Understanding the mechanism by which Gd(III) coiled coils achieve magnetic resonance relaxivity - a study into the water coordination chemistry

A class of Gd(III) coiled coils achieve high MRI relaxivity, in part due to their slow rotational correlation time. However, extending their length is unable to further enhance performance, as the mechanism by which relaxivity is achieved is dominated by the presence of three inner sphere waters in rapid exchange, through an associative mechanism.

Design of the elusive proteinaceous oxygen donor copper site suggests a promising future for copper for MRI contrast agents

We report the preparation and spectroscopic characterization of a highly elusive copper site bound exclusively to oxygen donor atoms within a protein scaffold. Despite copper generally being considered unsuitable for use in MRI contrast agents, which in the clinic are largely Gd(III) based, the designed copper coiled coil displays relaxivity values equal to, or superior than, those of the Gd(III) analog at clinical field strengths. The creation of this new-to-biology proteinaceous CuO_x-binding site demonstrates the power of the de novo peptide design approach to access chemistry for abiological applications, such as for the development of MRI contrast agents.

Correction: An in vitro method for inducing titan cells reveals novel features of yeast-to-titan switching in the human fungal pathogen Cryptococcus gattii

[This corrects the article DOI: 10.1371/journal.ppat.1010321.].

An in vitro method for inducing titan cells reveals novel features of yeast-to-titan switching in the human fungal pathogen Cryptococcus gattii

Cryptococcosis is a potentially lethal fungal infection of humans caused by organisms within the Cryptococcus neoformans/gattii species complex. Whilst C. neoformans is a relatively common pathogen of immunocompromised individuals, C. gattii is capable of acting as a primary pathogen of immunocompetent individuals. Within the host, both species undergo morphogenesis to form titan cells: exceptionally large cells that are critical for disease establishment. To date, the induction, defining attributes, and underlying mechanism of titanisation have been mainly characterized in C. neoformans. Here, we report the serendipitous discovery of a simple and robust protocol for in vitro induction of titan cells in C. gattii. Using this in vitro approach, we reveal a remarkably high capacity for titanisation within C. gattii, especially in strains associated with the Pacific Northwest Outbreak, and characterise strain-specific differences within the clade. In particular, this approach demonstrates for the first time that cell size changes, DNA amplification, and budding are not always synchronous during titanisation. Interestingly, however, exhibition of these cell cycle phenotypes was correlated with genes associated with cell cycle progression including CDC11, CLN1, BUB2, and MCM6. Finally, our findings reveal exogenous p-Aminobenzoic acid to be a key inducer of titanisation in this organism. Consequently, this approach offers significant opportunities for future exploration of the underlying mechanism of titanisation in this genus.

Location-Dependent Lanthanide Selectivity Engineered into Structurally Characterized Designed Coiled Coils

Herein we report unprecedented location-dependent, size-selective binding to designed lanthanide (Ln3+ ) sites within miniature protein coiled coil scaffolds. Not only do these engineered sites display unusual Ln3+ selectivity for moderately large Ln3+ ions (Nd to Tb), for the first time we demonstrate that selectivity can be location-dependent and can be programmed into the sequence. A 1 nm linear translation of the binding site towards the N-terminus can convert a selective site into a highly promiscuous one. An X-ray crystal structure, the first of a lanthanide binding site within a coiled coil to be reported, coupled with CD studies, reveal the existence of an optimal radius that likely stems from the structural constraints of the coiled coil scaffold. To the best of our knowledge this is the first report of location-dependent metal selectivity within a coiled coil scaffold, as well as the first report of location-dependent Ln3+ selectivity within a protein.

Stability of Cell-Penetrating Peptide anti-VEGF Formulations for the Treatment of Age-Related Macular Degeneration

AIM

The development of a polyarginine cell-penetrating peptide (CPP) could enable the treatment of age-related macular degeneration, with drugs like bevacizumab, to be administered using eye drops instead of intravitreal injections. Topical formulations have a vast potential impact on healthcare by increasing patient compliance while reducing the financial burden. However, as the ocular preparations may contain several doses, it is essential to understand the stability of the bevacizumab+CPP conjugate produced.

MATERIALS AND METHODS

In this work, we examine the stability of a bevacizumab solution with and without cell-penetrating peptide using dynamic light scattering and circular dichroism to assess the physical stability. We use HPLC to assess the chemical stability and ELISA to assess its biological activity. We also examine the potential of the CPP to be used as an antimicrobial agent in place of preservatives in the eye drop.

RESULTS

The structural stability of bevacizumab with and without the CPP was found not to be affected by temperature: samples stored at either 20°C or 4°C were identical in behavior. However, physical instability was observed after five weeks, leading to aggregation and precipitation. Further investigation revealed that the addition of the polypeptide led to increased aggregation, as revealed through dynamic light scattering and concentration analysis of the peptide through HPLC. Complexing the bevacizumab with CPP had no effect on biological stability or degradation.

