Left versus right: Exploring the effects of chiral threading intercalators using optical tweezers

Small-molecule DNA-binding drugs have shown promising results in clinical use against many types of cancer. Understanding the molecular mechanisms of DNA binding for such small molecules can be critical in advancing future drug designs. We have been exploring the interactions of ruthenium-based small molecules and their DNA-binding properties that are highly relevant in the development of novel metal-based drugs. Previously we have studied the effects of the right-handed binuclear ruthenium threading intercalator ΔΔ-[μ-bidppz(phen)4Ru2]4+, or ΔΔ-P for short, which showed extremely slow kinetics and high-affinity binding to DNA. Here we investigate the left-handed enantiomer ΛΛ-[μ-bidppz(phen)4Ru2]4+, or ΛΛ-P for short, to study the effects of chirality on DNA threading intercalation. We employ single-molecule optical trapping experiments to understand the molecular mechanisms and nanoscale structural changes that occur during DNA binding and unbinding as well as the association and dissociation rates. Despite the similar threading intercalation binding mode of the two enantiomers, our data show that the left-handed ΛΛ-P complex requires increased lengthening of the DNA to thread, and it extends the DNA more than double the length at equilibrium compared with the right-handed ΔΔ-P. We also observed that the left-handed ΛΛ-P complex unthreads three times faster than ΔΔ-P. These results, along with a weaker binding affinity estimated for ΛΛ-P, suggest a preference in DNA binding to the chiral enantiomer having the same right-handed chirality as the DNA molecule, regardless of their common intercalating moiety. This comparison provides a better understanding of how chirality affects binding to DNA and may contribute to the development of enhanced potential cancer treatment drug designs.

DNA threading intercalation of enantiopure [Ru(phen)2bidppz]2+ induced by hydrophobic catalysis

The enantiomers of a novel mononuclear ruthenium(ii) complex [Ru(phen)2bidppz]2+ with an elongated dppz moiety were synthesized. Surprisingly, the complex showed no DNA intercalating capability in an aqueous environment. However, by the addition of water-miscible polyethylene glycol ether PEG-400, self-aggregation of the hydrophobic ruthenium(ii) complexes was counter-acted, thus strongly promoting the DNA intercalation binding mode. This mild alteration of the environment surrounding the DNA polymer does not damage or alter the DNA structure but instead enables more efficient binding characterization studies of potential DNA binding drugs.

Including public health considerations in trade and investment agreements

There is widespread recognition that trade and investment agreements (TIAs) can affect health services, access to medicines, NCD prevention (particularly related to tobacco, alcohol and unhealthy food) and health systems structures. In addition, these binding international economic agreements can constrain the policy space available for innovative, evidence-based health policymaking. Although TIAs can have positive outcomes for employment and economic growth, these benefits are only likely to accrue when governments are pro-active in implementing complementary policies to mitigate impacts on other sectors and to address potential inequalities arising.

The aim of this panel session is to examine the ways in which TIAs can be designed to achieve economic goals while also protecting public health, and identify complementary policy measures that may be needed as well as strategies for strategic policy engagement. This panel will be hosted by the UK-PRP PETRA Network (Prevention of noncommunicable disease using trade agreements). The UK will be negotiating a range of new TIAs over the coming years, representing a window of opportunity for strategic engagement with policymakers regarding how public health can be protected and promoted within these agreements. There is an emerging global body of evidence regarding how consideration of health can be integrated into TIAs, both textually and through strategic engagement with policymakers before and during the negotiation phase. Experience to date indicates common global challenges and opportunities for health and trade, as well as significant potential for cross country learning regarding trade and health. The panel discussion will use the UK experience as a springboard to address these global issues.

Canonical DNA minor groove insertion of bisbenzamidine-Ru(ii) complexes with chiral selectivity

We report the first Ru(ii) coordination compounds that interact with DNA through a canonical minor groove insertion mode and with selectivity for A/T rich sites.

