JBIC Journal of Biological Inorganic Chemistry (v.23, #7)

β-Hydroxyaspartic acid in siderophores: biosynthesis and reactivity by Clifford D. Hardy; Alison Butler (957-967).
A growing number of siderophores are found to contain β-hydroxyaspartic acid (β-OH-Asp) as a functional group for Fe(III) coordination, along with the more common catechol and hydroxamic acid groups. This review covers the structures, biosynthesis, and reactions of peptidic β-OH-Asp siderophores. Hydroxylation of Asp in siderophore biosynthesis is predicted to be carried out either through discrete aspartyl β-hydroxylating enzymes or through hydroxylating domains within non-ribosomal peptide synthetases, both of which display sequence homology to known non-heme iron(II), α-ketoglutarate-dependent dioxygenases. Ferric complexes of β-OH-Asp siderophores are photoreactive, resulting in reduction of Fe(III) and oxidative cleavage of the siderophore to yield distinct types of photoproducts. Probing the photoreactivity of synthetic Fe(III)-α-hydroxycarboxylate clusters yields mechanistic insights into the different photoproducts observed for β-OH-Asp and other α-hydroxycarboxylate siderophore Fe(III) complexes.
Keywords: Siderophore; β-Hydroxyaspartic acid; Iron; Biosynthesis; Photoreactivity

The chemical biology and coordination chemistry of putrebactin, avaroferrin, bisucaberin, and alcaligin by Rachel Codd; Cho Zin Soe; Amalie A. H. Pakchung; Athavan Sresutharsan; Christopher J. M. Brown; William Tieu (969-982).
Dihydroxamic acid macrocyclic siderophores comprise four members: putrebactin (putH2), avaroferrin (avaH2), bisucaberin (bisH2), and alcaligin (alcH2). This mini-review collates studies of the chemical biology and coordination chemistry of these macrocycles, with an emphasis on putH2. These Fe(III)-binding macrocycles are produced by selected bacteria to acquire insoluble Fe(III) from the local environment. The macrocycles are optimally pre-configured for Fe(III) binding, as established from the X-ray crystal structure of dinuclear [Fe2(alc)3] at neutral pH. The dimeric macrocycles are biosynthetic products of two endo-hydroxamic acid ligands flanked by one amine group and one carboxylic acid group, which are assembled from 1,4-diaminobutane and/or 1,5-diaminopentane as initial substrates. The biosynthesis of alcH2 includes an additional diamine C-hydroxylation step. Knowledge of putH2 biosynthesis supported the use of precursor-directed biosynthesis to generate unsaturated putH2 analogues by culturing Shewanella putrefaciens in medium supplemented with unsaturated diamine substrates. The X-ray crystal structures of putH2, avaH2 and alcH2 show differences in the relative orientations of the amide and hydroxamic acid functional groups that could prescribe differences in solvation and other biological properties. Functional differences have been borne out in biological studies. Although evolved for Fe(III) acquisition, solution coordination complexes have been characterised between putH2 and oxido-V(IV/V), Mo(VI), or Cr(V). Retrosynthetic analysis of 1:1 complexes of [Fe(put)]+, [Fe(ava)]+, and [Fe(bis)]+ that dominate at pH < 5 led to a forward metal-templated synthesis approach to generate the Fe(III)-loaded macrocycles, with apo-macrocycles furnished upon incubation with EDTA. This mini-review aims to capture the rich chemistry and chemical biology of these seemingly simple compounds.
Keywords: Siderophores; Hydroxamic acid macrocycles; Putrebactin; Precursor-directed biosynthesis; Metal-templated synthesis

Multiple siderophores: bug or feature? by Darcy L. McRose; Mohammad R. Seyedsayamdost; François M. M. Morel (983-993).
It is common for bacteria to produce chemically diverse sets of small Fe-binding molecules called siderophores. Studies of siderophore bioinorganic chemistry have firmly established the role of these molecules in Fe uptake and provided great insight into Fe complexation. However, we still do not fully understand why microbes make so many siderophores. In many cases, the release of small structural variants or siderophore fragments has been ignored, or considered as an inefficiency of siderophore biosynthesis. Yet, in natural settings, microbes live in complex consortia and it has become increasingly clear that the secondary metabolite repertoires of microbes reflect this dynamic environment. Multiple siderophore production may, therefore, provide a window into microbial life in the wild. This minireview focuses on three biochemical routes by which multiple siderophores can be released by the same organism—multiple biosynthetic gene clusters, fragment release, and precursor-directed biosynthesis—and highlights emergent themes related to each. We also emphasize the plurality of reasons for multiple siderophore production, which include enhanced iron uptake via synergistic siderophore use, microbial warfare and cooperation, and non-classical functions such as the use of siderophores to take up metals other than Fe.
Keywords: Multiple siderophores; Secondary metabolites; Metallophores; Iron

