Synthetic peptides caged on histidine residues with a bisbipyridyl ruthenium(II) complex that can be photolyzed by visible light

Mitochondria-Targeted COUPY Photocages: Synthesis and Visible-Light Photoactivation in Living Cells

Releasing bioactive molecules in specific subcellular locations from the corresponding caged precursors offers great potential in photopharmacology, especially when using biologically compatible visible light. By taking advantage of the intrinsic preference of COUPY coumarins for mitochondria and their long wavelength absorption in the visible region, we have synthesized and fully characterized a series of COUPY-caged model compounds to investigate how the structure of the coumarin caging group affects the rate and efficiency of the photolysis process. Uncaging studies using yellow (560 nm) and red light (620 nm) in phosphate-buffered saline medium have demonstrated that the incorporation of a methyl group in a position adjacent to the photocleavable bond is particularly important to fine-tune the photochemical properties of the caging group. Additionally, the use of a COUPY-caged version of the protonophore 2,4-dinitrophenol allowed us to confirm by confocal microscopy that photoactivation can occur within mitochondria of living HeLa cells upon irradiation with low doses of yellow light. The new photolabile protecting groups presented here complement the photochemical toolbox in therapeutic applications since they will facilitate the delivery of photocages of biologically active compounds into mitochondria.

Photoactivatable o-Hydroxycinnamic Platforms for Bioimaging and Therapeutic Release

Photoactivatable or photoremovable protecting groups (PPGs) have become a powerful material and gained enormous interest in the field of biomedical applications. PPGs have been utilized for noninvasive, on-demand, spatio-temporal controlled release of biological effectors by irradiation with light to induce biochemical function. Over the past few years, o-hydroxycinnamate (oHC)-based PPGs have received considerable attention for the release of molecules of interest by either UV (one-photon) or near-IR (two-photon) irradiation. In this miniperspective, we have summarized the development of oHC PPGs for bioimaging and the controlled release of therapeutics, bioactive volatiles and other payloads with real-time monitoring. In addition, several future perspectives of oHC systems have been highlighted at the end of this miniperspective.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Transformation of COUPY Fluorophores into a Novel Class of Visible-Light-Cleavable Photolabile Protecting Groups

Although photolabile protecting groups (PPGs) have found widespread applications in several fields of chemistry, biology and materials science, there is a growing interest in expanding the photochemical toolbox to overcome some of the limitations of classical caging groups. In this work, the synthesis of a new class of visible-light-sensitive PPGs based on low-molecular weight COUPY fluorophores with several attractive properties, including long-wavelength absorption, is reported. Besides being stable to spontaneous hydrolysis in the dark, COUPY-based PPGs can be efficiently photoactivated with yellow (560 nm) and red light (620 nm) under physiological-like conditions, thereby offering the possibility of unmasking functional groups from COUPY photocages under irradiation conditions in which other PPGs remain stable. Additionally, COUPY photocages exhibit excellent cellular uptake and accumulate selectively in mitochondria, opening the door to the delivery of caged analogues of biologically active compounds into these organelles.

Stimuli-Responsive DNA Binding by Synthetic Systems

DNA is the molecule responsible for the storage and transmission of the genetic information in living organisms. The expression of this information is highly regulated. In eukaryotes, it is achieved mainly at the transcription level thanks to specialized proteins called transcription factors (TFs) that recognize specific DNA sequences, thereby promoting or inhibiting the transcription of particular genes. In many cases, TFs are present in the cell in an inactive form but become active in response to an external signal, which might modify their localization and DNA binding properties or modulate their interactions with the rest of the transcriptional machinery. As a result of the crucial role of TFs, the design of synthetic peptides or miniproteins that can emulate their DNA binding properties and eventually respond to external stimuli is of obvious interest. On the other hand, although the B-form double helix is the most common DNA secondary structure, it is not the only one with an essential biological function. Guanine quadruplexes (GQs) have received considerable attention due to their critical role in the regulation of gene expression, which is usually associated with a change in the GQ conformation. Thus, the development of GQ probes whose properties can be controlled using external signals is also of significant relevance.In this Account, we present a summary of the recent efforts toward the development of stimuli-responsive synthetic DNA binders with a particular emphasis on our own contributions. We first introduce the structure of B and GQ DNAs, and some of the main factors underlying their selective recognition. We then discuss some of the different approaches used for the design of stimulus-mediated DNA binders. We have organized our discussion according to whether the interaction takes place with duplex or guanine quadruplex DNAs, and each section is divided according to the nature of the stimulus (i.e., physical or chemical). Regarding physical stimuli, light (through the incorporation of photolabile protecting groups or photoisomerizable agents) is the most common input for the activation/deactivation of DNA binding events. With respect to chemical signals, the use of metals (through the incorporation of metal-coordinating groups in the DNA binding agent) has allowed the development of a wide range of stimuli-responsive DNA binders. More recently, redox-based systems have also been used to control DNA interactions.This Account ends with a "Conclusions and Outlook" section highlighting some of the general lessons that have been learned and future directions toward further advancing the field.

