Hairpin-shaped heterometallic luminescent lanthanide complexes for DNA intercalative recognition

Challenges and opportunities in the development of metal-based anticancer theranostic agents

Around 10 million fatalities were recorded worldwide in 2020 due to cancer and statistical projections estimate the number to increase by 60% in 2040. With such a substantial rise in the global cancer burden, the disease will continue to impose a huge socio-economic burden on society. Currently, the most widely used clinical treatment modality is cytotoxic chemotherapy using platinum drugs which is used to treat variety of cancers. Despite its clinical success, critical challenges like resistance, off-target side effects and cancer variability often reduce its overall therapeutic efficiency. These challenges require faster diagnosis, simultaneous therapy and a more personalized approach toward cancer management. To this end, small-molecule ‘theranostic’ agents have presented a viable solution combining diagnosis and therapy into a single platform. In this review, we present a summary of recent efforts in the design and optimization of metal-based small-molecule ‘theranostic’ anticancer agents. Importantly, we highlight the advantages of a theranostic candidate over the purely therapeutic or diagnostic agent in terms of evaluation of its biological properties.

Pentanuclear MII–MnII (M = Ni and Cu) complexes of N2O2 donor ligands with a variation of carboxylate anions: syntheses, structures, magnetic properties and catecholase-like activities

One NiII2MnII3 and two CuII2MnII3 complexes have been synthesized using N2O2 donor ligands. The former complex exhibits spin crossover at 2 K temperature. All the complexes exhibit catecholase-like activities.

Hetero-bimetallic Lanthanide-Coinage Metal Compounds Featuring Possible Metal-Metal Interactions in the Excited State

Heterometallic complexes, combining metals of the outer rims of the d-block, for example lanthanides(III) (Ln) and coinage metals(I) (M) are scarcely reported, synthetically challenging and highly interesting in terms of their interactions. In this context, we synthesized hetero-bimetallic Ln-M compounds ligated by the phosphine functionalized amidinate system (N,N'-bis[(2-diphenylphosphino)phenyl]formamidinate, "dpfam"). The resulting compounds [dpfam3 LnM][OTf] (Ln = La, Nd and M = Ag, Au) feature a close proximity of the two metal centres and were investigated experimentally by photoluminescence spectroscopy and quantum chemical calculations. The latter showed rare La-Au interactions for the first excited state.

Heterometal Grafted Metalla-ynes and Poly(metalla-ynes): A Review on Structure-Property Relationships and Applications

Metalla-ynes and poly(metalla-ynes) have emerged as unique molecular scaffolds with fascinating structural features and intriguing photo-luminescence (PL) properties. Their rigid-rod conducting backbone with tunable photo-physical properties has generated immense research interests for the design and development of application-oriented functional materials. Introducing a second d- or f-block metal fragment in the main-chain or side-chain of a metalla-yne and poly(metalla-yne) was found to further modulate the underlying features/properties. This review focuses on the photo-physical properties and opto-electronic (O-E) applications of heterometal grafted metalla-ynes and poly(metalla-ynes).

Luminescent lanthanide(III) complexes of DTPA-bis(amido-phenyl-terpyridine) for bioimaging and phototherapeutic applications