A reevaluation of iron binding by Mycobactin J by Courtney F. McQueen; John T. Groves (995-1007).
The complex stability constant (log β 110) and the free iron concentration (pM) are used to compare the relative strength of iron binding by siderophores. Direct measurements of these thermodynamic parameters are often not possible for siderophores due to very large log β 110 values ranging from 30 to 50. Instead, siderophore iron(III)-binding constants are determined by competitive experiments with other strong chelators with known values, such as EDTA. Iron(III) binding constants of water-insoluble siderophores, such as the mycobactins produced by the mycobacterium family, have never been directly measured. Since mycobactins contain two hydroxamic acid binding motifs, their log β 110 values have been assumed to be comparable to those of other hydroxamate-based siderophores like desferrioxamine B, at ~ 30. However, exochelin MN, another mycobacterial siderophore that contains two hydroxamic acid moieties, has a log β 110 of 39.1 and a pM of 31.1, which makes it among the strongest siderophores known. We have found that mycobactin J, the amphiphilic siderophore of Mycobacterium paratuberculosis, can remove iron(III) from TrenCAM (log β 110 = 43.6) within 1 min in methanol. This surprising result indicates that log β 110 for mycobactin J is ~ 43 and the ligand exchange kinetics in methanol is fast. The results imply that mycobactins are capable of removing iron quickly from very strongly binding siderophores in a cellular milieu. We propose a model mechanism for iron acquisition by pathogenic mycobacteria in vivo. This model explains how the host iron captured by siderophores can be returned to the invading pathogen even in the absence of active uptake mechanisms.
Keywords: Iron acquisition; Carboxymycobactin; Exochelin; Bacterial microvesicles

In the original publication, third author’s name was incorrectly published as Aneta L. Jelowicki. Vibrio campbellii BAA-1116 (formerly Vibrio harveyi) is a model organism for quorum sensing study and produces the siderophores anguibactin and amphi-enterobactin. This study examined the mechanisms and specificity of siderophore uptake in V. campbellii and V. harveyi, and surveyed the diversity of siderophore production in V. campbellii and V. harveyi strains. The amphi-enterobactin gene cluster of BAA-1116 harbors a gene, named fapA, that is a homologue of genes encoding Fe(III)-siderophore-specific outer membrane receptors. Another strain, V. campbellii HY01, a strain pathogenic to shrimp, also carries this cluster including fapA. Our siderophore bioassay results using HY01-derived indicator strains show that the FapA protein localized in the outer membrane fraction of V. campbellii HY01 is essential for the uptake of Fe(III)-amphi-enterobactin as well as exogenous siderophores, including enterobactin from E. coli, but not vanchrobactin from V. anguillarum RV22 while Fe(III)-amphi-enterobactin can be utilized by V. anguillarum. Electrospray ionization mass spectrometry as well as bioassay revealed that various V. campbellii and V. harveyi strains produce a suite of amphi-enterobactins with various fatty acid appendages, including several novel amphi-enterobactins, and these amphi-enterobactins can be taken up by V. campbellii HY01 via FapA, indicating that amphi-enterobactin production is a common phenotype among V. campbellii and V. harveyi, whereas our previous work, confirmed herein, showed that anguibactin is only produced by V. campbellii strains. These results along with the additional finding that a 2,3-dihydroxybenzoic acid biosynthesis gene, aebA, located in the amphi-enterobactin gene cluster, is essential for both anguibactin and amphi-enterobactin biosynthesis, suggest the possibility that amphi-enterobactin is a native siderophore of V. campbellii and V. harveyi, while the anguibactin system has been acquired by V. campbellii during evolution.
Keywords: Vibrio campbellii ; Vibrio harveyi ; Amphi-enterobactin; Anguibactin; Fe(III)-siderophore receptor

In the original publication, third author’s name was incorrectly published as Aneta L. Jelowicki.