CONCLUSIONS

Our findings suggest that the shelf life of CPP+bevacizumab complexes is at least 38 days from its initial formulation. Currently, the mechanism for aggregation is not fully understood but does not appear to occur through chemical degradation.

De novo designed coiled coils as scaffolds for lanthanides, including novel imaging agents with a twist

The design of artificial miniature lanthanide proteins, provide an opportunity to access new functional metalloproteins as well as insight into native lanthanide biochemistry.

Tuning coordination chemistry through the second sphere in designed metallocoiled coils

The metal hydration state within a designed coiled coil can be progressively tuned across the full integer range (3 → 0 aqua ligands), by careful choice of a second sphere terminal residue, including the lesser used Trp. Potential implications include a four-fold change in MRI relaxivity when applied to lanthanide coiled coils.

Light-controlled thrombin catalysis and clot formation using a photoswitchable G-quadruplex DNA aptamer

The reversible photocontrol of an enzyme governing blood coagulation is demonstrated. The thrombin binding aptamer (TBA), was rendered photochromic by modification with two anthracene groups. Light-triggered anthracene photodimerisation distorts its structure, inhibiting binding of the enzyme thrombin, which in turn triggers catalysis and the resulting clotting process.

Structural Determinants of the Stability of Enzyme-Responsive Polyion Complex Nanoparticles Targeting Pseudomonas aeruginosa's Elastase

Here, we report how the stability of polyion complex (PIC) particles containing Pseudomonas aeruginosa's elastase (LasB) degradable peptides and antimicrobial poly(ethylene imine) is significantly improved by careful design of the peptide component. Three LasB-degradable peptides are reported herein, all of them carrying the LasB-degradable sequence -GLA- and for which the number of anionic amino acids and cysteine units per peptide were systematically varied. Our results suggest that while net charge and potential to cross-link via disulfide bond formation do not have a predictable effect on the ability of LasB to degrade these peptides, a significant effect of these two parameters on particle preparation and stability is observed. A range of techniques has been used to characterize these new materials and demonstrates that increasing the charge and cross-linking potential of the peptides results in PIC particles with better stability in physiological conditions and upon storage. These results highlight the importance of molecular design for the preparation of PIC particles and should underpin the future development of these materials for responsive drug delivery.

pH dependent binding in de novo hetero bimetallic coiled coils

Herein the first example of a bimetallic coiled coil featuring a lanthanide binding site is reported, opening opportunities to exploit the attractive NMR and photophysical properties of the lanthanides in multi metallo protein design. In our efforts to fully characterise the system we identified for the first time that lanthanide binding to such sites is pH dependent, with optimal binding at neutral pH, and that the double AsnAsp site is more versatile in this regard than the single Asp site. Our second site featured the structural HgCys₃ site, the chemistry of which was essentially unaltered by the presence of the lanthanide site. In fact, both metal binding sites within the hetero bimetallic coiled coil displayed the same properties as their mononuclear single binding site controls, and operated independently of each other. Finally, pH can be used as an external trigger to control the binding of Hg(ii) and Tb(iii) to the two distinct sites within this coiled coil, and offers the opportunity to "activate" metal binding sites within complex multi metallo and multi-functional designs.

Topical Delivery of Anti-VEGF Drugs to the Ocular Posterior Segment Using Cell-Penetrating Peptides

Purpose

To evaluate the efficacy of anti-VEGF agents for treating choroidal neovascularization (CNV) when delivered topically using novel cell-penetrating peptides (CPPs) compared with delivery by intravitreal (ivit) injection.

Methods

CPP toxicity was investigated in cell cultures. Ivit concentrations of ranibizumab and bevacizumab after topical administration were measured using ELISA. The biological efficacy of topical anti-VEGF + CPP complexes was compared with ivit anti-VEGF injections using an established model of CNV.

Results

CPPs were nontoxic in vitro. In vivo, after topical eye drop delivery, CPPs were present in the rat anterior chamber within 6 minutes. A single application of CPP + bevacizumab eye drop delivered clinically relevant concentrations of bevacizumab to the posterior chamber of the rat eye in vivo. Similarly, clinically relevant levels of CPP + ranibizumab and CPP + bevacizumab were detected in the porcine vitreous and retina ex vivo. In an established model of CNV, mice treated with either a single ivit injection of anti-VEGF, twice daily CPP + anti-VEGF eye drops or daily dexamethasone gavage for 10 days all had significantly reduced areas of CNV when compared with lasered eyes without treatment.