We report here a series of coordinatively-saturated and thermodynamically stable luminescent [Ln(dtntp)(H₂O)] [Ln(III) = Eu (1), Tb (2), Gd (3), Sm (4) and Dy (5)] complexes using an aminophenyl-terpyridine appended-DTPA (dtntp) chelating ligand as cell imaging and photocytotoxic agents. The N,N″-bisamide derivative of H₅DTPA named as dtntp is based on 4'-(4-aminophenyl)-2,2':6',2″-terpyridine conjugated to diethylenetriamine-N,N',N″-pentaacetic acid. The structure, physicochemical properties, detailed photophysical aspects, interaction with DNA and serum proteins, and photocytotoxicity were studied. The intrinsic luminescence of Eu(III) and Tb(III) complexes due to f → f transitions used to evaluate their cellular uptake and distribution in cancer cells. The solid-state structure of [Eu(dtntp)(DMF)] (1·DMF) shows a discrete mononuclear molecule with nine-coordinated {EuN₃O₆} distorted tricapped-trigonal prism (TTP) coordination geometry around the Eu(III). The {EuN₃O₆} core results from three nitrogen atoms and three carboxylate oxygen atoms, and two carbonyl oxygen atoms of the amide groups of dtntp ligand. The ninth coordination site is occupied by an oxygen atom of DMF as a solvent from crystallization. The designed probes have two aromatic pendant phenyl-terpyridine (Ph-tpy) moieties as photo-sensitizing antennae to impart the desirable optical properties for cellular imaging and photocytotoxicity. The photostability, coordinative saturation, and energetically rightly poised triplet states of dtntp ligand allow the efficient energy transfer (ET) from Ph-tpy to the emissive excited states of the Eu(III)/Tb(III), makes them luminescent cellular imaging probes. The Ln(III) complexes show significant binding tendency to DNA (K ~ 10⁴ M^-1), and serum proteins (BSA and HSA) (K ~ 10⁵ M^-1). The luminescent Eu(III) (1) and Tb(III) (2) complexes were utilized for cellular internalization and cytotoxicity studies due to their optimal photophysical properties. The cellular uptake studies using fluorescence imaging displayed intracellular (cytosolic and nuclear) localization in cancer cells. The complexes 1 and 2 displayed significant photocytotoxicity in HeLa cells. These results offer a modular design strategy with further scope to utilize appended N,N,N-donor tpy moiety for developing light-responsive luminescent Ln(III) bioprobes for theranostic applications.

Efficient Sensitized Luminescence of Binuclear Ln(III) Complexes Based on a Chelating Bis-Carbacylamidophosphate

Binuclear rare earth complexes Ln₂L₃phen₂ (Ln^III = Nd^III, Sm^III, Eu^III, Tb^III, Dy^III, Yb^III and Y^III) with bis-CAPh type ligand - tetramethyl N,N'-(2,2,3,3,4,4-hexafluoro-1,5-dioxopentane-1,5-diyl)bis(phosphoramidate) (H₂L) and 1,10-phenanthroline (phen) were synthesized and characterized by elemental analysis, IR, NMR, absorption and luminescence spectroscopy. Luminescence measurements were performed for all the complexes in solid state and for the Eu^III, Tb^III and Y^III complexes - in solution in DMSO as well. The effective energy transfer from organic ligands to Ln^III ions strongly sensitizes the Ln^III ions emission and under excitation by UV light, the complexes exhibited bright characteristic emission of lanthanide metal centers. It was found that the energy level of the ligands lowest triplet state in the complexes matches better to resonance level of Eu^III rather than Tb^III ion. Depending on temperature the emission decay times of solid europium and terbium complexes were in the range of 1.5-2.0 ms. In solid state at room temperature the Eu^III complex possess intense luminescence with very high intrinsic quantum yield 91% and decay time equal 1.88 ms.

Modulation of Nuclearity in CuII -MnII Complexes of a N2 O2 Donor Ligand Depending upon Carboxylate Anions: Structures, Magnetic Properties and Catalytic Oxidase Activities

