivalents of NaOH are needed for optimum activity. The base is proposed to play a role in H2O2 deprotonation. The presence of an exogenic ligand (bpy) is important for the reactivity and may stabilize a reaction intermediate. Indeed, a brown intermediate with an absorption band centered at 433 nm can be detected at low temperature when 1 is treated with 10 equivalents of H2O2.

ACC-Oxidase like activity of a copper (II)–ACC complex in the presence of hydrogen peroxide. Detection of a reaction intermediate at low temperature

A Cu(II)-ACC complex [(Bpy)Cu(ACC)(H2O)]ClO4 (1) was prepared and its treatment with hydrogen peroxide gave rise to ethylene production in an ACC-Oxidase like activity. A brown species that could be a key intermediate in the reaction was detected at low temperature.

αRep A3: A versatile artificial scaffold for metalloenzyme design

αRep refers to a new family of artificial proteins based on a thermostable α-helical repeated motif. One of its members, αRep A3, forms a stable homo-dimer with a wide cleft that is able to accommodate metal complexes and thus appears to be suitable for generating new artificial biocatalysts. Based on the crystal structure of αRep A3, two positions (F119 and Y26) were chosen, and independently changed into cysteine residues. A phenanthroline ligand was covalently attached to the unique cysteine residue of each protein variant, and the corresponding biohybrids were purified and characterized. Once mutated and coupled to phenanthroline, the protein remained folded and dimeric. Copper(II) was specifically bound by the two biohybrids with two different binding modes. Furthermore, the holo-biohybrid A3F119NPH was found to be capable of enantioselectively catalyzing Diels-Alder (D-A) cycloadditions with up to 62 % ee. This study validates the choice of the αRep A3 dimer as a protein scaffold and provides a promising new route for the design and production of new enantioselective biohybrids based on entirely artificial proteins obtained from a highly diverse library.

Electrochemical DNA Biosensors for Bioterrorism Prevention

In the wake of letters containing anthrax spores terrifying the USA and other letters containing unidentified white powders circulating all over the world, the threat of bioterrorism attracts the attention of the general public as well as scientist. Therefore, it is urgent to develop rapid, sensitive, and high-throughput diagnostic methods able to counter attacks of bioterrorism by elucidating the suitable actions that should be implemented to prevent serious epidemic diseases. Numerous such methods are in development but Nucleic Acid Detection is the standard employed for identifying most biological agents that are used in bioterrorism. This method is based on PCR assays via the classical techniques of amplification and fluorescent detection. On the other hand, electrochemical biosensors are promising platforms that could achieve rapid highly sensitive and selective onsite detection of such agents. This chapter will present the recent developments in electrochemical biosensors for preparing DNA detection platforms that could be used to prevent attacks of bioterrorism.