Bacterial iron uptake machinery can be hijacked for the targeted delivery of antibiotics into pathogens by attaching antibiotics to siderophores, iron chelators that are employed by bacteria to obtain this essential nutrient. We synthesized and evaluated Ent–SS–Cipro, a siderophore–antibiotic conjugate comprised of the triscatecholate siderophore enterobactin and the fluoroquinolone antibiotic ciprofloxacin that contains a self-immolative disulfide linker. This linker is designed to be cleaved after uptake into the reducing environment of the bacterial cytoplasm. We show that the disulfide bond of Ent–SS–Cipro is cleaved by reducing agents, including the cellular reductant glutathione, which results in release of the unmodified fluoroquinolone antibiotic. Antibacterial activity assays against a panel of Escherichia coli show that Ent–SS–Cipro exhibits activity against some, but not all, E. coli. This work informs the design of siderophore–antibiotic conjugates, particularly those carrying antibiotics with cytoplasmic targets that require release after uptake into bacterial cells, and indicates that disulfide linkers may not be generally applicable for conjugation strategies of antibiotics.
Keywords: Siderophore; Antibiotic conjugate; Disulfide linker; Targeted delivery; Iron uptake

In methylotrophic bacteria, which use one-carbon (C1) compounds as a carbon source, methanol is oxidized by pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenase (MDH) enzymes. Methylotrophic genomes generally encode two distinct MDHs, MxaF and XoxF. MxaF is a well-studied, calcium-dependent heterotetrameric enzyme whereas XoxF is a lanthanide-dependent homodimer. Recent studies suggest that XoxFs are likely the functional MDHs in many environments. In methanotrophs, methylotrophs that utilize methane, interactions between particulate methane monooxygenase (pMMO) and MxaF have been detected. To investigate the possibility of interactions between pMMO and XoxF, XoxF was isolated from the methanotroph Methylomicrobium buryatense 5GB1C (5G-XoxF). Purified 5G-XoxF exhibits a specific activity of 0.16 μmol DCPIP reduced min−1 mg−1. The 1.85 Å resolution crystal structure reveals a La(III) ion in the active site, in contrast to the calcium ion in MxaF. The overall fold is similar to other MDH structures, but 5G-XoxF is a monomer in solution. An interaction between 5G-XoxF and its cognate pMMO was detected by biolayer interferometry, with a K D value of 50 ± 17 μM. These results suggest an alternative model of MDH-pMMO association, in which a XoxF monomer may bind to pMMO, and underscore the potential importance of lanthanide-dependent MDHs in biological methane oxidation.
Keywords: Lanthanide; Methanol dehydrogenase; Methanotroph; XoxF; Particulate methane monooxygenase

Crystal structure of VnfH, the iron protein component of vanadium nitrogenase by Michael Rohde; Christian Trncik; Daniel Sippel; Stefan Gerhardt; Oliver Einsle (1049-1056).
Nitrogenases catalyze the biological fixation of inert N2 into bioavailable ammonium. They are bipartite systems consisting of the catalytic dinitrogenase and a complementary reductase, the Fe protein that is also the site where ATP is hydrolyzed to drive the reaction forward. Three different subclasses of dinitrogenases are known, employing either molybdenum, vanadium or only iron at their active site cofactor. Although in all these classes the mode and mechanism of interaction with Fe protein is conserved, each one encodes its own orthologue of the reductase in the corresponding gene cluster. Here we present the 2.2 Å resolution structure of VnfH from Azotobacter vinelandii, the Fe protein of the alternative, vanadium-dependent nitrogenase system, in its ADP-bound state. VnfH adopts the same conformation that was observed for NifH, the Fe protein of molybdenum nitrogenase, in complex with ADP, representing a state of the functional cycle that is ready for reduction and subsequent nucleotide exchange. The overall similarity of NifH and VnfH confirms the experimentally determined cross-reactivity of both ATP-hydrolyzing reductases.
Keywords: Biological nitrogen fixation; Vanadium nitrogenase; Fe protein; Dinitrogenase reductase; X-ray crystallography