Three new hetero-metallic copper(II)-manganese(II) complexes, [(CuL)₂ Mn₃ (C₆ H₅ CO₂ )₆ ] (1), [(CuL)₂ Mn(CH₃ CO₂ )₂ ] (2), and {[(CuL)₂ Mn(C₆ H₅ CH₂ CO₂ )₂ ] ⋅ 2CH₃ CN} (3), have been synthesized using [CuL] as ''metalloligand'' (where H₂ L=N,N'-bis(2-hydroxynaphthyl-methylidene)-1,3-propanediamine). Single-crystal structural analyses show an almost linear penta-nuclear structure for complex 1 where a square planar [CuL] unit is connected to each of the two terminal Mn^II ions of a linear, centrosymmetric [Mn₃ (benzoate)₆ ] unit through the double phenoxido bridges. Both complexes 2 and 3 possess a linear tri-nuclear structure where two terminal square-pyramidal [CuL] units are bonded to the central Mn^II ion through double phenoxido oxygen atoms along with a syn-syn bridging acetate (for 2)/phenyl acetate (for 3). All three complexes exhibit catecholase, and phenoxazinone synthase-like activities under aerial conditions. For catecholase like activity, the turnover numbers (k_cat ) are 595, 40, and 205 h^-1 whereas, for phenoxazinone synthase like activity, the turnover numbers are 25, 4, and 11 h^-1 for complexes 1-3, respectively. The mechanism of both catalytic oxidase activities is proposed on the basis of mass spectral evidences. Variable-temperature (2-300 K) dc molar magnetic susceptibility measurements of 1 reveal antiferromagnetic interactions between the Cu-Mn centres (J₁ =-29.3 cm^-1 ), and also between the Mn-Mn centres of the [Mn₃ (benzoate)₆ ] unit (J₂ =-0.68 cm^-1 ). On increasing the magnetic field at 2 K, its ground spin state changes from S=3/2 to S=5/2 at 4 T, attributable to the low value of J₂ which makes the excited spin states close in energy with the ground spin state. Complexes 2 and 3 show antiferromagnetic coupling interactions between the Cu-Mn pairs with J values of -9.51, and -5.32 cm^-1 , respectively.

Quantification of energy transfer in bimetallic Pt(ii)-Ln(iii) complexes featuring an N^C^N-cyclometallating ligand

Cyclometallated Pt(ii) complexes with arylpolypyridyl ligands have impressive photophysical properties (high quantum yields, long lifetimes and tuneable emission) which can be readily tuned by modification of the organic ligand. Despite this, few examples of cyclometallated Pt(ii) complexes as sensitisers for Ln(iii) emission have been reported. Herein, we report the photophysical properties for a series of bimetallic complexes incorporating an N^C^N-coordinated Pt(ii) bearing an alkynyl terpyridine as a metalloligand for a Ln(iii) ion (where Ln = Nd, Gd, Er, Yb and Lu). Using a combination of steady state, time-resolved, and transient absorption experiments, the influence on the photophysical properties of the metalloligand exerted by the different Ln(iii) cations has been investigated, together with the energy transfer efficiency from the metalloligand to the Ln(iii) 4f* excited state.

Bioactive Heterometallic CuII-ZnII Complexes with Potential Biomedical Applications

A series of multinuclear heterometallic Cu-Zn complexes of molecular formula [(CuL)₂Zn(dca)₂] (1), [(CuL)₂Zn(NO₃)₂] (2), [(CuL)₂Zn₂(Cl)₄] (3), and [(CuL)₂Zn₂(NO₂)₄] (4) have been synthesized by reacting [CuL] as a "metalloligand (ML)" (where HL = N,N'-bis(5-chloro-2-hydroxybenzylidene)-2,2-dimethylpropane-1,3-diamine) and by varying the anions or coligands using the same molar ratios of the reactants. All of the four products including the ML have been characterized by infrared and UV-vis spectroscopies and elemental and single-crystal X-ray diffraction analyses. By varying the anions, different structures and topologies are obtained which we have tried to rationalize by means of thorough density functional theory calculations. All of the complexes (1-4) have now been applied for several biological investigations to verify their therapeutic worth. First, their cytotoxicity properties were assessed against HeLa human cervical carcinoma along with the determination of IC₅₀ values. The study was extended with extensive DNA and protein binding experiments followed by detailed fluorescence quenching study with suitable reagents to comprehend the mechanistic pathway. From all of these biological studies, it has been found that all of these heterometallic complexes show more than a few fold improvement of their therapeutic values as compared to the similar homometallic ones probably because of the simultaneous synergic effect of copper and zinc. Among all of the four heterometallic complexes, complex 3 exhibits highest binding constants and IC₅₀ values suggest for their better interaction toward the biological targets and hence have better clinical importance.