Conclusions

CPPs are nontoxic to ocular cells and can be used to deliver therapeutically relevant doses of ranibizumab and bevacizumab by eye drop to the posterior segment of mouse, rat, and pig eyes. The CPP + anti-VEGF drug complexes were cleared from the retina within 24 hours, suggesting a daily eye drop dosing regimen. Daily, topically delivered anti-VEGF with CPP was as efficacious as a single ivit injection of anti-VEGF in reducing areas of CNV in vivo.

d-Cysteine Ligands Control Metal Geometries within De Novo Designed Three-Stranded Coiled Coils

Although metal ion binding to naturally occurring l-amino acid proteins is well documented, understanding the impact of the opposite chirality (d-)amino acids on the structure and stereochemistry of metals is in its infancy. We examine the effect of a d-configuration cysteine within a designed l-amino acid three-stranded coiled coil in order to enforce a precise coordination number on a metal center. The d chirality does not alter the native fold, but the side-chain re-orientation modifies the sterics of the metal binding pocket. l-Cys side chains within the coiled-coil structure have previously been shown to rotate substantially from their preferred positions in the apo structure to create a binding site for a tetra-coordinate metal ion. However, here we show by X-ray crystallography that d-Cys side chains are preorganized within a suitable geometry to bind such a ligand. This is confirmed by comparison of the structure of Zn^II Cl(CSL16_D C)₃^2- to the published structure of Zn^II (H₂ O)(GRAND-CSL12AL16_L C)₃^- . Moreover, spectroscopic analysis indicates that the Cd^II geometry observed by using l-Cys ligands (a mixture of three- and four-coordinate Cd^II ) is altered to a single four-coordinate species when d-Cys is present. This work opens a new avenue for the control of the metal site environment in man-made proteins, by simply altering the binding ligand with its mirror-imaged d configuration. Thus, the use of non-coded amino acids in the coordination sphere of a metal promises to be a powerful tool for controlling the properties of future metalloproteins.

Preparation and antimicrobial evaluation of polyion complex (PIC) nanoparticles loaded with polymyxin B

•

We report the preparation of PIC particles loaded with clinically relevant antibiotic Polymyxin B.

•

PIC particle formulation is optimised to accommodate different loadings of Polymyxin B.

•

PIC particles inhibit the growth of Pseudomonas aeruginosa in a particle dependent manner.

Here, we describe novel polyion complex (PIC) particles for the delivery of Polymyxin B (Pol-B), an antimicrobial peptide currently used in the clinic as a last resort antibiotic against multidrug-resistant gram-negative bacteria. A range of conditions for the controlled assembly of Pol-B with poly(styrene sulphonate) (PSS) has been identified which let us prepare stable colloidal PIC particles. This way, PIC particles containing different Pol-B:PSS ratios have been prepared and their stability under simulated physiological conditions (i.e. pH, osmotic pressure and temperature) characterised. Furthermore, preliminary evaluation of the antimicrobial activity of these Pol-B containing PIC particles has been performed, by monitoring their effect on the growth of Pseudomonas aeruginosa, an opportunistic gram-negative bacterium.

Antimicrobial peptide coatings for hydroxyapatite: electrostatic and covalent attachment of antimicrobial peptides to surfaces

The interface between implanted devices and their host tissue is complex and is often optimized for maximal integration and cell adhesion. However, this also gives a surface suitable for bacterial colonization. We have developed a novel method of modifying the surface at the material-tissue interface with an antimicrobial peptide (AMP) coating to allow cell attachment while inhibiting bacterial colonization. The technology reported here is a dual AMP coating. The dual coating consists of AMPs covalently bonded to the hydroxyapatite surface, followed by deposition of electrostatically bound AMPs. The dual approach gives an efficacious coating which is stable for over 12 months and can prevent colonization of the surface by both Gram-positive and Gram-negative bacteria.

Enzyme-responsive polyion complex (PIC) nanoparticles for the targeted delivery of antimicrobial polymers

Here we present new enzyme-responsive polyion complex (PIC) nanoparticles prepared from antimicrobial poly(ethylene imine) and an anionic enzyme-responsive peptide targeting Pseudomonas aeruginosa's elastase.

Here we present new enzyme-responsive polyion complex (PIC) nanoparticles prepared from antimicrobial poly(ethylene imine) and an anionic enzyme-responsive peptide targeting Pseudomonas aeruginosa's elastase. The synthetic conditions used to prepare these nanomaterials allowed us to optimise particle size and charge, and their stability under physiological conditions. We demonstrate that these enzyme responsive PIC nanoparticles are selectively degraded in the presence of P. aeruginosa elastase without being affected by other endogenous elastases. This enzyme-responsive PIC particle can exert an elastase-specific antimicrobial effect against P. aeruginosa without affecting non-pathogenic strains of these bacteria. These targeted enzyme-responsive PIC nanoparticles constitute a novel platform for the delivery of antimicrobial peptides and polymers, and can be a powerful tool in the current race against antimicrobial resistance.

