Luminescent rhenium(I) polypyridine complexes appended with an α-D-glucose moiety as novel biomolecular and cellular probes

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Neutral Re(I) Complex Platform for Live Intracellular Imaging

Luminescent metal complexes are a valuable platform for the generation of cell imaging agents. However, many metal complexes are cationic, a factor that can dominate the intracellular accumulation to specific organelles. Neutral Re(I) complexes offer a more attractive platform for the development of bioconjugated imaging agents, where charge cannot influence their intracellular distribution. Herein, we report the synthesis of a neutral complex (ReAlkyne), which was used as a platform for the generation of four carbohydrate-conjugated imaging agents via Cu(I)-catalyzed azide-alkyne cycloaddition. A comprehensive evaluation of the physical and optical properties of each complex is provided, together with a determination of their utility as live cell imaging agents in H9c2 cardiomyoblasts. Unlike their cationic counterparts, many of which localize within mitochondria, these neutral complexes have localized within the endosomal/lysosomal network, a result consistent with examples of dinuclear carbohydrate-appended neutral Re(I) complexes that have been reported. This further demonstrates the utility of these neutral Re(I) complex imaging platforms as viable imaging platforms for the development of bioconjugated cell imaging agents.

2-Azabutadiene complexes of rhenium(I): S,N-chelated species with photophysical properties heavily governed by the ligand hidden traits

The reaction of [Re(CO)3(THF)(μ-Br)]2 or [Re(CO)5X] (X = Cl, Br, I) with the diaryl-2-azabutadienes [(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2] containing two thioether arms at the 4,4-position forms the luminescent S,N-chelate complexes fac-[(OC)3ReX{(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2}] (1a-h). The halide abstraction by silver triflate converts [(OC)3ReCl{(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1c) to [(OC)3Re(OS([double bond, length as m-dash]O)2CF3){(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1j) bearing a covalently bound triflate ligand. The cyclic voltammograms reveal reversible S^N ligand-centred reduction and irreversible oxidation waves for all complexes. The crystal structures of nine octahedral complexes have been determined along with that of (NaphtylS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2 (L6). A rich system of weak non-covalent intermolecular secondary interactions through CHX(Cl, Br)Re, CHO, COπ(Ph), CHπCO, CHO and CHS contacts has been evidenced. The photophysical properties have been investigated by steady-state and time-resolved absorption (fs transient absorption, fs-TAS) and emission (ns-TCSPC and ps-Streak camera) spectroscopy in 2-MeTHF solution at 298 and 77 K. The emission bands are composed of either singlet (450 < λmax < 535 nm) and/or triplet emissions (at 77 K only, λmax < 640 nm, or appearing as a tail at λ > 600 nm), which decay in a multiexponential manner for the fluorescence (short ps (i.e. <IRF) < τF < 1.9 ns at 298 and 77 K) and monoexponentially for the phosphorescence (4.0 < τP < 7.0 ns at 77 K). The fs-TAS data reveal the presence of 2 to 4 transient species decaying in four narrow time windows (generally 125-165 fs, 370-685 fs, 3-6 ps, 30-45 ps). The complexity of these kinetics was explained by studying the photophysical behaviour of ligand L6. Its behaviour is the same as the complexes thus indicating that the ligand dictates the kinetic traits of the Re-species, except for the triplet emission as L6 is not phosphorescent. The triplet lifetime (4.0 < τP < 7.0 ns) is considered very short but not unprecedented. Furthermore, the nature of the lowest energy excited states of these chelate compounds and L6 has been addressed using DFT and TDDFT calculations and been assigned to metal-to-ligand (MLCT) and/or intraligand charge-transfer (ILCT).

Rhenium carbonyl complexes bearing methylated triphenylphosphonium cations as antibody-free mitochondria trackers for X-ray fluorescence imaging

A convenient rhenium-based multimodal mitochondrial-targeted probe compatible with Synchrotron Radiation X-ray Fluorescence nano-imaging.

Glycoconjugated Metal Complexes as Cancer Diagnostic and Therapeutic Agents

The possibility of selectively delivering metal complexes to a defined cohort of cells on the basis of their metabolic features is a highly challenging goal, which may be extremely useful for a series of purposes, including diagnosis and therapy of pathological states, such as cancer. Tumor cells display augmented requests for carbohydrates and, in particular, for glucose in order to sustain their high proliferation rate, which causes an increased glycolytic process (Warburg effect). Since several metal complexes display diagnostic and/or therapeutic properties, their conjugation to carbohydrate portions often induce their preferential accumulation in cancer cells, similarly to what is observed with fluorodeoxyglucose (FDG). In this review we have considered the latest developments of glycoconjugates containing metal complexes in their structures. These compounds are classified as diagnostic or therapeutic agents and are further systematically discussed on the basis of the metal atom they contain. Several diagnostic techniques are possible with these probes, since, depending on the metal species included in their structures, they may be employed in nuclear medicine (PET, SPECT), magnetic resonance imaging, luminescence and phosphorescence. At the same time, the lack of selective cytotoxicity displayed by several metal-based chemotherapeutic agents, may also be solved by the conjugation of these agents to carbohydrate portions. Overall, data so far available reveal the great potential of this chemical class in the early detection and in the cure of severe neoplastic diseases, which still needs to be fully explored in the clinic.