The Asp99–Arg188 salt bridge of the Pseudomonas aeruginosa HemO is critical in allowing conformational flexibility during catalysis by Geoffrey A. Heinzl; Weiliang Huang; Elizabeth Robinson; Fengtian Xue; Pierre Möenne-Loccoz; Angela Wilks (1057-1070).
In the original publication, fifth author’s name was incorrectly published as Pierre Moenne-Loccoz.The P. aeruginosa iron-regulated heme oxygenase (HemO) is required within the host for the utilization of heme as an iron source. As iron is essential for survival and virulence, HemO represents a novel antimicrobial target. We recently characterized small molecule inhibitors that bind to an allosteric site distant from the heme pocket, and further proposed binding at this site disrupts a nearby salt bridge between D99 and R188. Herein, through a combination of site-directed mutagenesis and hydrogen–deuterium exchange mass spectrometry (HDX-MS), we determined that the disruption of the D99–R188 salt bridge leads to significant decrease in conformational flexibility within the distal and proximal helices that form the heme-binding site. The RR spectra of the resting state Fe(III) and reduced Fe(II)-deoxy heme-HemO D99A, R188A and D99/R188A complexes are virtually identical to those of wild-type HemO, indicating no significant change in the heme environment. Furthermore, mutation of D99 or R188 leads to a modest decrease in the stability of the Fe(II)-O2 heme complex. Despite this slight difference in Fe(II)-O2 stability, we observe complete loss of enzymatic activity. We conclude the loss of activity is a result of decreased conformational flexibility in helices previously shown to be critical in accommodating variation in the distal ligand and the resulting chemical intermediates generated during catalysis. Furthermore, this newly identified allosteric binding site on HemO represents a novel alternative drug-design strategy to that of competitive inhibition at the active site or via direct coordination of ligands to the heme iron.
Keywords: Pseudomonas aeruginosa ; Heme oxygenase; Biliverdin; Oxygen activation; Protein dynamics

Correction to: The Asp99–Arg188 salt bridge of the Pseudomonas aeruginosa HemO is critical in allowing conformational flexibility during catalysis by Geoffrey A. Heinzl; Weiliang Huang; Elizabeth Robinson; Fengtian Xue; Pierre Möene-Loccoz; Angela Wilks (1071-1071).
In the original publication, fifth author’s name was incorrectly published as Pierre Moenne-Loccoz.

Influence of heme c attachment on heme conformation and potential by Jesse G. Kleingardner; Benjamin D. Levin; Giorgio Zoppellaro; K. Kristoffer Andersson; Sean J. Elliott; Kara L. Bren (1073-1083).
Heme c is characterized by its covalent attachment to a polypeptide. The attachment is typically to a CXXCH motif in which the two Cys form thioether bonds with the heme, “X” can be any amino acid other than Cys, and the His serves as a heme axial ligand. Some cytochromes c, however, contain heme attachment motifs with three or four intervening residues in a CX3CH or CX4CH motif. Here, the impacts of these variations in the heme attachment motif on heme ruffling and electronic structure are investigated by spectroscopically characterizing CX3CH and CX4CH variants of Hydrogenobacter thermophilus cytochrome c 552. In addition, a novel CXCH variant is studied. 1H and 13C NMR, EPR, and resonance Raman spectra of the protein variants are analyzed to deduce the extent of ruffling using previously reported relationships between these spectral data and heme ruffling. In addition, the reduction potentials of these protein variants are measured using protein film voltammetry. The CXCH and CX4CH variants are found to have enhanced heme ruffling and lower reduction potentials. Implications of these results for the use of these noncanonical motifs in nature, and for the engineering of novel heme peptide structures, are discussed.
Keywords: Heme distortion; Heme ruffling; Reduction potential; Cytochrome c ; Cytochrome c maturation

Spectroscopic evidence supporting neutral thiol ligation to ferrous heme iron by Masanori Sono; Shengfang Sun; Anuja Modi; Mark S. Hargrove; Bastian Molitor; Nicole Frankenberg-Dinkel; John H. Dawson (1085-1092).
The binding of neutral thiol (ethanethiol, EtSH) or thioether (tetrahydrothiophene, THT) to two types of heme proteins in their ferrous state has been investigated with UV–visible (UV–Vis) absorption and magnetic circular dichroism spectroscopy. For the second GAF (cGMP-specific phosphodiesterases, adenylyl cyclases, and FhlA) domain from the sensory kinase MsmS (sGAF2), stepwise additions of these respective two sulfur-donor ligands to its dithionite-reduced ferrous form generate homogeneous six-coordinate low-spin ferrous complexes at both pHs 7.0 and 5.4. Similar complexes were partially formed for deoxyferrous soybean leghemoglobin with EtSH or THT within their solubility limits in water. The titrations cause significant UV–Vis spectra changes attributable to a five-coordinate to six-coordinate heme iron coordination change. For sGAF2, the resulting spectra are essentially identical for the both ligands, clearly indicating the direct binding of neutral thiol/thioether to ferrous heme iron as the distal ligand. On the other hand, the thiol EtSH binds to ferric sGAF2 in the anionic thiolate form, while thioether THT forms its ferric sGAF2 complex as a neutral ligand. These observations provide compelling evidence that neutral cysteine is a plausible ligand for ferrous heme proteins.
Keywords: Thiols as heme axial ligands; Magnetic circular dichroism spectroscopy; Soybean leghemoglobin; Sensory kinase MsmS; The second GAF domain (sGAF2)