Toward Optimal Ru(II) Photocages: Balancing Photochemistry, Stability, and Biocompatibility Through Fine Tuning of Steric, Electronic, and Physiochemical Features

Ru(II) complex photocages are used in a variety of biological applications, but the thermal stability, photosubstitution quantum yield, and biological compatibility of the most commonly used Ru(II) systems remain unoptimized. Here, multiple compounds used in photocaging applications were analyzed and found to have several unsatisfactory characteristics. To address these deficiencies, three new scaffolds were designed to improve key properties through modulation of a combination of electronic, steric, and physiochemical features. One of these new systems, containing the 2,2'-biquinoline-4,4'-dicarboxylic acid (2,2'-bicinchoninic acid) ligand, fulfills several of the requirements for an optimal photocage. Another complex, containing the 2-benzothiazol-2-yl-quinoline ligand, provides a scaffold for the creation of "dual action" agents.

Near-Infrared-Triggered Photodynamic, Photothermal, and on Demand Chemotherapy by Multifunctional Upconversion Nanocomposite

In an attempt to integrate photodynamic therapy (PDT) with photothermal therapy and chemotherapy for enhanced anticancer activity, we have rationally synthesized a multifunctional upconversion nanoplatform using NaYF₄:Yb/Tm/Er/Fe nanoparticles (NPs) as the core and NaYbF₄:1% Tm as a shell. The as-synthesized core-shell upconversion (CSU) NPs exhibited diverse and enhanced photoluminescence emissions in a wide range (UV to NIR) consequent upon Fe³⁺ doping in the core and fabrication of an active shell. Subsequently, CSU was first decorated with titania NPs as photosensitizers. Next, the mesoporous silica (MS) shell loaded with doxorubicin (DOX) via a photocleavable Ru complex as the gating molecule was developed around titania-containing CSU. Finally, gold nanorods (GNRs) with localized surface plasmon resonance (LSPR) at 800 nm were incorporated around the MS layer to obtain the multifunctional nanoplatform. We demonstrated that the UV, blue, and NIR emissions from the CSU produced ROS-mediated PDT through titania activation, induced DOX release through photocleavage of the Ru complex, and generated hyperthermia by LSPR activity of GNRs, respectively, upon a single NIR excitation through FRET. The therapeutic efficacy was validated on HeLa cell lines in vitro by various microscopic and biochemical studies under a significantly milder NIR irradiation and lower dosage of the nanoplatforms, which have been further demonstrated as diagnostic nanoprobes for cell imaging.

Light-activated ruthenium (II)-bicalutamide prodrugs for prostate cancer

Targeted delivery of clinically approved anticancer drug to tumor sites is an effective way to achieve enhanced drug efficacy as well as reduced side effects and toxicity. Here bicalutamide is caged by the Ru(II) center through the nitrile group, and three photoactive Ru(II) complexes were designed and synthesized. Docking study showed that the ruthenium(II) fragments can effectively block the binding of complexes 1-3 with AR (androgen receptor) owing to the large steric structures, thus bicalutamide in complexes 1-3 could not interact with AR-LBD (ligand binding domain). Once irradiation with blue light (465nm), complexes 1-3 can release bicalutamide and anticancer Ru(II) fragments, which possesses dual-action of AR binding and DNA interaction simultaneously. In vitro cytotoxicity study on these complexes further confirmed that complexes 1-3 exhibited considerable cytotoxicity upon irradiation with blue light. Significantly, complex 3 could be activated at 660nm, which greatly increases the scope of complex 3 to treat deeper within tissue. Theoretical calculations showed that the lowest singlet excitation energy of complex 3 is lower than those of complexes 1-2, which explains the experimental results well. Moreover, the ³MC (metal centered) states of these complexes are more stable than their ³MLCT (metal to ligand charge transfer) states, indicating that the photoactive processes of these complexes are likely to result in ligand dissociation.