Boosting two-photon photodynamic therapy with mitochondria-targeting ruthenium-glucose conjugates

Herein, we present a series of dual-targeted ruthenium-glucose conjugates that can function as two-photon absorption (TPA) PDT agents to effectively destroy tumors by preferentially targeting both tumor cells and mitochondria. The in vivo experiments revealed an excellent tumor inhibitory efficiency of the dual-targeted TPA PSs.

Fluorescent glycoconjugates and their applications

Fluorescent glycoconjugates are discussed for their applications in biology in vitro, in cell assays and in animal models. Advantages and limitations are presented for each design using a fluorescent core conjugated with glycosides, or vice versa.

Carbohydrate-based heteronuclear complexes as topoisomerase Iα inhibitor: approach toward anticancer chemotherapeutics

Due to the critical role of cellular enzymes necessary for cell proliferation by deciphering topological hurdles in the process of DNA replication, topoisomerases have been one of the major targets in the anticancer drug development area. A need, therefore, arises for new metallodrugs that specifically recognizes DNA and inhibits the activity of topoisomerase enzymes, herein, we report the synthesis and characterization of new metal-based glycoconjugate entities containing heterobimetallic core Cu^II-Sn^IV (1) and Ni^II-Sn^IV (2) derived from N-glycoside ligand (L). The optimized structure of complex 1 and other significant vibrational modes have been explained using dispersion corrected B3LYP/DFT calculations. In vitro DNA binding profile of the L and both the complexes 1 and 2 were done by various biophysical studies. Complex 1 breaks pBR322 DNA via a hydrolytic means which was validated by T4 DNA enzymatic assay. To get a mechanistic insight of mode of action topoisomerase I (Topo I) inhibition assay was carried out. Also, we have taken the help of molecular modeling studies in accordance with experimental findings. In vitro cytotoxicity of the complex 1 was evaluated against a panel of cancer cells which exhibited remarkably good anticancer activity (GI₅₀ values <10 μg/ml). Moreover, intracellular localization of the complex 1 was visualized by confocal microscopy against HeLa cells.

The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight

How can the intimate nature of the triazole moiety govern the geometry and luminescence properties of a rhenium complex?

Anticancer activity of complexes of the third row transition metals, rhenium, osmium, and iridium

A summary of recent developments on the anticancer activity of complexes of rhenium, osmium, and iridium is described.

In Vitro Anticancer Activity and in Vivo Biodistribution of Rhenium(I) Tricarbonyl Aqua Complexes

Seven rhenium(I) complexes of the general formula fac-[Re(CO)3(NN)(OH2)]+ where NN = 2,2'-bipyridine (8), 4,4'-dimethyl-2,2'-bipyridine (9), 4,4'-dimethoxy-2,2'-bipyridine (10), dimethyl 2,2'-bipyridine-4,4'-dicarboxylate (11), 1,10-phenanthroline (12), 2,9-dimethyl-1,10-phenanthroline (13), or 4,7-diphenyl-1,10-phenanthroline (14), were synthesized and characterized by 1H NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray crystallography. With the exception of 11, all complexes exhibited 50% growth inhibitory concentration (IC50) values that were less than 20 μM in HeLa cells, indicating that these compounds represent a new potential class of anticancer agents. Complexes 9, 10, and 13 were as effective in cisplatin-resistant cells as wild-type cells, signifying that they circumvent cisplatin resistance. The mechanism of action of the most potent complex, 13, was explored further by leveraging its intrinsic luminescence properties to determine its intracellular localization. These studies indicated that 13 induces cytoplasmic vacuolization that is lysosomal in nature. Additional in vitro assays indicated that 13 induces cell death without causing an increase in intracellular reactive oxygen species or depolarization of the mitochondrial membrane potential. Further studies revealed that the mode of cell death does not fall into one of the canonical categories such as apoptosis, necrosis, paraptosis, and autophagy, suggesting that a novel mode of action may be operative for this class of rhenium compounds. The in vivo biodistribution and metabolism of complex 13 and its 99mTc analogue 13* were also evaluated in naı̈ve mice. Complexes 13 and 13* exhibited comparable biodistribution profiles with both hepatic and renal excretion. High-performance liquid chromatography inductively coupled plasma mass-spectrometry (HPLC-ICP-MS) analysis of mouse blood plasma and urine postadministration showed considerable metabolic stability of 13, rendering this potent complex suitable for in vivo applications. These studies have shown the biological properties of this class of compounds and demonstrated their potential as promising theranostic anticancer agents that can circumvent cisplatin resistance.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