Single Site Discrimination of Cytosine, 5-Methylcytosine, and 5-Hydroxymethylcytosine in Target DNA Using Anthracene-Tagged Fluorescent Probes

The ability to discriminate between epigenetic variants in DNA is a necessary tool if we are to increase our understanding of the roles that they play in various biological processes and medical conditions. Herein, it is demonstrated how a simple two-step fluorescent probe assay can be used to differentiate all three major epigenetic variants of cytosine at a single locus site in a target strand of DNA.

Location dependent coordination chemistry and MRI relaxivity, in de novo designed lanthanide coiled coils

Herein, we establish for the first time the design principles for lanthanide coordination within coiled coils, and the important consequences of binding site translation. By interrogating design requirements and by systematically translating binding site residues, one can influence coiled coil stability and more importantly, the lanthanide coordination chemistry. A 10 Å binding site translation along a coiled coil, transforms a coordinatively saturated Tb(Asp)3(Asn)3 site into one in which three exogenous water molecules are coordinated, and in which the Asn layer is no longer essential for binding, Tb(Asp)3(H2O)3. This has a profound impact on the relaxivity of the analogous Gd(iii) coiled coil, with more than a four-fold increase in the transverse relaxivity (21 to 89 mM-1 s-1), by bringing into play, in addition to the outer sphere mechanism present for all Gd(iii) coiled coils, an inner sphere mechanism. Not only do these findings warrant further investigation for possible exploitation as MRI contrast agents, but understanding the impact of binding site translation on coordination chemistry has important repercussions for metal binding site design, taking us an important step closer to the predictable and truly de novo design of metal binding sites, for new functional applications.

Exploiting anthracene photodimerization within peptides: light induced sequence-selective DNA binding

Here we detail the first example of anthracene photodimerisation in peptides, and use it to trigger a selective biomolecular recognition event.

Metal-ion-regulated miniature DNA-binding proteins based on GCN4 and non-native regulation sites

The design of artificial peptide dimers containing polypyridine switching domains, for which metal-ion coordination is shown to regulate DNA binding, is reported. Short peptides, based on the basic domain of the GCN4 transcription factor (GCN4bd), dimerised with either 2,2'-bipyridine (bipy(GCN4bd)2 ) or 2,2':6',2''-terpyridine (terpy(GCN4bd)2 ) linker units, undergo a conformational rearrangement on Cu(II) and Zn(II) coordination. Depending on the linker substitution pattern, this is proposed to alter the relative alignment of the two peptide moieties, and in turn regulate DNA binding. Circular dichroism and UV-visible spectroscopy reveal that Cu(II) and Zn(II) coordination promotes binding to DNA containing the CRE target site, but to a differing and opposite degree for the two linkers, and that the metal-ion affinity for terpy(GCN4bd)2 is enhanced in the presence of CRE DNA. Binding to DNA containing the shorter AP1 target site, which lacks a single nucleobase pair compared to CRE, as well as half-CRE, which contains only half of the CRE target site, was also investigated. Cu(II) and Zn(II) coordination to terpy(GCN4bd)2 promotes binding to AP1 DNA, and to a lesser extent half-CRE DNA. Whereas, bipy(GCN4bd)2 , for which interpeptide distances are largely independent of metal-ion coordination and less suitable for binding to these shorter sites, displays allosteric ineffective behaviour in these cases. These findings for the first time demonstrate that biomolecular recognition, and specifically sequence-selective DNA binding, can be controlled by metal-ion coordination to designed switching units, non-native regulation sites, in artificial biomolecules. We believe that in the future these could find a wide range of applications in biotechnology.

De novo design of Ln(III) coiled coils for imaging applications

A new peptide sequence (MB1) has been designed which, in the presence of a trivalent lanthanide ion, has been programmed to self-assemble to form a three stranded metallo-coiled coil, Ln(III)(MB1)₃. The binding site has been incorporated into the hydrophobic core using natural amino acids, restricting water access to the lanthanide. The resulting terbium coiled coil displays luminescent properties consistent with a lack of first coordination sphere water molecules. Despite this the gadolinium coiled coil, the first to be reported, displays promising magnetic resonance contrast capabilities.

De novo design of peptide scaffolds as novel preorganized ligands for metal-ion coordination

This chapter describes how de novo designed peptides can be used as novel preorganized ligands for metal ion coordination. The focus is on the design of peptides which are programmed to spontaneously self-assemble into α-helical coiled coils in aqueous solution, and how metal ion binding sites can be engineered onto and into these structures. In addition to describing the various design principles, some key examples are covered illustrating the success of this approach, including a more detailed example in the case study.