Light-triggered release of photocaged therapeutics - Where are we now?

The current available therapeutics face several challenges such as the development of ideal drug delivery systems towards the goal of personalized treatments for patients benefit. The application of light as an exogenous activation mechanism has shown promising outcomes, owning to the spatiotemporal confinement of the treatment in the vicinity of the diseased tissue, which offers many intriguing possibilities. Engineering therapeutics with light responsive moieties have been explored to enhance the bioavailability, and drug efficacy either in vitro or in vivo. The tailor-made character turns the so-called photocaged compounds highly desirable to reduce the side effects of drugs and, therefore, have received wide research attention. Herein, we seek to highlight the potential of photocaged compounds to obtain a clear understanding of the mechanisms behind its use in therapeutic delivery. A deep overview on the progress achieved in the design, fabrication as well as current and possible future applications in therapeutics of photocaged compounds is provided, so that novel formulations for biomedical field can be designed.

New Ru(ii) complex for dual photochemotherapy: release of cathepsin K inhibitor and 1O2 production

A new complex, [Ru(tpy)(dppn)(Cbz-Leu-NHCH2CN)]2+ (1, tpy = 2,2':6',2''-terpyridine, dppn = benzo[i]dipyrido[3,2-a:2',3'-c]phenazine) was synthesized and its photochemical properties were investigated. This complex undergoes photorelease of the Cbz-Leu-NHCH2CN ligand, a known cathepsin K inhibitor, with a quantum yield, Φ450, of 0.0012(4) in water (λirr = 450 nm). In addition, 1 sensitizes the production of singlet oxygen upon visible light irradiation with quantum yield, ΦΔ, of 0.64(3) in CH3OH. The photophysical properties of 1 were compared with those of [Ru(tpy)(bpy)(Cbz-Leu-NHCH2CN)]2+ (2, bpy = 2,2'-bipyridine), [Ru(tpy)(dppn)(CH3CN)]2+ (3), and [Ru(tpy)(bpy)(CH3CN)]2+ (4) to evaluate the effect of the release of the Cbz-Leu-NHCH2CN inhibitor relative to the CH3CN ligand, as well as the role of dppn as the bidentate ligand for 1O2 production instead of bpy. Nanosecond transient absorption spectroscopy confirms the formation of the long-lived dppn-centered 3ππ* state in 1 and 3 with a maximum at ∼540 nm and τ ∼20 μs in deaerated acetonitrile. Complexes 1 and 3 are able to cause photoinduced damage to DNA (λirr ≥ 395 nm), whereas 2 and 4 do not photocleave DNA under similar experimental conditions. These results suggest that 1 is a promising agent for dual activity, both releasing a drug and producing singlet oxygen, and is poised to exhibit enhanced biological activity in phototochemotherapy upon irradiation with visible light.

Ru(ii) complexes with diazine ligands: electronic modulation of the coordinating group is key to the design of "dual action" photoactivated agents

Coordination complexes can be used to photocage biologically active ligands, providing control over the location, time, and dose of a delivered drug. Dual action agents can be created if both the ligand released and the ligand-deficient metal center effect biological processes. Ruthenium(ii) complexes coordinated to pyridyl ligands generally are only capable of releasing one ligand in H2O, wasting equivalents of drug molecules, and producing a Ru(ii) center that is not cytotoxic. In contrast, Ru(ii) polypyridyl complexes containing diazine ligands eject both monodentate ligands, with the quantum yield (φPS) of the second phase varying as a function of ligand pKa and the pH of the medium. This effect is general, as it is effective with different Ru(ii) structures, and demonstrates that diazine-based drugs are the preferred choice for the development of light-activated dual action Ru(ii) agents.