We report the first Ru(ii) coordination compounds that interact with DNA through a canonical minor groove insertion mode and with selectivity for A/T rich sites. This was made possible by integrating a bis-benzamidine minor groove DNA-binding agent with a ruthenium(ii) complex. Importantly, one of the enantiomers (Δ-[Ru(bpy)₂b4bpy]²⁺, Δ-4Ru) shows a considerably higher DNA affinity than the parent organic ligand and the other enantiomer, particularly for the AATT sequence, while the other enantiomer preferentially targets long AAATTT sites with overall lower affinity. Finally, we demonstrate that the photophysical properties of these new binders can be exploited for DNA cleavage using visible light.

Diastereomeric bactericidal effect of Ru(phenanthroline)2 dipyridophenazine

Metal susceptibility assays and spot plating were used to investigate the antimicrobial activity of enantiopure [Ru(phen)₂ dppz]²⁺ (phen =1,10-phenanthroline and dppz = dipyrido[3,2-a:2´,3´-c]phenazine) and [μ-bidppz(phen)₄ Ru₂ ]⁴⁺ (bidppz =11,11´-bis(dipyrido[3,2-a:2´,3´-c]phenazinyl)), on Gram-negative Escherichia coli and Gram-positive Bacillus subtilis as bacterial models. The minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) were determined for both complexes: while [μ-bidppz(phen)₄ Ru₂ ]⁴⁺ only showed a bactericidal effect at the highest concentrations tested, the antimicrobial activity of [Ru(phen)₂ dppz]²⁺ against B. subtilis was comparable to that of tetracyline. In addition, the Δ-enantiomer of [Ru(phen)₂ dppz]²⁺ showed a 2-fold higher bacteriostatic and bactericidal effect compared to the Λ-enantiomer. This was in accordance with the enantiomers relative binding affinity for DNA, thus strongly indicating DNA binding as the mode of action.

Competitive DNA binding of Ru(bpy)2dppz2+ enantiomers studied with isothermal titration calorimetry (ITC) using a direct and general binding isotherm algorithm

While isothermal titration calorimetry (ITC) is widely used and sometimes referred to as the "gold standard" for quantitative measurements of biomolecular interactions, its usage has so far been limited to the analysis of the binding to isolated, non-cooperative binding sites. Studies on more complicated systems, where the binding sites interact, causing either cooperativity or anti-cooperativity between neighboring bound ligands, are rare, probably due to the complexity of the methods currently available. Here we have developed a simple algorithm not limited by the complexity of a binding system, meaning that it can be implemented by anyone, from analyzing systems of simple, isolated binding sites to complicated interactive multiple-site systems. We demonstrate here that even complicated competitive binding calorimetric isotherms can be properly analyzed, provided that ligand-ligand interactions are taken into account. As a practical example, the competitive binding interactions between the two enantiomers of Ru(bpy)₂dppz²⁺ (Ru-bpy) and poly(dAdT)₂ (AT-DNA) are analyzed using our new algorithm, which provided an excellent global fit for the ITC experimental data.

Reshaping the Energy Landscape Transforms the Mechanism and Binding Kinetics of DNA Threading Intercalation

Molecules that bind DNA via threading intercalation show high binding affinity as well as slow dissociation kinetics, properties ideal for the development of anticancer drugs. To this end, it is critical to identify the specific molecular characteristics of threading intercalators that result in optimal DNA interactions. Using single-molecule techniques, we quantify the binding of a small metal-organic ruthenium threading intercalator (Δ,Δ-B) and compare its binding characteristics to a similar molecule with significantly larger threading moieties (Δ,Δ-P). The binding affinities of the two molecules are the same, while comparison of the binding kinetics reveals significantly faster kinetics for Δ,Δ-B. However, the kinetics is still much slower than that observed for conventional intercalators. Comparison of the two threading intercalators shows that the binding affinity is modulated independently by the intercalating section and the binding kinetics is modulated by the threading moiety. In order to thread DNA, Δ,Δ-P requires a "lock mechanism", in which a large length increase of the DNA duplex is required for both association and dissociation. In contrast, measurements of the force-dependent binding kinetics show that Δ,Δ-B requires a large DNA length increase for association but no length increase for dissociation from DNA. This contrasts strongly with conventional intercalators, for which almost no DNA length change is required for association but a large DNA length change must occur for dissociation. This result illustrates the fundamentally different mechanism of threading intercalation compared with conventional intercalation and will pave the way for the rational design of therapeutic drugs based on DNA threading intercalation.

