Cell-specific and nuclear targeting with [M(CO)(3)](+) (M=(99m)Tc, Re)-based complexes conjugated to acridine orange and bombesin

Receptor-specific nuclear targeting requires trifunctional metal complexes. We have synthesized [M(L(2)-pept)(L(1)-acr)(CO)(3)] (pept=peptide; acr=acridine-based agent) in which the fac-[M(CO)(3)](+) moiety (1st function, M=(99m)Tc, Re) couples an acridine-based nuclear-targeting agent (2nd function, L(1)-acr) and the specific cell-receptor-binding peptide bombesin (3rd function, L(2)-pept). The metal-mediated coupling is based on the mixed ligand [2+1] principle. The nuclear targeting agents have been derivatised with an isocyanide group for monodentate (L(1)) and bombesin (BBN) with a bidentate ligand (L(2)) for complexation to fac-[M(CO)(3)](+). For nuclear uptake studies, the model complexes [Re(L(2))(L(1)-acr)(CO)(3)] (L(2)=pyridine-2-carboxylic acid and pyridine-2,4-dicarboxylic acid) were synthesized and structurally characterized. We selected acridine derivatives as nuclear-targeting agents, because they are very good nucleus-staining agents and exhibit strong fluorescence. Despite the bulky metal complexes attached to acridine, all [Re(L(2))(L(1)-acr)(CO)(3)] showed high accumulation in the nuclei of PC3 and B16F1 cells, as evidenced by fluorescence microscopy. For radiopharmaceutical purposes, the (99m)Tc analogues have been prepared and radioactivity distribution confirmed the fluorescence results. Coupling of BBN to L(2) gave the receptor-selective complexes [M(L(2)-BBN)(L(1)-acr)(CO)(3)]. Whereas no internalization was found with B16F1 cells, fluorescence microscopy on PC3 cells bearing the BBN receptor showed high and rapid uptake by receptor-mediated endocytosis into the cytoplasm, but not into the nucleus.     

Nickelatetrazoles as Intermediates in the Course of the Photolysis of Nickel(II) Azido Complexes? – A Theoretical Study

The primary photoreactions due to charge transfer excitation of d⁸ transition metal complexes of [MP₂(N₃)₂] constitution (P₂: mono‐ or diphosphane ligands) are strongly influenced by the central ion. While [MP₂(N₃)₂] complexes of both palladium(II) and platinum(II) yield primarily the corresponding metal(I) species and azidyl radicals, nickel(II) complexes of the same constitution lead to intermediate generation of nitrenes as has been indirectly shown by various scavenging reactions. During the course of the succeeding reactions, the intermediate generation of nickelatetrazoles is assumed. Both nitrene and metallatetrazole intermediates could not be identified directly so far. Therefore, the proposed reaction pathway has been modeled theoretically to get indications regarding the probability of the formation of such intermediates. Based on density functional theory (DFT) calculations, the energetics of the proposed pathway is compared with those of possible other pathways. The calculations support the proposed reaction pathway and point to possible reasons for the different behaviour of nickel(II) compared to palladium(II) and platinum(II).     

UV-Broadband Light Scattering Measurements During Metallic Particle Formation in a Combustion-Like Environment

