Mono-, di- and trinuclear 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz)-containing copper(II) complexes: syntheses, crystal structures and magnetic properties

Three new copper(ii) complexes of formula [Cu(tppz)(NCO)(2)].0.4H(2)O (1), [Cu(2)(tppz)Br(4)](2) and [Cu(3)(tppz)(C(5)O(5))(3)(H(2)O)(3)].7H(2)O (3)[tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine; C(5)O(5)(2-) = croconate, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione] have been synthesised and structurally characterized by X-ray diffraction methods. The structure of complex is made up of neutral [Cu(tppz)(NCO)(2)] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper-copper distance of 3.9244(4) angstroms. The structure of complex 1 consists of neutral tppz-bridged [Cu(2)(tppz)Br(4)] dinuclear units, the copper-copper separation across tppz being 6.6198(1) angstroms. The dinuclear units are further connected through weak, double out-of-plane Cu-Br...Cu bridges [Br(1)...Cu(1a) 4.0028(17) angstroms] creating tetranuclear entities, the copper-copper separation through this interaction being 4.3299(21) angstroms. The structure of complex 3 is built of neutral [Cu(3)(tppz)(C(5)O(5))(3)(H(2)O)(3)] trinuclear units and uncoordinated water molecules. Tppz and one of the croconate groups act as bridging ligands, the former exhibiting the bis-terdentate coordination mode and the latter adopting an unusual asymmetrical bis-bidentate bridging mode through three adjacent oxygen atoms. The other two croconate groups exhibit the bidentate coordination mode. The intramolecular copper-copper separations are 6.5417(9)(across tppz) and 4.3234(9) angstroms (through bis-bidentate croconato). The magnetic properties of 2 and 3 have been investigated in the temperature range 1.9-300 K. The magnetic behaviour of complex 2 is that of an antiferromagnetically coupled copper(II) dimer (J = -40.9 cm(-1), the Hamiltonian being H = -JS(A).S(B)). In the case of compound , the chi(M) T vs. T plot is typical of an overall antiferromagnetic coupling with a low-lying spin doublet being fully populated at T < 10 K. The values of the intramolecular antiferromagnetic interactions in 3 are -19.9 (across tppz) and -32.9 cm(-1)(through bridging croconato). Density functional type calculations were performed on model dinuclear fragments of 3 in order to analyze the efficiency of the exchange pathways involved and also to substantiate the coupling parameters.     

Stereoselective synthesis of farnesylphosphoryl β-d-arabinofuranose

Decaprenylphosphoryl β-d-arabinofuranose (DPA) is a key arabinose donor in mycobacteria. In an effort to establish a practical synthetic scheme for DPA, the synthesis of nerylphosphoryl and farnesylphosphoryl β-d-arabinofuranoses has been developed. The products were obtained by coupling of a suitably protected β-d-arabinofuranosyl phosphate intermediate with activated forms of the C₁₀ nerol and C₁₅trans,trans-farnesol and subsequent deprotection.     

Direct and indirect monitoring of peptide-silica interactions using time-resolved fluorescence anisotropy

The present work extends the application of time-resolved fluorescence anisotropy (TRFA) of a cationic probe rhodamine 6G (R6G) in aqueous Ludox to in situ monitoring of peptide adsorption onto the silica particles. Steady-state anisotropy and TRFA of R6G in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles in the presence of varying levels of tripeptides of varying charge, including Lys-Trp-Lys (KWK), N-acetylated Lys-Trp-Lys (Ac-KWK), Glu-Trp-Glu (EWE), and N-acetylated Glu-Trp-Glu (Ac-EWE). The results were compared to those obtained by direct observation of peptide adsorption using the steady-state anisotropy of the intrinsic tryptophan residue. Ionic binding of the peptides to Ludox particles produced an increase in the steady-state Trp anisotropy that was dependent on the number of cationic groups present, but the limiting anisotropy values were relatively low, indicating significant rotational freedom of the indole residue in the adsorbed peptides. On the other hand, R6G showed significant decreases in anisotropy in the presence of cationic peptides, consistent with the cationic peptides blocking the adsorption of the dye to the silica surface. Thus, R6G is able to indirectly report on the binding of peptides to Ludox particles. It was noteworthy that, while there were similar trends in the data obtained from steady-state anisotropy and TRFA studies of R6G, the use of steady-state anisotropy to assess binding of peptides overestimated the degree of peptide adsorption relative to the value obtained by TRFA. The study shows that the competitive binding method can be used to assess the binding of various biologically relevant compounds onto silica surfaces and demonstrates the potential of TRFA for probing peptide-silica and protein-silica interactions.     

The pK(a) values of N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylenediamine

Aqueous solutions of the industrially important chelating agent N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine exhibit basic properties. The proton dissociation constants were determined to be 8.99 +/- 0.04 (pK(1)) and 4.30 +/- 0.04 (pK(2)) by potentiometric titration at 25 degrees in 0.15M sodium chloride.     