Effects of methyl substitution on DNA binding enthalpies of enantiopure Ru(phenanthroline)2dipyridophenazine2+ complexes

An augmented McGhee – von Hippel binding model allows deconvolution of the intercalation enthalpy from the significant ligand aggregation enthalpies.

Guanine Can Direct Binding Specificity of Ru-dipyridophenazine (dppz) Complexes to DNA through Steric Effects

X-ray crystal structures of three Λ-[Ru(L)2 dppz]2+ complexes (dppz=dipyridophenazine; L=1,10-phenanthroline (phen), 2,2'-bipyridine (bpy)) bound to d((5BrC)GGC/GCCG) showed the compounds intercalated at a 5'-CG-3' step. The compounds bind through canted intercalation, with the binding angle determined by the guanine NH2 group, in contrast to symmetrical intercalation previously observed at 5'-TA-3' sites. This result suggests that canted intercalation is preferred at 5'-CG-3' sites even though the site itself is symmetrical, and we hypothesise that symmetrical intercalation in a 5'-CG-3' step could give rise to a longer luminescence lifetime than canted intercalation.

Dissecting the Dynamic Pathways of Stereoselective DNA Threading Intercalation

DNA intercalators that have high affinity and slow kinetics are developed for potential DNA-targeted therapeutics. Although many natural intercalators contain multiple chiral subunits, only intercalators with a single chiral unit have been quantitatively probed. Dumbbell-shaped DNA threading intercalators represent the next order of structural complexity relative to simple intercalators, and can provide significant insights into the stereoselectivity of DNA-ligand intercalation. We investigated DNA threading intercalation by binuclear ruthenium complex [μ-dppzip(phen)4Ru2](4+) (Piz). Four Piz stereoisomers are defined by the chirality of the intercalating subunit (Ru(phen)2dppz) and the distal subunit (Ru(phen)2ip), respectively, each of which can be either right-handed (Δ) or left-handed (Λ). We used optical tweezers to measure single DNA molecule elongation due to threading intercalation, revealing force-dependent DNA intercalation rates and equilibrium dissociation constants. The force spectroscopy analysis provided the zero-force DNA binding affinity, the equilibrium DNA-ligand elongation Δxeq, and the dynamic DNA structural deformations during ligand association xon and dissociation xoff. We found that Piz stereoisomers exhibit over 20-fold differences in DNA binding affinity, from a Kd of 27 ± 3 nM for (Δ,Λ)-Piz to a Kd of 622 ± 55 nM for (Λ,Δ)-Piz. The striking affinity decrease is correlated with increasing Δxeq from 0.30 ± 0.02 to 0.48 ± 0.02 nm and xon from 0.25 ± 0.01 to 0.46 ± 0.02 nm, but limited xoff changes. Notably, the affinity and threading kinetics is 10-fold enhanced for right-handed intercalating subunits, and 2- to 5-fold enhanced for left-handed distal subunits. These findings demonstrate sterically dispersed transition pathways and robust DNA structural recognition of chiral intercalators, which are critical for optimizing DNA binding affinity and kinetics.