The thermal evolution of aqueous solution droplets of lead and nickel nitrate was studied experimentally in a drop-tube furnace operated up to 1300 K. Dimensions and physico-chemical properties of the droplets/particles were obtained by coupling the analysis of the spectra of ultraviolet light scattered by the produced aerosol with scanning electron microscopy and numerical simulation of the scattering spectra by Mie theory. Lead nitrate forms solid hollow particles with sizes of the order of the original droplets during the drying process, whereas at higher temperature it decomposes, forming spherical micrometer-sized particles of lead oxide and even submicrometer-sized particles of pure lead. Nickel nitrate never forms solid particles owing to its high solubility in water, but precipitates as nickel hydroxide particles in the temperature range where this intermediate decomposition product is formed. At higher temperatures the decomposition of nickel hydroxide and the formation of oxide particles in the micrometer size range is observed. The mutual interaction of the salt properties were analyzed by studying the behavior of a lead–nickel nitrate mixture in the drop-tube reactor. The main peculiarity of the mixture evolution is the formation of composite particles of lead nitrate in a nickel hydroxide shell. The combined use of in situ ultraviolet spectral scattering and ex-situ scanning electron microscopy, along with the simulations of the scattering spectra by Mie theory, allows us to compile a database of scattering spectra attributed to specific droplets or particles of given chemical properties and size which may be useful for the continuous detection and speciation of metallic aerosols at the exit of real plants.     

Hydrolytische Spaltung von Nucleinsäuren-vom Enzymmechanismus zum Enzymmodell

Detailed knowledge of enzyme mechanisms has previously led to the design of „enzyme mimetics”︁, small synthetic compounds that use the same principles of catalysis as enzymes but need a less complicated framework than proteins. The development of enzyme mimetics for the hydrolysis of RNA is of special interest because of their potential use as drugs in gene therapy. Field development from the first active compounds discovered in the late 80s to the application of highly specific artificial RNases in vitro is described.     

The Molecular and Electronic Structure of Nickelatetrazole – A Theoretical Study

Nickelatetrazoles have been proposed as intermediates in the course of the photoreaction of Ni^II complexes of [NiP₂(N₃)₂] constitution (P₂: mono‐ or diphosphane ligands). However, any metallatetrazoles as well as their organic analogue, 5 H‐carbatetrazole, could neither be prepared nor identified up to now. Based on density functional theory (DFT) calculations, predictions are given concerning the molecular and electronic structure of tetrazoles. While 5 H‐tetrazole is indeed a rather unstable species, metallatetrazole moieties in square‐planar d⁸ transition metal complexes should be experimentally accessible.     

Lipophilic oligonucleotides spontaneously insert into lipid membranes, bind complementary DNA strands, and sequester into lipid-disordered domains

For the development of surface functionalized bilayers, we have synthesized lipophilic oligonucleotides to combine the molecular recognition mechanism of nucleic acids and the self-assembly characteristics of lipids in planar membranes. A lipophilic oligonucleotide consisting of 21 thymidine units and two lipophilic nucleotides with an alpha-tocopherol moiety as a lipophilic anchor was synthesized using solid-phase methods with a phosphoramadite strategy. The interaction of the water soluble lipophilic oligonucleotide with vesicular lipid membranes and its capability to bind complementary DNA strands was studied using complementary methods such as NMR, EPR, DSC, fluorescence spectroscopy, and fluorescence microscopy. This oligonucleotide inserted stably into preformed membranes from the aqueous phase. Thereby, no significant perturbation of the lipid bilayer and its stability was observed. However, the non-lipidated end of the oligonucleotide is exposed to the aqueous environment, is relatively mobile, and is free to interact with complementary DNA strands. Binding of the complementary single-stranded DNA molecules is fast and accomplished by the formation of Watson-Crick base pairs, which was confirmed by 1H NMR chemical shift analysis and fluorescence resonance energy transfer. The molecular structure of the membrane bound DNA double helix is very similar to the free double-stranded DNA. Further, the membrane bound DNA double strands also undergo regular melting. Finally, in raft-like membrane mixtures, the lipophilic oligonucleotide was shown to preferentially sequester into liquid-disordered membrane domains.     