Two new neutral fac-[Re(CO)₃(phen)L] compounds (1,2), with phen = 1,10-phenanthroline and L = O₂C(CH₂)₅CH₃ or O₂C(CH₂)₄C≡CH, were synthetized in one-pot procedures from fac-[Re(CO)₃(phen)Cl] and the corresponding carboxylic acids, and were fully characterized by IR and UV-Vis absorption spectroscopy, ¹H- and ¹³C-NMR, mass spectrometry and X-ray crystallography. The compounds, which display orange luminescence, were used as probes for living cancer HeLa cell staining. Confocal microscopy revealed accumulation of both dyes in mitochondria. To investigate the mechanism of mitochondrial staining, a new non-emissive compound, fac-[Re(CO)₃(phen)L], with L = O₂C(CH₂)₃((C₅H₅)Fe(C₅H₄), i.e., containing a ferrocenyl moiety, was synthetized and characterized (3). 3 shows the same mitochondrial accumulation pattern as 1 and 2. Emission of 3 can only be possible when ferrocene-containing ligand dissociates from the metal center to produce a species containing the luminescent fac-[Re(CO)₃(phen)]⁺ core. The release of ligands from the Re center was verified in vitro through the conjugation with model proteins. These findings suggest that the mitochondria accumulation of compounds 1-3 is due to the formation of luminescent fac-[Re(CO)₃(phen)]⁺ products, which react with cellular matrix molecules giving secondary products and are uptaken into the negatively charged mitochondrial membranes. Thus, reported compounds feature a rare dissociation-driven mechanism of action with great potential for biological applications.

Luminescent Re(I) Carbonyl Complexes as Trackable PhotoCORMs for CO delivery to Cellular Targets

A family of Re(I) carbonyl complexes of general formula [ReX(CO)3(phen)]0/1+ (where X = Cl-, CF3SO3-, MeCN, PPh3, and methylimidazole) derived from 1,10-phenanthroline (phen) exhibits variable emission characteristics depending on the presence of the sixth ancillary ligand/group (X). All complexes but with X = MeCN exhibit moderate CO release upon irradiation with low-power UV light and are indefinitely stable in anaerobic/aerobic environment in solution as well as in solid state when kept under dark condition. These CO donors liberate three, one, or no CO depending on the nature of sixth ligand upon illumination as studied with the aid of time-dependent IR spectroscopy. Results of excited-state density functional theory (DFT) and time-dependent DFT calculations provided insight into the origin of the emission characteristics of these complexes. The luminescent rheinum(I) photoCORMs uniformly displayed efficient cellular internalization by the human breast adenocarcinoma cells, MDA-MB-231, while the complex with PPh3 as ancillary ligand showed moderate nuclear localization in addition to the cytosolic distribution. These species hold significant promise as theranostic photoCORMs (photoinduced CO releasing molecules), where the entry of the pro-drug can be tracked within the cellular matrices.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.

Glyco-functionalized dinuclear rhenium(i) complexes for cell imaging

New luminescent glycosilated rhenium complexes were synthesized and evaluated as dyes for optical imaging.

Novel glycoconjugated squaraine dyes for selective optical imaging of cancer cells

Glycoconjugated squaraine dyes for selective internalisation in cancer cell lines are reported. The cancer cell selectivity was achieved through the “Warburg effect”.

New [(η5-C5H5)Ru(N–N)(PPh3)][PF6] compounds: colon anticancer activity and GLUT-mediated cellular uptake of carbohydrate-appended complexes

Ruthenium glycoconjugates, with privileged passage through HCT116 colon cancer cell membranes via glucose transporters, are reported.