Ru(II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging

Metal complexes have many proven applications in the caging and photochemical release of biologically active compounds. Photocaging groups derived from Ru(II) traditionally have been composed of ancillary ligands that are planar and bi- or tridentate, such as 2,2'-bipyridine (bpy), 2,2':6',2″-terpyridine (tpy), and 1,10-phenanthroline (phen). Complexes bearing ancillary ligands with denticities higher than three represent a new class of Ru(II)-based photocaging groups that are grossly underdeveloped. Because high-denticity ancillary ligands provide the ability to increase the structural rigidity and control the stereochemistry, our groups initiated a research program to explore the applications of such ligands in Ru(II)-based photocaging. Ru(TPA), bearing the tetradentate ancillary ligand tris(2-pyridylmethyl)amine (TPA), has been successfully utilized to effectively cage nitriles and aromatic heterocycles. Nitriles and aromatic heterocycles caged by the Ru(TPA) group show excellent stability in aqueous solutions in the dark, and the complexes can selectively release the caged molecules upon irradiation with light. Ru(TPA) is applicable as a photochemical agent to offer precise spatiotemporal control over biological activity without undesired toxicity. In addition, Ru(II) polypyridyl complexes with desired photochemical properties can be synthesized and identified by solid-phase synthesis, and the resulting complexes show properties to similar to those of complexes obtained by solution-phase synthesis. Density functional theory (DFT) calculations reveal that orbital mixing between the π* orbitals of the ancillary ligand and the Ru-N dσ* orbital is essential for ligand photodissociation in these complexes. Furthermore, the introduction of steric bulk enhances the photoliability of the caged molecules, validating that steric effects can largely influence the quantum efficiency of photoinduced ligand exchange in Ru(II) polypyridyl complexes. Recently, two new photocaging groups, Ru(cyTPA) and Ru(1-isocyTPQA), have been designed and synthesized for caging of nitriles and aromatic heterocycles, and these complexes exhibit unique photochemical properties distinct from those derived from Ru(TPA). Notably, the unusually greater quantum efficiency for the ligand exchange in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂, Φ₄₀₀ = 0.033(3), uncovers a trans-type effect in the triplet metal-to-ligand charge transfer (³MLCT) state that enhances photoinduced ligand exchange in a new manner. DFT calculations and ultrafast transient spectroscopy reveal that the lowest-energy triplet state in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is a highly mixed ³MLCT/³ππ* excited state rather than a triplet metal-centered ligand-field (³LF) excited state; the latter is generally accepted for ligand photodissociation. In addition, Mulliken spin density calculations indicate that a majority of the spin density in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is localized on the isoquinoline arm, which is opposite to the cis MeCN, rather than on the ruthenium center. This significantly weakens the Ru-N6 ( cis MeCN) bond, which then promotes the ligand photodissociation. This newly discovered effect gives a clearer perception of the interplay between the ³MLCT and ³LF excited states of Ru(II) polypyridyl complexes, which may be useful in the design and applications of ruthenium complexes in the areas of photoactivated drug delivery and photosensitizers.

Photoactivation of imatinib-antibody conjugate using low-energy visible light from Ru(ii)-polypyridyl cages

Ru(ii)-polypyridyl cages with sterically bulky bidentate ligands provide efficient photochemical release of the anticancer drug imatinib using low energy visible light, imparting spatiotemporal control over drug bioavailability. The light-activated drug release is maintained when the Ru(ii) cage is covalently coupled to an antibody, which is expected to localize selectively on the tumor.

Illuminating cytochrome P450 binding: Ru(ii)-caged inhibitors of CYP17A1

New Ru(ii)-caged abiraterone complexes were synthesized that exhibit strong absorption in the visible region and release the steroidal CYP17A1 inhibitor abiraterone upon exposure to low energy visible light in buffer and prostate cancer cells. Photoinduced release results in abiraterone binding to its CYP17A1 target in an inhibitory mode.

Ru(ii) polypyridyl complexes as photocages for bioactive compounds containing nitriles and aromatic heterocycles

Photocaging allows for precise spatiotemporal control over the release of biologically active compounds with light. Most photocaged molecules employ organic photolabile protecting groups; however, biologically active compounds often contain functionalities such as nitriles and aromatic heterocycles that cannot be caged with organic groups. Despite their prevalence, only a few studies have reported successful caging of nitriles and aromatic heterocycles. Recently, Ru(ii)-based photocaging has emerged as a powerful method for the release of bioactive molecules containing these functional groups, in many cases providing high levels of spatial and temporal control over biological activity. This Feature Article discusses recent developments in applying Ru(ii)-based photocaging towards biological problems. Our groups designed and synthesized Ru(ii)-based platforms for the photoinduced delivery of cysteine protease and cytochrome P450 inhibitors in order to achieve selective control over enzyme inhibition. We also reported Ru(ii) photocaging groups derived from higher-denticity ancillary ligands that possess photophysical and photochemical properties distinct from more traditional Ru(ii)-based caging groups. In addition, for the first time, we are able to rapidly synthesize and screen Ru(ii) polypyridyl complexes that elicit desired properties by solid-phase synthesis. Finally, our work also defined steric and orbital mixing effects that are important factors in controlling photoinduced ligand exchange.