Conformational Study of an Artificial Metal-Dependent Regulation Site for Use in Designer Proteins

This report describes the dimerisation of glutathione, and by extension, other cysteine-containing peptides or protein fragments, with a 5, 5'-disubstituted-2, 2'-bipyridine or 6, 6"-disubstituted-2, 2':6',2"-terpyridine unit. The resulting bipy-GS2 and terpy-GS2 were investigated as potential metal ion dependent switches in aqueous solution, and were found to predominantly adopt the transoïd conformation at physiological pH. Metal complexation with CuII and ZnII at this pH has been studied by UV/Vis, CD, NMR and ion-mobility mass spectrometry. ZnII titrations are consistent with the formation of a 1:1 ZnII:terpy-GS2 complex at pH 7.4, but bipy-GS2 was shown to form both 1:1 and 1:2 complexes with the former being predominant under dilute micromolar conditions. Formation constants for the resulting 1:1 complexes were determined to be log KM 6.86 (bipy-GS2 ) and 6.22 (terpy-GS2 ), consistent with a higher affinity for the unconstrained bipyridine, compared to the strained terpyridine. CuII coordination involves the initial formation of 1:1 complexes, followed by 1.5Cu:1bipy-GS2 and 2Cu:1terpy-GS2 complexes at micromolar concentrations. Binding constants for formation of the 1:1 complexes (log KM 12.5 (bipy-GS2 ); 8.04 and 7.14 (terpy-GS2 )) indicate a higher affinity for CuII than ZnII. Finally, ion-mobility MS studies detected the free ligands in their protonated form, and were consistent with the formation of two different Cu adducts with different conformations in the gas-phase. We illustrate that the bipyridine and terpyridine dimerisation units can behave like conformational switches in response to Cu/Zn complexation, and propose that in future these can be employed in synthetic biology with larger peptide or protein fragments, to control large scale folding and related biological function.

Structural comparisons of apo- and metalated three-stranded coiled coils clarify metal binding determinants in thiolate containing designed peptides

Over the past two decades, designed metallopeptides have held the promise for understanding a variety of fundamental questions in metallobiochemistry; however, these dreams have not yet been realized because of a lack of structural data to elaborate the protein scaffolds before metal complexation and the resultant metalated structures which ultimately exist. This is because there are few reports of structural characterization of such systems either in their metalated or nonmetalated forms and no examples where an apo structure and the corresponding metalated peptide assembly have both been defined by X-ray crystallography. Herein we present X-ray structures of two de novo designed parallel three-stranded coiled coils (designed using the heptad repeat (a → g)) CSL9C (CS = Coil Ser) and CSL19C in their nonmetalated forms, determined to 1.36 and 2.15 A resolutions, respectively. Leucines from either position 9 (a site) or 19 (d site) are replaced by cysteine to generate the constructs CSL9C and CSL19C, respectively, yielding thiol-rich pockets at the hydrophobic interior of these peptides, suitable to bind heavy metals such as As(III), Hg(II), Cd(II), and Pb(II). We use these structures to understand the inherent structural differences between a and d sites to clarify the basis of the observed differential spectroscopic behavior of metal binding in these types of peptides. Cys side chains of (CSL9C)(3) show alternate conformations and are partially preorganized for metal binding, whereas cysteines in (CSL19C)(3) are present as a single conformer. Zn(II) ions, which do not coordinate or influence Cys residues at the designed metal sites but are essential for forming X-ray quality crystals, are bound to His and Glu residues at the crystal packing interfaces of both structures. These "apo" structures are used to clarify the changes in metal site organization between metalated As(CSL9C)(3) and to speculate on the differential basis of Hg(II) binding in a versus d peptides. Thus, for the first time, one can establish general rules for heavy metal binding to Cys-rich sites in designed proteins which may provide insight for understanding how heavy metals bind to metallochaperones or metalloregulatory proteins.

Harnessing natures ability to control metal ion coordination geometry using de novo designed peptides

Advances in protein chemistry and molecular and structural biology have empowered modern chemists to build complex biological architectures using a "first principles" approach, which is known as de novo protein design. In this Perspective we demonstrate how simple three-stranded alpha-helical constructs can be prepared by the sole consideration of the primary amino acid sequence of a peptide. With these well defined systems, we then demonstrate that metal binding cavities can be carved out of the hydrophobic cores of these aggregates in order to bind metal ions such as cadmium with well defined coordination geometries. Examples will be given of homoleptic CdS(3) complexes, CdS(3)O sites and proteins which contain equilibrium mixtures of these two species. We will provide a description of a strategy that allows us to build heterochromic peptides (small proteins that complex two metals in nearly identical environments but which result in different physical properties and allow for metal site selectivity). We conclude with a new class of designed peptides, diastereopeptides, which can exploit changes in amino acid chirality to control metal ion coordination number and lead to an alternative path towards heterochromic systems. The constructs described herein represent the initial steps of preparing protein structures that may simultaneous contain structural and catalytic metal binding centers. These studies inform the community on a developing field, which promises new opportunities for the study of bioinorganic chemistry.