Luminescent Rhenium(I) and Iridium(III) Polypyridine Complexes as Biological Probes, Imaging Reagents, and Photocytotoxic Agents

Although the interactions of transition metal complexes with biological molecules have been extensively studied, the use of luminescent transition metal complexes as intracellular sensors and bioimaging reagents has not been a focus of research until recently. The main advantages of luminescent transition metal complexes are their high photostability, long-lived phosphorescence that allows time-resolved detection, and large Stokes shifts that can minimize the possible self-quenching effect. Also, by the use of transition metal complexes, the degree of cellular uptake can be readily determined using inductively coupled plasma mass spectrometry. For more than a decade, we have been interested in the development of luminescent transition metal complexes as covalent labels and noncovalent probes for biological molecules. We argue that many transition metal polypyridine complexes display triplet charge transfer ((3)CT) emission that is highly sensitive to the local environment of the complexes. Hence, the biological labeling and binding interactions can be readily reflected by changes in the photophysical properties of the complexes. In this laboratory, we have modified luminescent tricarbonylrhenium(I) and bis-cyclometalated iridium(III) polypyridine complexes of general formula [Re(bpy-R(1))(CO)3(py-R(2))](+) and [Ir(ppy-R(3))2(bpy-R(4))](+), respectively, with reactive functional groups and used them to label the amine and sulfhydryl groups of biomolecules such as oligonucleotides, amino acids, peptides, and proteins. Additionally, using a range of biological substrates such as biotin, estradiol, and indole, we have designed luminescent rhenium(I) and iridium(III) polypyridine complexes as noncovalent probes for biological receptors. The interesting results generated from these studies have prompted us to investigate the possible applications of luminescent transition metal complexes in intracellular systems. Thus, in the past few years, we have developed an interest in the cytotoxic activity, cellular uptake, and bioimaging applications of these complexes. Additionally, we and other research groups have demonstrated that many transition metal complexes have facile cellular uptake and organelle-localization properties and that their cytotoxic activity can be readily controlled. For example, complexes that can target the nucleus, nucleolus, mitochondria, lysosomes, endoplasmic reticulum, and Golgi apparatus have been identified. We anticipate that this selective localization property can be utilized in the development of intracellular sensors and bioimaging reagents. Thus, we have functionalized luminescent rhenium(I) and iridium(III) polypyridine complexes with various pendants, including molecule-binding moieties, sugar molecules, bioorthogonal functional groups, and polymeric chains such as poly(ethylene glycol) and polyethylenimine, and examined their potentials as biological reagents. This Account describes our design of luminescent rhenium(I) and iridium(III) polypyridine complexes and explains how they can serve as a new generation of biological reagents for diagnostic and therapeutic applications.

Solid-state (185/187)Re NMR and GIPAW DFT study of perrhenates and Re2(CO)10: chemical shift anisotropy, NMR crystallography, and a metal-metal bond

Advances in solid-state nuclear magnetic resonance (SSNMR) methods, such as dynamic nuclear polarization (DNP), intricate pulse sequences, and increased applied magnetic fields, allow for the study of systems which even very recently would be impractical. However, SSNMR methods using certain quadrupolar probe nuclei (i.e., I > 1/2), such as (185/187)Re remain far from fully developed due to the exceedingly strong interaction between the quadrupole moment of these nuclei and local electric field gradients (EFGs). We present a detailed high-field (B0 = 21.1 T) experimental SSNMR study on several perrhenates (KReO4, AgReO4, Ca(ReO4)2·2H2O), as well as ReO3 and Re2(CO)10. We propose solid ReO3 as a new rhenium SSNMR chemical shift standard due to its reproducible and sharp (185/187)Re NMR resonances. We show that for KReO4, previously poorly understood high-order quadrupole-induced effects (HOQIE) on the satellite transitions can be used to measure the EFG tensor asymmetry (i.e., ηQ) to nearly an order-of-magnitude greater precision than competing SSNMR and nuclear quadrupole resonance (NQR) approaches. Samples of AgReO4 and Ca(ReO4)2·2H2O enable us to comment on the effects of counter-ions and hydration upon Re(vii) chemical shifts. Calcium-43 and (185/187)Re NMR tensor parameters allow us to conclude that two proposed crystal structures for Ca(ReO4)2·2H2O, which would be considered as distinct, are in fact the same structure. Study of Re2(CO)10 provides insights into the effects of Re-Re bonding on the rhenium NMR tensor parameters and rhenium oxidation state on the Re chemical shift value. As overtone NQR experiments allowed us to precisely measure the (185/187)Re EFG tensor of Re2(CO)10, we were able to measure rhenium chemical shift anisotropy (CSA) for the first time in a powdered sample. Experimental observations are supported by gauge-including projector augmented-wave (GIPAW) density functional theory (DFT) calculations, with NMR tensor calculations also provided for NH4ReO4, NaReO4 and RbReO4. These calculations are able to reproduce many of the experimental trends in rhenium δiso values and EFG tensor magnitudes. Using KReO4 as a prototypical perrhenate-containing system, we establish a correlation between the tetrahedral shear strain parameter (|ψ|) and the nuclear electric quadrupolar coupling constant (CQ), which enables the refinement of the structure of ND4ReO4. Shortcomings in traditional DFT approaches, even when including relativistic effects via the zeroth-order regular approximation (ZORA), for calculating rhenium NMR tensor parameters are identified for Re2(CO)10.