Synthesis, structure, and photoluminescent behaviour of molecular lanthanide–2-thiophenecarboxylate–2,2′:6′,2′′-terpyridine materials

A lanthanide series incorporating 2-thiophenecarboxylate and terpyridine is presented. Four structure types are observed with differences in the coordination number and nuclearity of the complexes attributed to the effects of the lanthanide contraction.

A luminescent europium(iii)–platinum(ii) heterometallic complex as a theranostic agent: a proof-of-concept study

We present a luminescent heterometallic multifunctional theranostic Eu–Pt₂ complex, [{cis-PtCl₂(DMSO)}₂Eu(L)(H₂O)], possessing two cytotoxic Pt-centers with four DNA-binding sites, which shows intracellular Eu-based red luminescence sensitized by platinum based MLCT excited states.

Targeting of DNA molecules, BSA/c-Met tyrosine kinase receptors and anti-proliferative activity of bis(terpyridine)copper(ii) complexes

The newly synthesized bis(terpyridine)copper(ii) complexes may act as DNA-targeting metallo-anticancer agents to overcome cisplatin resistance.

Fluorescent Labeling of Plasmid DNA and mRNA: Gains and Losses of Current Labeling Strategies

Live-cell imaging has provided the life sciences with insights into the cell biology and dynamics. Fluorescent labeling of target molecules proves to be indispensable in this regard. In this Review, we focus on the current fluorescent labeling strategies for nucleic acids, and in particular mRNA (mRNA) and plasmid DNA (pDNA), which are of interest to a broad range of scientific fields. By giving a background of the available techniques and an evaluation of the pros and cons, we try to supply scientists with all the information needed to come to an informed choice of nucleic acid labeling strategy aimed at their particular needs.

Two series of novel 3D potentially porous heterometallic Cu–Ln coordination frameworks assembled by 3,4-pyridinedicarboxylic acid with different topologies and channels: syntheses, structures, luminescence and magnetic properties

Two series of heterometallic Cu–Ln coordination frameworks assembled by 3,4-pyridinedicarboxylic acid with different topologies and channels.

A new heterometallic terbium(iii)–ruthenium(ii) complex and its terbium(iii)–zinc(ii) analog: syntheses, characterization, luminescence, and electrochemical properties

Tb^IIIsensitized Ru^IIluminescence by intramolecular energy transfer from the luminescent⁵D₄MC excited state of Tbiiito the³MLCT state of Ru^IIat RT in the d–f heterometallic assembly [Tb(NO₃)₂(L2){Ru(ttpy)}₂](PF₆)₅is reported.

Photocytotoxic luminescent lanthanide complexes of DTPA–bisamide using quinoline as photosensitizer

Luminescent lanthanide(iii) complexes of DTPA–bisamide quinoline as photosensitizer were studied for their structures, luminescent properties, binding with DNA and protein, photo-induced DNA cleavage, photocytotoxicity and cellular uptake studies.

Sensitized near-infrared luminescence from Nd(III), Yb(III) and Er(III) complexes by energy-transfer from ruthenium 1,3-Bis([1,10]phenanthroline-[5,6-d]-imidazol-2 -yl)benzene

The luminescent ruthenium 1,3 -bis([1,10]phenanthroline-[5,6 -d]- imidazol-2 -yl)benzene (bpibH2) complex, a potentially useful bridging ligand with a vacant diimine site, has been used as 'metallo ligand' to make heterodinuclear d-f complexes by attachment of a {Ln(dik)3} fragment (dik = 1,3-diketonate) at the vacant site. When Ln = Nd, Yb, or Er the lanthanide centre has low-energy f-f excited states capable of accepting energy from the (3)MLCT excited state of the Ru(II) centre, there is quenching in the (3)MLCT luminescence of the Ru(II) centre, that affords sensitized lanthanide(III) based luminescence in the near-IR region. Nd(III) was found to be the most effective at quenching the (3)MLCT luminescence of the ruthenium component because of the high density of f-f excited states of the appropriate energy which make it as effective energy-acceptor compared to Er and Yb complexes.