Mn(III) species formed by the multi-copper oxidase MnxG investigated by electron paramagnetic resonance spectroscopy by Lizhi Tao; Troy A. Stich; Alexandra V. Soldatova; Bradley M. Tebo; Thomas G. Spiro; William H. Casey; R. David Britt (1093-1104).
The multi-copper oxidase (MCO) MnxG from marine Bacillus bacteria plays an essential role in geochemical cycling of manganese by oxidizing Mn2+(aq) to form manganese oxide minerals at rates that are three to five orders of magnitude faster than abiotic rates. The MCO MnxG protein is isolated as part of a multi-protein complex, denoted as Mnx, which includes one MnxG unit and a hexamer of MnxE3F3 subunit. During the oxidation of Mn2+(aq) catalyzed by the Mnx protein complex, an enzyme-bound Mn(III) species was trapped recently in the presence of pyrophosphate (PP) and analyzed using parallel-mode electron paramagnetic resonance (EPR) spectroscopy. Herein, we provide a full analysis of this enzyme-bound Mn(III) intermediate via temperature dependence studies and spectral simulations. This Mnx-bound Mn(III) species is characterized by a hyperfine-coupling value of A(55Mn) = 4.2 mT (corresponding to 120 MHz) and a negative zero-field splitting (ZFS) value of D = − 2.0 cm−1. These magnetic properties suggest that the Mnx-bound Mn(III) species could be either six-coordinate with a 5B1g ground state or square-pyramidal five-coordinate with a 5B1 ground state. In addition, as a control, Mn(III)PP is also analyzed by parallel-mode EPR spectroscopy. It exhibits distinctly different magnetic properties with a hyperfine-coupling value of A(55Mn) = 4.8 mT (corresponding to 140 MHz) and a negative ZFS value of D = − 2.5 cm−1. The different ZFS values suggest differences in ligand environment of Mnx-bound Mn(III) and aqueous Mn(III)PP species. These studies provide further insights into the mechanism of biological Mn2+(aq) oxidation.
Keywords: Parallel-mode EPR; Multi-copper oxidase MnxG; Mnx protein complex; Mn(II) oxidation; Zero-field splitting

Mercury-induced aggregation of human lens γ-crystallins reveals a potential role in cataract disease by J. A. Domínguez-Calva; M. L. Pérez-Vázquez; E. Serebryany; J. A. King; L. Quintanar (1105-1118).
Cataract disease results from non-amyloid aggregation of eye lens proteins and is the leading cause of blindness in the world. A variety of studies have implicated both essential and xenobiotic metals as potential etiological agents in cataract disease. Essential metal ions, such as copper and zinc, are known to induce the aggregation in vitro of human γD crystallin, one of the more abundant γ-crystallins in the core of the lens. In this study, we expand the investigation of metal–crystallin interactions to heavy metal ions, such as divalent lead, cadmium and mercury. The impact of these metal ions in the non-amyloid aggregation, protein folding and thermal stability of three homologous human lens γ-crystallins has been evaluated using turbidity assays, electron microscopy, electronic absorption and circular dichroism spectroscopies. Our results show that Hg(II) ions can induce the non-amyloid aggregation of human γC and γS crystallins, but not γD crystallin. The mechanism of Hg-induced aggregation involves direct metal–protein interactions, loss of thermal stability, partial unfolding of the N-terminal domain of these proteins, and formation of disulfide-bridged dimers. Putative Hg(II) binding sites in γ-crystallins involved in metal-induced aggregation are discussed. This study reveals that mercury ions can induce the aggregation of human lens proteins, uncovering a potential role of this heavy metal ion in the bioinorganic chemistry of cataract disease.
Keywords: Lens crystallins; Human gamma crystallin; Mercury; Heavy metal ions; Cataract disease