Impact of molecular charge on GLUT-specific cellular uptake of glucose bioprobes and in vivo application of the glucose bioprobe, GB2-Cy3

The molecular charge of fluorescent bioprobes has recently received much attention due to its influence on cellular uptake. Herein, we demonstrate the effect of the molecular charge of glucose bioprobes on their GLUT-specific cellular uptake. We also applied GB2-Cy3 to in vivo imaging in the zebrafish model.

Synthesis, characterization and cellular location of cytotoxic constitutional organometallic isomers of rhenium delivered on a cyanocobalmin scaffold

Constitutional isomers based on vitamin B₁₂ and a fluorescent rhenium diimine complex were prepared, characterized, tested against PC-3 prostate cancer cells and investigated via IR spectromicroscopy for cellular uptake by live 3T3 fibroblasts.

A phosphorescent rhenium(i) histone deacetylase inhibitor: mitochondrial targeting and paraptosis induction

A phosphorescent rhenium(i) tricarbonyl polypyridine complex with mitochondria-specific localization, HDAC inhibitory activity and paraptosis-inducing capability has been explored as a multifunctional agent.

Phosphorescent biscyclometallated iridium(iii) ethylenediamine complexes functionalised with polar ester or carboxylate groups as bioimaging and visualisation reagents

Four new phosphorescent biscyclometallated iridium(iii) ethylenediamine complexes were designed as bioimaging and visualization reagents.

Amidino ligands obtained from the coupling of 1-methylcytosine and nitrile: a new method to incorporate biomolecules into luminescent Re(CO)3 complexes

Amidino chelating ligands obtained from the coupling of 1-methylcytosine with nitriles allow the incorporation of biologically relevant substrates into Re(CO)₃ complexes.

Modification of 1,2,4,5-tetrazine with cationic rhenium(i) polypyridine units to afford phosphorogenic bioorthogonal probes with enhanced reaction kinetics

New phosphorogenic bioorthogonal probes derived from rhenium(i) polypyridine tetrazine complexes have been designed.

Metal-carbonyl units for vibrational and luminescence imaging: towards multimodality

Metal-carbonyl complexes are attractive structures for bio-imaging. In addition to unique vibrational properties due to the CO moieties enabling IR and Raman cell imaging, the appropriate choice of ancillary ligands opens up the opportunity for luminescence detection. Through a classification by techniques, past and recent developments in the application of metal-carbonyl complexes for vibrational and luminescence bio-imaging are reviewed. Finally, their potential as bimodal IR and luminescent probes is addressed.

Luminescent oligo(ethylene glycol)-functionalized cyclometalated platinum(II) complexes: cellular characterization and mitochondria-specific localization

A readily tunable series of non-planar oligo(ethylene glycol)-substituted phosphorescent Pt(II) complexes has been investigated as live cell imaging agents; suitable structural modifications can give good cellular uptake, traceable mitochondria-specific localization and potent cytotoxic characteristics towards HeLa cells.

Enhanced Cytotoxicity through Conjugation of a "Clickable" Luminescent Re(I) Complex to a Cell-Penetrating Lipopeptide

Re(I) tricarbonyl polypyridine-based complexes are particularly attractive metal complexes in the field of inorganic chemical biology due to their luminescent properties, ease of conjugation to targeting biomolecules, and the possibility to prepare their "hot" (99m)Tc analogues for radioimaging. In this study, we prepared and characterized a novel, "clickable" complex, [Re(2,2'-bipyridine)(3-ethynylpyridine)(CO)3](BF4) ([Re(CO) 3 (bipy)(py-alkyne)](BF 4 )), exhibiting the characteristic luminescent properties and moderate cytotoxicity of this general class of compound. Using Cu(I)-catalyzed "click" chemistry, the complex was efficiently attached to a lipidated peptide known to increase cell permeability, namely, the myristoylated HIV-1 Tat peptide (myr-Tat), to give Re-myr-Tat. Fluorescence microscopy localization in human cervical cancer cells (HeLa) confirmed enhanced cellular uptake of Re-myr-Tat compared with [Re(CO) 3 (bipy)(py-alkyne)](BF 4 ), and cytotoxicity studies showed that this resulted in an increase in potency to a level comparable with cisplatin (13.0 ± 2.0 μM).