The synthesis of monobactam analogues bearing carbapenemside-chains on C(3)

A series of monobactam analogues lacking 3-acylamino side chains has been synthesised from the photoisomer of 4-methyl-2-pyridone; none of these analogues showed any useful biological activity.     

Swollen poly(dimethylsiloxane) (PDMS) as a template for inorganic morphologies

We report a series of silica, titania, and zirconia microstructures synthesized within swollen poly(dimethylsiloxane) (PDMS). Voids created by solvent-swelling the polymer are used to template the product. The inorganic morphologies range from spheres to networks, depending upon the nature of the polymer, its degree of swelling, and the synthetic conditions. Organic solvents as well as pure metal alkoxide liquids have been used to swell the polymer. Once the alkoxide precursor is inside the swollen polymer, water is introduced to bring about hydrolysis and condensation polymerization. The product is a textured metal oxide within a PDMS matrix. Scanning electron microscopy (SEM), optical microscopy, nuclear magnetic resonance (NMR), and powder X-ray diffraction (PXRD) were used to characterize the products. Microstructures formed in this manner have potential use as an inexpensive route to catalysts, fillers, capsules, or membranes for separations.     

Lanthanide-transition metal chalcogenido cluster materials

[(THF)3Sm(SePh)2Zn(SePh)2]n decomposes to give a variety of products, including [(THF)8Sm4Se(SePh)8](2+)[Zn8Se(SePh)16](2-), an ionic cluster that can also be prepared in more than 60% yield by stoichiometric addition of Se to a mixture of Sm(SePh)3 and Zn(SePh)2. The isostructural Nd compound [(THF)8Nd4Se(SePh)8](2+)[Zn8Se(SePh)16](2-) was also prepared by the stoichiometric route to establish the viability of this cluster type with redox-inactive Ln. In addition, the salt [Yb(THF)6](3+)[Fe4Se4(SePh)4](3-) was isolated and structurally characterized. These ionic cluster materials illustrate the difficulties associated with doping Ln ions into covalent metal chalcogenido matrixes.     

Selenoxanthones via directed metalations in 2-arylselenobenzamide derivatives

The reaction of N, N-diethyl 4-(N', N'-dimethylamino)-2-phenylselenobenzamide (7a), N, N-diethyl 4-methoxy-2-phenylselenobenzamide (7b), N, N-diethyl 4-(N', N'-dimethylamino)-2-[(3-(N, N-dimethylamino)phenylseleno]benzamide (7c), and N, N-diethyl 4-methoxy-2-(3-methoxyphenylseleno)-benzamide (7d) with excess lithium diisopropylamide (LDA) gave 2-N, N-(dimethylamino)-9H-selenoxanthone (9a), 2-methoxy-9H-selenoxanthone (9b), 2,7-bis-N, N-(dimethylamino)-9H-selenoxanthone (9c), and 2,5-dimethoxy-9H-selenoxanthone (9d) in 70%, 59%, 23%, and 90% isolated yields, respectively. N, N-Diethyl 2-phenylselenobenzamide (2-Se) gave no reaction with LDA, t-BuLi, MeLi, or lithium 2,2,6,6-tetramethylpiperidide as base. Electron donation from the 4-substituent of benzamide derivatives 7a-7d may increase the directing ability of the carbonyl oxygen to metalate the 2-position of the arylseleno group.     

Participation of the photosensitizer alpha-terthienyl in the peroxidase-catalyzed oxidation of indole-3-acetic acid

The plant photosensitizer alpha-terthienyl (alpha T) is toxic toward a variety of organisms, and normally requires exposure to ultraviolet-A radiation for activation and singlet molecular oxygen formation. However, some toxicity has also been reported to occur in the dark. One hypothesis that has been proposed to account for this light-independent toxicity is that the sensitizer becomes activated by energy transfer from the excited-state products of enzymatic reactions. We have investigated this hypothesis using the horseradish peroxidase (HRP)-catalyzed oxidation of indole-3-acetic acid (IAA), which generates indole-3-aldehyde in an excited triplet state. Light is emitted during the IAA/HRP reaction at acidic pH, is increased by inclusion of alpha T and is not observed with heat-denatured HRP. The rates of both the oxidation of IAA and the subsidence of light emission are more rapid in the IAA/alpha T/HRP system than with IAA and HRP alone, indicating that the presence of alpha T accelerates the reaction. Bleaching occurs at the wavelength of maximal alpha T absorbance and is promoted by the inclusion of IAA. Readdition of both IAA and alpha T to a spent reaction mixture is required to restore light emission after it has subsided, further suggesting that both are consumed in the reaction. We were unable to detect measurable quantities of singlet molecular oxygen formation in this system. These results do not support the energy transfer hypothesis, but instead are more compatible with a model proposed by Krylov and Chebotareva [Krylov, S. N. and A. B. Chebotareva (1993) FEBS Lett. 324, 6-8] for the co-oxidation of IAA and xanthene dyes.