Medicinal organometallic chemistry: designing metal arene complexes as anticancer agents

The field of medicinal inorganic chemistry is rapidly advancing. In particular organometallic complexes have much potential as therapeutic and diagnostic agents. The carbon-bound and other ligands allow the thermodynamic and kinetic reactivity of the metal ion to be controlled and also provide a scaffold for functionalization. The establishment of structure-activity relationships and elucidation of the speciation of complexes under conditions relevant to drug testing and formulation are crucial for the further development of promising medicinal applications of organometallic complexes. Specific examples involving the design of ruthenium and osmium arene complexes as anticancer agents are discussed.

Switching the chirality of the metal environment alters the coordination mode in designed peptides

The effects of switching the chirality of a single layer of amino acids in a three stranded coiled coil has been investigated. X-ray crystallography reveals that this modification is well tolerated and does not alter the designed structure. In contrast, spectroscopic studies of cadmium binding to both the L- and D- enantiomers of the penicillamine, provide evidence that this switch dramatically alters the metal binding capability, the resulting coordination environment and the position of binding.

Using diastereopeptides to control metal ion coordination in proteins

Here, we report a previously undescribed approach for controlling metal ion coordination geometry in biomolecules by reorientating amino acid side chains through substitution of L- to D-amino acids. These diastereopeptides allow us to manipulate the spatial orientation of amino acid side chains to alter the sterics of metal binding pockets. We have used this approach to design the de novo metallopeptide, Cd(TRIL12L(D)L16C)(3)(-), which is an example of Cd(II) bound to 3 L-Cys as exclusively trigonal CdS(3), as characterized by a combination of (113)Cd NMR and (111m)Cd PAC spectroscopy. We subsequently show that the physical properties of such a site, such as the high pK(a2) for Cd(II) binding of 15.1, is due to the nature of the coordination number and not the ligating group. Further more this approach allowed for the design of a construct, GRANDL12L(D)L16CL26AL30C, capable of independently binding 2 equivalents of Cd(II) to 2 very similar Cys sites as exclusively 3- and 4-, CdS(3) and CdS(3)O, respectively. Demonstrating that we are capable of controlling the Cd(II) coordination number in these 2 sites solely by varying the nature of a noncoordinating second coordination sphere amino acid, with D-leucine and L-alanine resulting in exclusively 3- and 4-coordinate structures, respectively. Cd(II) was found to selectively bind to the 4-coordinate CdS(3)O site, demonstrating that a protein can be designed that displays metal-binding selectivity based solely on coordination number control and not on the chemical identity of coordinating ligands.

DNA interactions of monofunctional organometallic osmium(II) antitumor complexes in cell-free media

This work is the first in-depth study of osmium binding to DNA and confirms the pharmacological activity of a new class of anticancer metallodrugs. We investigated the interactions between the potential biological target DNA and four osmium(II) arene complexes, of the type [(eta 6-arene)Os(LL)Cl]n+, where arene = biphenyl or p-cymene and LL = ethylenediamine, picolinate, or oxinate in an effort to understand their mechanism of action. Most notably we show that these complexes bind to DNA. DNA adducts of the OsII complexes that exhibit promising cytotoxic effects in ovarian tumor cell lines largely distort its conformation. The data are consistent with DNA binding of the complexes containing biphenyl as the arene ligand that involves combined coordination to guanine residues and noncovalent interactions between the arene ligand and DNA. The results also indicate both a mechanism of action and a detoxification mechanism for OsII arene compounds different from those of cisplatin.

Chloro Half-Sandwich Osmium(II) Complexes: Influence of Chelated N,N-Ligands on Hydrolysis, Guanine Binding, and Cytotoxicity