A ruthenium dimer complex with a flexible linker slowly threads between DNA bases in two distinct steps

Several multi-component DNA intercalating small molecules have been designed around ruthenium-based intercalating monomers to optimize DNA binding properties for therapeutic use. Here we probe the DNA binding ligand [μ-C4(cpdppz)₂(phen)₄Ru₂]⁴⁺, which consists of two Ru(phen)₂dppz²⁺ moieties joined by a flexible linker. To quantify ligand binding, double-stranded DNA is stretched with optical tweezers and exposed to ligand under constant applied force. In contrast to other bis-intercalators, we find that ligand association is described by a two-step process, which consists of fast bimolecular intercalation of the first dppz moiety followed by ∼10-fold slower intercalation of the second dppz moiety. The second step is rate-limited by the requirement for a DNA-ligand conformational change that allows the flexible linker to pass through the DNA duplex. Based on our measured force-dependent binding rates and ligand-induced DNA elongation measurements, we are able to map out the energy landscape and structural dynamics for both ligand binding steps. In addition, we find that at zero force the overall binding process involves fast association (∼10 s), slow dissociation (∼300 s), and very high affinity (K_d ∼10 nM). The methodology developed in this work will be useful for studying the mechanism of DNA binding by other multi-step intercalating ligands and proteins.

Binding of Ru(terpyridine)(pyridine)dipyridophenazine to DNA studied with polarized spectroscopy and calorimetry

Achiral Ru(tpy)(py)dppz²⁺ intercalated into DNA has similar intermolecular interactions as opposite enantiomers of its structural isomer, the “light-switch” complex Ru(bpy)₂dppz²⁺.

Enhanced cellular uptake of antisecretory peptide AF-16 through proteoglycan binding

Peptide AF-16, which includes the active site of Antisecretory Factor protein, has antisecretory and anti-inflammatory properties, making it a potent drug candidate for treatment of secretory and inflammatory diseases such as diarrhea, inflammatory bowel diseases, and intracranial hypertension. Despite remarkable physiological effects and great pharmaceutical need for drug discovery, very little is yet understood about AF-16 mechanism of action. In order to address interaction mechanisms, we investigated the binding of AF-16 to sulfated glycosaminoglycan, heparin, with focus on the effect of pH and ionic strength, and studied the influence of cell-surface proteoglycans on cellular uptake efficiency. Confocal laser scanning microscopy and flow cytometry experiments on wild type and proteoglycan-deficient Chinese hamster ovary cells reveal an endocytotic nature of AF-16 cellular uptake that is, however, less efficient for the cells lacking cell-surface proteoglycans. Isothermal titration calorimetry provides quantitative thermodynamic data and evidence for that the peptide affinity to heparin increases at lower pH and ionic strength. Experimental data, supported by theoretical modeling, of peptide-glycosaminoglycan interaction indicate that it has a large electrostatic contribution, which will be enhanced in diseases accompanied by decreased pH and ionic strength. These observations show that cell-surface proteoglycans are of general and crucial importance for the antisecretory and anti-inflammatory activities of AF-16.

Strong DNA deformation required for extremely slow DNA threading intercalation by a binuclear ruthenium complex

DNA intercalation by threading is expected to yield high affinity and slow dissociation, properties desirable for DNA-targeted therapeutics. To measure these properties, we utilize single molecule DNA stretching to quantify both the binding affinity and the force-dependent threading intercalation kinetics of the binuclear ruthenium complex Δ,Δ-[μ‐bidppz‐(phen)₄Ru₂]⁴⁺ (Δ,Δ-P). We measure the DNA elongation at a range of constant stretching forces using optical tweezers, allowing direct characterization of the intercalation kinetics as well as the amount intercalated at equilibrium. Higher forces exponentially facilitate the intercalative binding, leading to a profound decrease in the binding site size that results in one ligand intercalated at almost every DNA base stack. The zero force Δ,Δ-P intercalation K_d is 44 nM, 25-fold stronger than the analogous mono-nuclear ligand (Δ-P). The force-dependent kinetics analysis reveals a mechanism that requires DNA elongation of 0.33 nm for association, relaxation to an equilibrium elongation of 0.19 nm, and an additional elongation of 0.14 nm from the equilibrium state for dissociation. In cells, a molecule with binding properties similar to Δ,Δ-P may rapidly bind DNA destabilized by enzymes during replication or transcription, but upon enzyme dissociation it is predicted to remain intercalated for several hours, thereby interfering with essential biological processes.