Emission of volatile halogenated compounds, speciation and localization of bromine and iodine in the brown algal genome model Ectocarpus siliculosus by Frithjof C. Küpper; Eric P. Miller; Stephen J. Andrews; Claire Hughes; Lucy J. Carpenter; Wolfram Meyer-Klaucke; Chiaki Toyama; Yasuyuki Muramatsu; Martin C. Feiters; Carl J. Carrano (1119-1128).
This study explores key features of bromine and iodine metabolism in the filamentous brown alga and genomics model Ectocarpus siliculosus. Both elements are accumulated in Ectocarpus, albeit at much lower concentration factors (2-3 orders of magnitude for iodine, and < 1 order of magnitude for bromine) than e.g. in the kelp Laminaria digitata. Iodide competitively reduces the accumulation of bromide. Both iodide and bromide are accumulated in the cell wall (apoplast) of Ectocarpus, with minor amounts of bromine also detectable in the cytosol. Ectocarpus emits a range of volatile halogenated compounds, the most prominent of which by far is methyl iodide. Interestingly, biosynthesis of this compound cannot be accounted for by vanadium haloperoxidase since the latter have not been found to catalyze direct halogenation of an unactivated methyl group or hydrocarbon so a methyl halide transferase-type production mechanism is proposed.
Keywords: Energy-dispersive X-ray analysis; Halocarbons; Methyl iodide; Phaeophyta; X-ray absorption spectroscopy

Isosteres of hydroxypyridinethione as drug-like pharmacophores for metalloenzyme inhibition by Rebecca N. Adamek; Cy V. Credille; Benjamin L. Dick; Seth M. Cohen (1129-1138).
Hydroxypyridinethiones (HOPTOs) are strong ligands for metal ions and potentially useful pharmacophores for inhibiting metalloenzymes relevant to human disease. However, HOPTOs have been sparingly used in drug discovery efforts due, in part, to concerns that this scaffold will act as a promiscuous, non-selective metalloenzyme inhibitor, as well as possess poor pharmacokinetics (PK), which may undermine drug candidates containing this functional group. To advance HOPTOs as a useful pharmacophore for metalloenzyme inhibitors, a library of 22 HOPTO isostere compounds has been synthesized and investigated. This library demonstrates that it is possible to maintain the core metal-binding pharmacophore (MBP) while generating diversity in structure, electronics, and PK properties. This HOPTO library has been screened against a set of four different metalloenzymes, demonstrating that while the same metal-binding donor atoms are maintained, there is a wide range of activity between metalloenzyme targets. Overall, this work shows that HOPTO isosteres are useful MBPs and valuable scaffolds for metalloenzyme inhibitors.
Keywords: Fragment-based drug discovery; Hydroxypyridinethione; Inhibitor; Isostere; Metal-binding pharmacophore; Metalloenzyme

Rhenium and technetium complexes of thioamide derivatives of pyridylhydrazine that bind to amyloid-β plaques by Scott P. Fletcher; Asif Noor; James L. Hickey; Catriona A. McLean; Jonathan M. White; Paul S. Donnelly (1139-1151).
Age-associated deposition of amyloid-β in cerebral blood vessels, a condition referred to as cerebral amyloid angiopathy, can contribute to stroke and dementia. This research aimed to design new radioactive technetium-99 m complexes that bind to amyloid-β plaques that have the potential to assist in diagnosis of cerebral amyloid angiopathy using single-photon-emitted computed tomography (SPECT) imaging. Six new pyridylthiosemicarbazide ligands containing either benzofuran or styrylpyridyl functional groups that are known to selectively bind to amyloid plaques were prepared. Non-radioactive isotopes of technetium are not available so rhenium was used as a surrogate for exploratory chemistry. The new ligands were used to prepare well-defined [Re-oxo]3+ complexes where two pyridylthiosemicarbazide ligands were coordinated to a single metal ion to give bivalent complexes with two amyloid-β targeting functional groups. The interaction of the [Re-oxo]3+ complexes with synthetic amyloid-β1-42 and with amyloid plaques in human brain tissue was investigated. Two ligands were selected to develop methods to prepare their [99mTc-oxo]3+ complexes at the tracer level. These technetium-99 m complexes are likely to be isostructural to their rhenium-oxo analogues.
Keywords: Technetium; Rhenium; Amyloid-β; Imaging