Dihydroxy-Acid Dehydratases From Pathogenic Bacteria: Emerging Drug Targets to Combat Antibiotic Resistance

There is an urgent global need for the development of novel therapeutics to combat the rise of various antibiotic-resistant superbugs. Enzymes of the branched-chain amino acid (BCAA) biosynthesis pathway are an attractive target for novel anti-microbial drug development. Dihydroxy-acid dehydratase (DHAD) is the third enzyme in the BCAA biosynthesis pathway. It relies on an Fe−S cluster for catalytic activity and has recently also gained attention as a catalyst in cell-free enzyme cascades. Two types of Fe−S clusters have been identified in DHADs, i.e. [2Fe−2S] and [4Fe−4S], with the latter being more prone to degradation in the presence of oxygen. Here, we characterise two DHADs from bacterial human pathogens, Staphylococcus aureus and Campylobacter jejuni (SaDHAD and CjDHAD). Purified SaDHAD and CjDHAD are virtually inactive, but activity could be reversibly reconstituted in vitro (up to ∼19,000-fold increase with k_cat as high as ∼6.7 s⁻¹). Inductively-coupled plasma-optical emission spectroscopy (ICP-OES) measurements are consistent with the presence of [4Fe−4S] clusters in both enzymes. N-isopropyloxalyl hydroxamate (IpOHA) and aspterric acid are both potent inhibitors for both SaDHAD (K_i=7.8 and 51.6 μM, respectively) and CjDHAD (K_i=32.9 and 35.1 μM, respectively). These compounds thus present suitable starting points for the development of novel anti-microbial chemotherapeutics.     

Molecular distribution of some commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Nonylphenol ethoxylates (NPEs) belong to a group of nonionic surfactants that are collectively referred to as alkylphenol ethoxylates (APEs). APEs find widespread use in heavy-duty commercial and household cleaning formulations, shampoos, and industrial processing, i.e. textile manufacture. Their environmental impact depends on the molecular distribution and the extent of their biodegradation in municipal sewage systems, waterways and rivers. We have established two sample preparation methods that have enabled the determination of the molecular distributions of six commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). In both methods, alpha-cyano-4-hydroxycinnamic acid, dissolved in acetonitrile/tetrahydrofuran, was used as the matrix. In one set of experiments, the NPEs were dissolved in an acetonitrile/tetrahydrofuran solvent system prior to mixing with the matrix solution, and the resulting MALDI-TOF mass spectra produced mostly sodiated molecules [M + Na](+). The NPEs, all having the formula 4-(C(9)H(19))-C(6)H(4)-(OCH(2)CH(2))(n)-OH, are Surfonic (R)N-95, N-100, N-102, N-120, N-150 and N-300. Surfonic N-95 and N-100 gave n values of 5-20; Surfonic N-102, N-120, N-150, and N-300 gave n values of 5-21, 5-22, 8-25 and 15-40, respectively. In order to develop a sample preparation method that could be used with less polar NPEs, we dissolved the NPEs (except N-300) in pentane prior to mixing with the matrix solution, and found that the MALDI spectral quality was unaffected by the solvent systems. Copyright 1999 John Wiley & Sons, Ltd.     

Quenched Lewis Acidity: Studies on the Medium Dependent Fluorescence of Zinc(II) Complexes

Three new zinc(II) coordination units [Zn(1–3)] based on planar-directing tetradentate Schiff base-like ligands H₂(1 – 3) were synthesized. Their solid-state structures were investigated by single crystal X-ray diffraction, showing the tendency to overcome the square-planar coordination sphere by axial ligation. Affinity in solution towards axial ligation has been tested by extended spectroscopic studies, both in the absorption and emission mode. The electronic spectrum of the pyridine complex [Zn(1)(py)] has been characterized by MC-PDFT to validate the results of extended TD-DFT studies. Green emission of non-emissive solutions of [Zn(1–3)] in chloroform could be switched on in the presence of potent Lewis-bases. While interpretation in terms of an equilibrium of stacked/non-fluorescent and destacked/fluorescent species is in line with precedents from literature, the sensitivity of [Zn(1–3)] was greatly reduced. Results of a computation-based structure search allow to trace the hidden Lewis acidity of [Zn(1–3)] to a new stacking motif, resulting in a strongly enhanced stability of the dimers.