Bridging ligand length controls at selectivity and enantioselectivity of binuclear ruthenium threading intercalators

The slow dissociation of DNA threading intercalators makes them interesting as model compounds in the search for new DNA targeting drugs, as there appears to be a correlation between slow dissociation and biological activity. Thus, it would be of great value to understand the mechanisms controlling threading intercalation, and for this purpose we have investigated how the length of the bridging ligand of binuclear ruthenium threading intercalators affects their DNA binding properties. We have synthesised a new binuclear ruthenium threading intercalator with slower dissociation kinetics from ct-DNA than has ever been observed for any ruthenium complex with any type of DNA, a property that we attribute to the increased distance between the ruthenium centres of the new complex. By comparison with previously studied ruthenium complexes, we further conclude that elongation of the bridging ligand reduces the sensitivity of the threading interaction to DNA flexibility, resulting in a decreased AT selectivity for the new complex. We also find that the length of the bridging ligand affects the enantioselectivity with increasing preference for the ΔΔ enantiomer as the bridging ligand becomes longer.

Initial DNA interactions of the binuclear threading intercalator Λ,Λ-[μ-bidppz(bipy)4Ru2]4+: an NMR study with [d(CGCGAATTCGCG)]2

Binuclear polypyridine ruthenium compounds have been shown to slowly intercalate into DNA, following a fast initial binding on the DNA surface. For these compounds, intercalation requires threading of a bulky substituent, containing one Ru^II, through the DNA base-pair stack, and the accompanying DNA duplex distortions are much more severe than with intercalation of mononuclear compounds. Structural understanding of the process of intercalation may greatly gain from a characterisation of the initial interactions between binuclear Ru^II compounds and DNA. We report a structural NMR study on the binuclear Ru^II intercalator Λ,Λ-B (Λ,Λ-[μ-bidppz(bipy)₄Ru₂]⁴⁺; bidppz=11,11′-bis(dipyrido[3,2-a:2′,3′-c]phenazinyl, bipy = 2,2′-bipyridine) mixed with the palindromic DNA [d(CGCGAATTCGCG)]₂. Threading of Λ,Λ-B depends on the presence and length of AT stretches in the DNA. Therefore, the latter was selected to promote initial binding, but due to the short stretch of AT base pairs, final intercalation is prevented. Structural calculations provide a model for the interaction: Λ,Λ-B is trapped in a well-defined surface-bound state consisting of an eccentric minor-groove binding. Most of the interaction enthalpy originates from electrostatic and van der Waals contacts, whereas intermolecular hydrogen bonds may help to define a unique position of Λ,Λ-B. Molecular dynamics simulations show that this minor-groove binding mode is stable on a nanosecond scale. To the best of our knowledge, this is the first structural study by NMR spectroscopy on a binuclear Ru compound bound to DNA. In the calculated structure, one of the positively charged Ru²⁺ moieties is near the central AATT region; this is favourable in view of potential intercalation as observed by optical methods for DNA with longer AT stretches. Circular dichroism (CD) spectroscopy suggests that a similar binding geometry is formed in mixtures of Λ,Λ-B with natural calf thymus DNA. The present minor-groove binding mode is proposed to represent the initial surface interactions of binuclear Ru^II compounds prior to intercalation into AT-rich DNA.