Relatively little is known about the kinetics or the pharmacological potential of organometallic complexes of osmium compared to its lighter congeners, iron and ruthenium. We report the synthesis of seven new complexes, [(eta6-arene)Os(NN)Cl]+, containing different bidentate nitrogen (N,N) chelators, and a dichlorido complex, [(eta6-arene)Os(N)Cl2]. The X-ray crystal structures of seven complexes are reported: [(eta6-bip)Os(en)Cl]PF6 (1PF6), [(eta6-THA)Os(en)Cl]BF4 (2BF4), [(eta6-p-cym)Os(phen)Cl]PF6 (5PF6), [(eta6-bip)Os(dppz)Cl]PF6 (6PF6), [(eta6-bip)Os(azpy-NMe2)Cl]PF6 (7PF6), [(eta6-p-cym)Os(azpy-NMe2)Cl]PF6 (8PF6), and [(eta6-bip)Os(NCCH3-N)Cl2] (9), where THA = tetrahydroanthracene, en = ethylenediamine, p-cym = p-cymene, phen = phenanthroline, bip = biphenyl, dppz = [3,2-a: 2',3'-c]phenazine and azpy-NMe2 = 4-(2-pyridylazo)-N,N-dimethylaniline. The chelating ligand was found to play a crucial role in enhancing aqueous stability. The rates of hydrolysis at acidic pH* decreased when the primary amine N-donors (NN = en, t1/2 = 0.6 h at 318 K) are replaced with pi-accepting pyridine groups (e.g., NN = phen, t1/2 = 9.5 h at 318 K). The OsII complexes hydrolyze up to 100 times more slowly than their RuII analogues. The pK*a of the aqua adducts decreased with a similar trend (pK*a = 6.3 and 5.8 for en and phen adducts, respectively). [(eta6-bip)Os(en)Cl]PF6/BF4 (1PF6/BF4) and [(eta6-THA)Os(en)Cl]BF4 (2BF4) were cytotoxic toward both the human A549 lung and A2780 ovarian cancer cell lines, with IC50 values of 6-10 microM, comparable to the anticancer drug carboplatin. 1BF4 binds to both the N7 and phosphate of 5'-GMP (ratio of 2:1). The formation constant for the 9-ethylguanine (9EtG) adduct [(eta6-bip)M(en)(9EtG)]2+ was lower for OsII (log K = 3.13) than RuII (log K = 4.78), although the OsII adduct showed some kinetic stability. DNA intercalation of the dppz ligand in 6PF6 may play a role in its cytotoxicity. This work demonstrates that the nature of the chelating ligand can play a crucial role in tuning the chemical and biological properties of [(eta6-arene)Os(NN)Cl]+ complexes.

Osmium(II) and Ruthenium(II) Arene Maltolato Complexes: Rapid Hydrolysis and Nucleobase Binding

Density functional calculations show that aquation of [Os(eta6-arene)(XY)Cl]n+ complexes is more facile for complexes in which XY=an anionic O,O-chelated ligand compared to a neutral N,N-chelated ligand, and the mechanism more dissociative in character. The O,O-chelated XY=maltolato (mal) [M(eta6-p-cym)(mal)Cl] complexes, in which p-cym=p-cymene, M=OsII (1) and RuII (2), were synthesised and the X-ray crystal structures of 1 and 22 H2O determined. Their hydrolysis rates were rapid (too fast to follow by NMR spectroscopy). The aqua adduct of the OsII complex 1 was 1.6 pKa units more acidic than that of the RuII complex 2. Dynamic NMR studies suggested that O,O-chelate ring opening occurs on a millisecond timescale in coordinating proton-donor solvents, and loss of chelated mal in aqueous solution led to the formation of the hydroxo-bridged dimers [(eta6-p-cym)M(mu-OH)3M(eta6-p-cym)]+. The proportion of this dimer in solutions of the OsII complex 1 increased with dilution and it predominated at micromolar concentrations, even in the presence of 0.1 M NaCl (conditions close to those used for cytotoxicity testing). Although 9-ethylguanine (9-EtG) binds rapidly to Os(II) in 1 and more strongly (log K=4.4) than to RuII in 2 (log K=3.9), the OsII adduct [Os(eta6-p-cym)(mal)(9EtG)]+ was unstable with respect to formation of the hydroxo-bridged dimer at micromolar concentrations. Such insights into the aqueous solution chemistry of metal-arene complexes under biologically relevant conditions will aid the rational design of organometallic anticancer agents.

Tuning the Hydrolytic Aqueous Chemistry of Osmium Arene Complexes with N,O-Chelating Ligands to Achieve Cancer Cell Cytotoxicity