Sequential Uncaging with Green Light can be Achieved by Fine-Tuning the Structure of a Dicyanocoumarin Chromophore

We report the synthesis and photochemical properties of a series of dicyanocoumarinylmethyl (DEAdcCM)- and dicyanocoumarinylethyl (DEAdcCE)-based photocages of carboxylic acids and amines with absorption maximum around 500 nm. Photolysis studies with green light have demonstrated that the structure of the coumarin chromophore as well as the nature of the leaving group and the type of bond to be photocleaved (ester or carbamate) have a strong influence on the rate and efficiency of the uncaging process. These experimental observations were also supported by DFT calculations. Such differences in deprotection kinetics have been exploited to sequentially photolyze two dicyanocoumarin-caged model compounds (e.g., benzoic acid and ethylamine), and open the way to increasing the number of functional levels that can be addressed with light in a single system, particularly when combining dicyanocoumarin caging groups with other photocleavable protecting groups, which remain intact under green light irradiation.

Development of Green/Red-Absorbing Chromophores Based on a Coumarin Scaffold That Are Useful as Caging Groups

We report the design, synthesis, and spectroscopic characterization of a series of push-pull chromophores based on a novel coumarin scaffold in which the carbonyl of the lactone function of the original coumarin dyes has been replaced by the cyano(4-nitrophenyl)methylene moiety. The skeleton of the compounds was synthesized by condensation of a thiocoumarin precursor with the corresponding arylacetonitrile derivatives, and their photophysical properties were fine-tuned through the incorporation of electron-withdrawing groups (EWGs) like nitro and cyano at the phenyl ring, leading to absorption in the green to red region. Although fluorescence emission was weakened or even canceled upon introduction of two or three strong EWGs, the emission of the mononitro-containing coumarin derivatives in the red region upon excitation with green light is noticeable, as are their significantly large Stokes shifts. The new coumarin derivatives can be useful as photocleavable protecting groups, as demonstrated through the synthesis and characterization of a series of coumarin-based photocages of benzoic acid. Preliminary photolysis studies with green light have demonstrated that the structure of the coumarin chromophore influences the rate of the uncaging process, opening the way to exploiting these new coumarin scaffolds as caging groups that can be removed with visible light.

Influence of the Steric Bulk and Solvent on the Photoreactivity of Ruthenium Polypyridyl Complexes Coordinated to l-Proline

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)₂(l-prol)]PF₆ ([1a]PF₆; bpy = 2,2′-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF₆ ([2a]PF₆ and [2b]PF₆; dmbpy = 6,6′-dimethyl-2,2′-bipyridine), and Λ-[Ru(dmbpy)₂(l-prol)]PF₆ ([3a]PF₆). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF₆ and [2b]PF₆. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF₆, [2a]PF₆, [2b]PF₆, and [3a]PF₆ was studied in water (H₂O) and acetonitrile (MeCN) using UV–vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and ¹H NMR spectroscopy. In H₂O, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF₆ was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF₆) or four ([3a]PF₆) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF₆ was photostable in PBS, [2a]PF₆ quantitatively isomerized to [2b]PF₆ upon light irradiation. In pure MeCN, [2a]PF₆ and [3a]PF₆ showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF₆ was photostable. Finally, in H₂O–MeCN mixtures, [3a]PF₆ showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed.

Three ruthenium polypyridyl l-proline complexes with increasing strain (R, R′ = H or Me) were synthesized and their photoreactivities studied in phosphate-buffered saline, pure acetonitrile (MeCN), and water−MeCN mixtures. Depending on the number of methyl groups, on the presence of air, and on the nature of the solvent, either photoisomerization, photooxidation of l-proline, selective photosubstitution, or nonselective photosubstitution was observed.

An Overview Of Photosubstitution Reactions Of Ru(II) Imine Complexes And Their Application In Photobiology And Photodynamic Therapy

This article is a short review that presents a short review of photosubstitution reactions of Ru(II) imine complexes and illustrates their use in the development of therapeutic agents. The review begins with an overview of the photophysical behavior and common photoreactions of Ru(II) imine complexes, with select examples from the literature since the 1960s. It is followed by a more detailed picture of the application of knowledge gained over the years in the development of Ru(II) complexes for photobiology and photodynamic therapy.