Interactions of binuclear ruthenium(II) complexes with oligonucleotides in hydrogel matrix: enantioselective threading intercalation into GC context

A stretched poly(vinyl alcohol) (PVA) film provides a unique matrix that enables also short DNA oligonucleotide duplex to be oriented and studied by linear dichroism (LD). This matrix further allows controlling DNA secondary structure by proper hydration (A or B form), and such humid films could potentially also mimic the molecular crowding in cellular contexts. However, early attempts to study intercalators and groove binders for probing DNA in PVA failed due to competitive matrix binding. Here we report the successful orientation in PVA of DNA oligonucleotide duplex hairpins with thread-intercalated binuclear complex [μ-(11,11'-bidppz)(phen)4Ru2](4+), and how LD depends on oligonucleotide sequence and metal center chirality. Opposite enantiomers of the ruthenium complex, ΔΔ and ΛΛ, were investigated with respect to enantioselectivity toward GC stretches as long as 22 bp. LD, supported by emission kinetics, reveals that threading intercalation occurs only with ΔΔ whereas ΛΛ remains externally bound, probably in either or both of the grooves of the GC-DNA. Enantioselective binding properties of sterically rigid DNA probes such as the ruthenium complexes could find applications for targeting nucleic acids, e.g., to inhibit transcription in therapeutic context such as treatment of malaria or cancer.

Lifetime heterogeneity of DNA-bound dppz complexes originates from distinct intercalation geometries determined by complex-complex interactions

Despite the extensive interest in structurally explaining the photophysics of DNA-bound [Ru(phen)(2)dppz](2+) and [Ru(bpy)(2)dppz](2+), the origin of the two distinct emission lifetimes of the pure enantiomers when intercalated into DNA has remained elusive. In this report, we have combined a photophysical characterization with a detailed isothermal titration calorimetry study to investigate the binding of the pure Δ and Λ enantiomers of both complexes with [poly(dAdT)](2). We find that a binding model with two different binding geometries, proposed to be symmetric and canted intercalation from the minor groove, as recently reported in high-resolution X-ray structures, is required to appropriately explain the data. By assigning the long emission lifetime to the canted binding geometry, we can simultaneously fit both calorimetric data and the binding-density-dependent changes in the relative abundance of the two emission lifetimes using the same binding model. We find that all complex-complex interactions are slightly unfavorable for Δ-[Ru(bpy)(2)dppz](2+), whereas interactions involving a complex canted away from a neighbor are favorable for the other three complexes. We also conclude that Δ-[Ru(bpy)(2)dppz](2+) preferably binds isolated, Δ-[Ru(phen)(2)dppz](2+) preferably binds as duplets of canted complexes, and that all complexes are reluctant to form longer consecutive sequences than triplets. We propose that this is due to an interplay of repulsive complex-complex and attractive complex-DNA interactions modulated by allosteric DNA conformation changes that are largely affected by the nature of the ancillary ligands.

Spectroscopic study on the interaction of ct-DNA with manganese Salen complex containing triphenyl phosphonium groups

The DNA binding properties of a bulky and hydrophobic Schiff base complex of manganese(III) [N,N'-bis(5-(triphenyl phosphonium methyl)salicylidene)-1,2-ethylene diamine chloride Mn(III) acetate] was examined by spectroscopic techniques. UV-vis titration data indicate both hypo and hyperchromic effect with addition of DNA to complex. A competitive binding study showed that the enhanced emission intensity of ethidium bromide (EB) in the presence of DNA was quenched by adding Mn Salen complex. This finding indicates that Mn Salen complex displaces EB from its binding site in DNA. Helix melting studies indicate improvement in the helix stability, and an increase in the melting temperature. The analysis of CD spectra represents the structural changes in DNA due to the binding of Mn Salen complex. The binding constant has been calculated using absorbance and fluorescence data. The results also represent that the binding process proceeds by strong electrostatic and hydrophobic interactions.