Potential biological and medical applications of organometallic complexes are hampered by a lack of knowledge of their aqueous solution chemistry. We show that the hydrolytic and aqueous solution chemistry of half-sandwich OsII arene complexes of the type [(eta6-arene)Os(XY)Cl] can be tuned with XY chelating ligands to achieve cancer cell cytoxicity comparable to carboplatin. Complexes containing arene = p-cymene, XY = N,O-chelating ligands glycinate (1), L-alaninate (2), alpha-aminobutyrate (3), beta-alaninate (4), picolinate (5), or 8-hydroxyquinolinate (7) were synthesized. Although, 1-4 and 7 hydrolyzed rapidly (<min), complexes with pi-acceptor pyridine as N-donor and carboxylate as O-donor (5 and 6) hydrolyzed much more slowly (t1/2 = 0.20 and 0.52 h, 298 K). The aqua picolinate complexes were more acidic (pKa* = 6.67, 6.33) than the other aqua adducts (pKa* = 7.17-7.71). At biological test concentrations (micromolar), the chelating ligands dissociated from complexes 1-4 to give the inert hydroxo-bridged dinuclear species [(eta6-arene)Os(mu-OH)3Os(eta6-arene)]+ (8), and these complexes were inactive toward human lung A549 and ovarian A2780 cancer cells. In contrast, 5-7 were cytotoxic, especially 6 (IC50 values of 8 and 4.2 microM). The X-ray structures of 9-ethylguanine, [(eta6-p-cym)Os(pico)(9EtG-N7)]PF6, and 9-ethyladenine, [(eta6-p-cym)Os(pico)(9EtA-N7)]PF6, adducts of 5 are reported (the first reported for G or A adducts of OsII). Crystals of the 9EtA complex contain homoadenine base pairing. The 9EtG adduct in particular exhibits remarkable aqueous kinetic stability. This work shows how the rational control of chemical reactivity (hydrolysis, acidity, formation of hydroxo-bridged dinuclear species) can allow the design of cytotoxic anticancer OsII arene complexes.

Catalysis of regioselective reduction of NAD+ by ruthenium(II) arene complexes under biologically relevant conditions

Ruthenium(II) arene anticancer complexes [(eta6-arene)Ru(en)Cl]PF6 (arene is hexamethylbenzene, p-cymene, indan; en is ethylenediamine) can catalyse regioselective reduction of NAD+ by formate in water to form 1,4-NADH, at pD 7.2, 37 degrees C, and in the presence of air. The catalytic activity is markedly dependent on the arene, with the hexamethylbenzene (hmb) complex showing the highest activity. For [(eta 6-hmb)Ru(en)Cl]PF6, the rate of reaction is independent of NAD+ concentration and shows saturation kinetics with respect to formate concentration. A Km value of 58 mM and a turnover frequency at saturation of 1.46 h(-1) were observed. Removal of chloride and performing the reaction under argon led to higher reaction rates. Lung cancer cells (A549) were found to be remarkably tolerant to formate even at millimolar concentrations. The possibility of using ruthenium arene complexes coadministered with formate as catalytic drugs is discussed.

Tuning the Reactivity of Osmium(II) and Ruthenium(II) Arene Complexes under Physiological Conditions

The Os(II) arene ethylenediamine (en) complexes [(eta(6)-biphenyl)Os(en)Cl][Z], Z = BPh(4) (4) and BF(4) (5), are inactive toward A2780 ovarian cancer cells despite 4 being isostructural with an active Ru(II) analogue, 4R. Hydrolysis of 5 occurred 40 times more slowly than 4R. The aqua adduct 5A has a low pK(a) (6.3) compared to that of [(eta(6)-biphenyl)Ru(en)(OH(2))](2+) (7.7) and is therefore largely in the hydroxo form at physiological pH. The rate and extent of reaction of 5 with 9-ethylguanine were also less than those of 4R. We replaced the neutral en ligand by anionic acetylacetonate (acac). The complexes [(eta(6)-arene)Os(acac)Cl], arene = biphenyl (6), benzene (7), and p-cymene (8), adopt piano-stool structures similar to those of the Ru(II) analogues and form weak dimers through intermolecular (arene)C-H...O(acac) H-bonds. Remarkably, these Os(II) acac complexes undergo rapid hydrolysis to produce not only the aqua adduct, [(eta(6)-arene)Os(acac)(OH(2))](+), but also the hydroxo-bridged dimer, [(eta(6)-arene)Os(mu(2)-OH)(3)Os(eta(6)-arene)](+). The pK(a) values for the aqua adducts 6A, 7A, and 8A (7.1, 7.3, and 7.6, respectively) are lower than that for [(eta(6)-p-cymene)Ru(acac)(OH(2))](+) (9.4). Complex 8A rapidly forms adducts with 9-ethylguanine and adenosine, but not with cytidine or thymidine. Despite their reactivity toward nucleobases, complexes 6-8 were inactive toward A549 lung cancer cells. This is attributable to rapid hydrolysis and formation of unreactive hydroxo-bridged dimers which, surprisingly, were the only species present in aqueous solution at biologically relevant concentrations. Hence, the choice of chelating ligand in Os(II) (and Ru(II)) arene complexes can have a dramatic effect on hydrolysis behavior and nucleobase binding and provides a means of tuning the reactivity and the potential for discovery of anticancer complexes.