Ruthenation of Non-stacked Guanines in DNA G-Quadruplex Structures: Enhancement of c-MYC Expression

Guanine quadruplexes (GQs) are compact four-stranded DNA structures that play a key role in the control of a variety of biological processes, including gene transcription. Bulky ruthenium complexes featuring a bipyridine, a terpyridine, and one exchangeable ligand ([Ru(terpy)(bpy)X]n+ ) are able to metalate exposed guanines present in the GQ of the c-MYC promoter region that are not involved in quadruplex base pairing. qRT-PCR and western-blot experiments indicated that the complexes promote a remarkable increase in the expression of this oncogene. We also show that exchangeable thioether ligands (X=RSR', Met) allow regulation of the metalating activity of the complex with visible light.

A Green Light-Triggerable RGD Peptide for Photocontrolled Targeted Drug Delivery: Synthesis and Photolysis Studies

We describe for the first time the synthesis and photochemical properties of a coumarin-caged cyclic RGD peptide and demonstrate that uncaging can be efficiently performed with biologically compatible green light. This was accomplished by using a new dicyanocoumarin derivative (DEAdcCE) for the protection of the carboxyl function at the side chain of the aspartic acid residue, which was selected on the basis of Fmoc-tBu SPPS compatibility and photolysis efficiency. The shielding effect of a methyl group incorporated in the coumarin derivative near the ester bond linking both moieties in combination with the use of acidic additives such as HOBt or Oxyma during the basic Fmoc-removal treatment were found to be very effective for minimizing aspartimide-related side reactions. In addition, a conjugate between the dicyanocoumarin-caged cyclic RGD peptide and ruthenocene, which was selected as a metallodrug model cargo, has been synthesized and characterized. The fact that green-light triggered photoactivation can be efficiently performed both with the caged peptide and with its ruthenocenoyl bioconjugate reveals great potential for DEAdcCE-caged peptide sequences as selective drug carriers in the context of photocontrolled targeted anticancer strategies.

Effects of Methyl Substitution in Ruthenium Tris(2-pyridylmethyl)amine Photocaging Groups for Nitriles

Four complexes of the general formula [Ru(L)(CH3CN)2](PF6)2, [L = TPA (5), MeTPA (6), Me2TPA (7), and Me3TPA (8)] [TPA = tris[(pyridin-2-yl)methyl]amine, where methyl groups were introduced consecutively onto the 6-position of py donors of TPA, were prepared and characterized by various spectroscopic techniques and mass spectrometry. While 5 and 8 were isolated as single stereoisomers, 6 and 7 were isolated as mixtures of stereoisomers in 2:1 and 1.5:1 ratios, respectively. Steric effects on ground state stability and thermal and photochemical reactivities were studied for all four complexes using (1)H NMR and electronic absorption spectroscopies and computational studies. These studies confirmed that the addition of steric bulk accelerates photochemical and thermal nitrile release.

Selective Release of Aromatic Heterocycles from Ruthenium Tris(2-pyridylmethyl)amine with Visible Light

Three complexes of the general formula [Ru(TPA)L2](PF6)2 [TPA = tris(2-pyridylmethyl)amine], where L = pyridine (1), nicotinamide (2), and imidazole (3), were prepared and characterized spectroscopically. X-ray crystallographic data were obtained for 1 and 3. Complexes 1-3 show strong absorption in the visible region and selective release of heterocycles upon irradiation with visible light. Time-dependent density functional theory calculations are consistent with the presence of singlet metal-to-ligand charge-transfer bands in the visible region in 1-3. Caged heterocycles 1-3 are highly stable in solution in the dark, including in cell growth media. Cell viability data show no signs of toxicity of 1-3 against PC-3 cells at concentrations up to 100 μM under light and dark conditions, consistent with Ru(TPA) acting as a nontoxic and effective photocaging group for aromatic heterocycles.

Light-Controlled Cellular Internalization and Cytotoxicity of Nucleic Acid-Binding Agents: Studies in Vitro and in Zebrafish Embryos

We synthesized octa-arginine conjugates of DNA-binding agents (bisbenzamidine, acridine and Thiazole Orange) and demonstrated that their DNA binding and cell internalization can be inhibited by appending a (negatively charged) oligoglutamic tail through a photolabile linker. UV irradiation released the parent conjugates, thus restoring cell internalization and biological activity. Assays with zebrafish embryos demonstrates the potential of this prodrug strategy for controlling in vivo cytotoxicity.