Effects of chirality on the intracellular localization of binuclear ruthenium(II) polypyridyl complexes

Interest in binuclear ruthenium(II) polypyridyl complexes as luminescent cellular imaging agents and for biomedical applications is increasing rapidly. We have investigated the cellular localization, uptake, and biomolecular interactions of the pure enantiomers of two structural isomers of [μ-bipb(phen)(4)Ru(2)](4+) (bipb is bis(imidazo[4,5-f]-1,10-phenanthrolin-2-yl)benzene and phen is 1,10-phenanthroline) using confocal laser scanning microscopy, emission spectroscopy, and linear dichroism. Both complexes display distinct enantiomeric differences in the staining pattern of fixed cells, which are concluded to arise from chiral discrimination in the binding to intracellular components. Uptake of complexes in live cells is efficient and nontoxic at 5 μM, and occurs through an energy-dependent mechanism. No differences in uptake are observed between the structural isomers or the enantiomers, suggesting that the interactions triggering uptake are rather insensitive to structural variations. Altogether, these findings show that the complexes investigated are promising for future applications as cellular imaging probes. In addition, linear dichroism shows that the complexes exhibit DNA-condensing properties, making them interesting as potential gene delivery vectors.

Nanofabrication yields. Hybridization and click-fixation of polycyclic DNA nanoassemblies

We demonstrate the stepwise assembly of a fully addressable polycyclic DNA hexagon nanonetwork for the preparation of a four-ring system, one of the biggest networks yet constructed from tripodal building blocks. We find that the yield exhibits a distinct upper level <100%, a fundamental problem of thermodynamic DNA assembly that appears to have been overlooked in the DNA nanotechnology literature. A simplistic model based on a single step-yield parameter y can quantitatively describe the total yield of DNA assemblies in one-pot reactions as Y = y(duplex)(n), with n the number of hybridization steps. Experimental errors introducing deviations from perfect stoichiometry and the thermodynamics of hybridization equilibria contribute to decreasing the value of y(duplex) (on average y = 0.96 for our 10 base pair hybridization). For the four-ring system (n = 31), the total yield is thus less than 30%, which is clearly unsatisfactory if bigger nanoconstructs of this class are to be designed. Therefore, we introduced site-specific click chemistry for making and purifying robust building blocks for future modular constructs of larger assemblies. Although the present yield of this robust module was only about 10%, it demonstrates a first step toward a general fabrication approach. Interestingly, we find that the click yields follow quantitatively a binomial distribution, the predictability of which indicates the usefulness of preparing pools of pure and robust building blocks in this way. The binomial behavior indicates that there is no interference between the six simultaneous click reactions but that step-yield limiting factors such as topological constraints and Cu(I) catalyst concentration are local and independent.

Sequential one-pot ruthenium-catalyzed azide-alkyne cycloaddition from primary alkyl halides and sodium azide

An experimentally simple sequential one-pot RuAAC reaction, affording 1,5-disubstituted 1H-1,2,3-triazoles in good to excellent yields starting from an alkyl halide, sodium azide, and an alkyne, is reported. The organic azide is formed in situ by treating the primary alkyl halide with sodium azide in DMA under microwave heating. Subsequent addition of [RuClCp*(PPh(3))(2)] and the alkyne yielded the desired cycloaddition product after further microwave irradiation.

Ruthenium(II) Complex Enantiomers as Cellular Probes for Diastereomeric Interactions in Confocal and Fluorescence Lifetime Imaging Microscopy

Ruthenium dipyridophenazine (dppz) complexes are sensitive luminescent probes for hydrophobic environments. Here, we apply multiple-frequency fluorescence lifetime imaging microscopy (FLIM) to Δ and Λ enantiomers of lipophilic ruthenium dppz complexes in live and fixed cells, and their different lifetime staining patterns are related to conventional intensity-based microscopy. Excited state lifetimes of the enantiomers determined from FLIM measurements correspond well with spectroscopically measured emission decay curves in pure microenvironments of DNA, phospholipid membrane or a model protein. We show that FLIM can be applied to monitor the long-lived excited states of ruthenium complex enantiomers and, combined with confocal microscopy, give new insight into their biomolecular binding and reveal differences in the microenvironment probed by the complexes.