A metallo-biopolymer conjugate of elastin-like polypeptide: photoluminescence enhancement in the coacervate microenvironment by Koushik Ghosh; Katherine C. Elbert; Eva Rose M. Balog; Jennifer S. Martinez; Reginaldo C. Rocha (1153-1157).
An optically active metallo-polymer assembly is demonstrated via conjugation of a genetically engineered elastin-like polypeptide (ELP) and a ruthenium(II) polypyridyl complex. By taking advantage of the phase transition of ELPs in water, photophysical properties of the resultant conjugate are investigated for both phases, below and above the critical transition temperature. Upon coacervation, the luminescence of the metallo-ELP is greatly enhanced as a consequence of local effects on the metal–ligand luminophore. These findings open a possibility to harness the temperature control of stimuli-responsive properties of biopolymers.
Keywords: Bioconjugation; Elastin-like polymers; Luminescence; Metal complex; Optical biomaterials

Understanding the interactions of diruthenium anticancer agents with amino acids by Alexey A. Nazarov; Maria-Grazia Mendoza-Ferri; Muhammad Hanif; Bernhard K. Keppler; Paul J. Dyson; Christian G. Hartinger (1159-1164).
The dinuclear anticancer agents 1,n-bis{chlorido[3-(oxo-κO)-2-methyl-4-(1H)-pyridinonato-κO4](η6-p-cymene)-ruthenium(II)}alkane (PyRu2 n ) exhibit high antiproliferative activity in human cancer cell. Reactivity studies with DNA and protein revealed uncommon protein–DNA and DNA–DNA crosslinking ability. We report here studies on the reactions of the diruthenium organometallics PyRu26 and PyRu28 in comparison with a mononuclear analogue PyRu3 with amino acids using mass spectrometry and NMR spectroscopy. The compounds behave very similarly, independent of the spacer length between the metal center and of the nuclearity of the complexes. Incubation with l-cysteine (Cys) results in fast release of the pyridone ligand, with the Ru complexes able to form Cys adducts. In contrast, l-methionine forms, initially, adducts with the metal centers, but over time, the adducts decompose. Similar behavior was observed for the reaction with l-histidine with [Ru(η6-p-cymene)(l-histidine)] species detected.
Keywords: Amino acid interaction; Bioorganometallics; Dinuclear compounds; Mass spectrometry; NMR spectroscopy; Ruthenium(arene) complexes

Efficient copper-based DNA cleavers from carboxylate benzimidazole ligands by Víctor A. Barrera-Guzmán; Edgar O. Rodríguez-Hernández; Naytzé Ortíz-Pastrana; Ricardo Domínguez-González; Ana B. Caballero; Patrick Gamez; Norah Barba-Behrens (1165-1183).
Four copper(II) coordination compounds from 2-benzimidazole propionic acid (Hbzpr) and 4-(benzimidazol-2-yl)-3-thiobutanoic acid (Hbztb) were synthesized and fully characterized by elemental analyses, electronic spectroscopy, FT-IR and mass spectrometry. The molecular structure for the four complexes was confirmed by single-crystal X-ray crystallography. The DNA-interacting properties of the two trinuclear and two mononuclear compounds were investigated using different spectroscopic techniques including absorption titration experiments, fluorescence spectroscopy and circular dichroism spectroscopy. Trinuclear [Cu3(bzpr)4(H2O)2](NO3)2·3H2O·CH3OH (2) and [Cu3(bzpr)4Cl2]·3H2O (3) bind to DNA through non-intercalative interactions, while for mononuclear [Cu(bzpr)2(H2O)]·2H2O (1) and [Cu(bztb)2]·2H2O (4), at minor concentrations in relation to the DNA, a groove binding interaction is favored, while at higher concentrations an intercalative mode is preferred. The nuclease properties of all complexes were studied by gel electrophoresis, which showed that they were able to cleave supercoiled plasmid DNA (form I) to the nicked form (form II). Compound 4 is even capable of generating linear form III (resulting from double-strand cleavage). The proposed mechanism of action involves an oxidative pathway (Fenton-type reaction), which produces harmful reactive species, like hydroxyl radicals.
Keywords: Copper(II) complexes; 2-Carboxylate benzimidazoles; ct-DNA; pBr322 DNA; DNA-cleaving properties