Claudia Turro

Light activation of a cysteine protease inhibitor: caging of a peptidomimetic nitrile with Ru(II)(bpy)2.

A novel method for caging protease inhibitors is described. The complex [Ru(II)(bpy)(2)(1)(2)](PF(6))(2) (2) was prepared from the nitrile-based peptidomimetic inhibitor Ac-Phe-NHCH(2)CN (1). (1)H NMR, UV-vis, and IR spectroscopic and mass spectrometric data confirmed that 2 equiv of inhibitor 1 bind to Ru(II) through the nitrile functional group. Complex 2 shows excellent stability in aqueous solution in the dark and fast release of 1 upon irradiation with visible light. As a result of binding to the Ru(II) center, the nitriles of complex 2 are caged, and 2 does not act as a potent enzyme inhibitor. However, when 2 is irradiated, it releases 1, which inhibits the cysteine proteases papain and cathepsins B, K and L up to 2 times more potently than 1 alone. Ratios of the IC(50) values in the dark versus in the light ranged from 6:1 to 33:1 for inhibition by 2 against isolated enzymes and in human cell lysates, confirming that a high level of photoinduced enzyme inhibition can be obtained using this method.

Ultrafast ligand exchange: detection of a pentacoordinate Ru(II) intermediate and product formation.

The ultrafast kinetics of ligand exchange of cis-[Ru(bpy)(2)(CH(3)CN)(2)](2+) were measured in H(2)O and CH(3)CN. The formation of the (3)MLCT excited-state and a five-coordinate intermediate are observed in both solvents within 2 ps after excitation (310 nm, fwhm approximately 300 fs). The (3)MLCT excited-state undergoes vibrational cooling (5-6 ps), then decays to regenerate the ground-state with a lifetime of approximately 50 ps. In CH(3)CN, ligand recombination takes place in 28 ps, while the formation of cis-[Ru(bpy)(2)(CH(3)CN)(H(2)O)](2+) in H(2)O takes place with tau = 77 ps.

Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)]2+ (1; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru(tpy)(Me2bpy)(py)]2+ (2; Me2bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru(tpy)(biq)(py)]2+ (3; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH3CN (λirr = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH3)]2+ (NN = Me2bpy, biq) complex with quantum yields, Φ500, of 0.16(1) and 0.033(1) for 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of 1, Φ500 < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)]2+ (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes 1–3 are stable in the dark in both CH3CN and H2O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me2bpy in 2 and biq in 3, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in 1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of 2 and 3 to undergo ligand exchange with λirr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.

The remarkable influence of M2δ to thienyl π conjugation in oligothiophenes incorporating MM quadruple bonds

Oligothiophenes incorporating MM quadruple bonds have been prepared from the reactions between Mo2(TiPB)4 (TiPB = 2,4,6-triisopropyl benzoate) and 3′,4′-dihexyl-2,2′-:5′,2″-terthiophene-5,5″-dicarboxylic acid. The oligomers of empirical formula Mo2(TiPB)2(O2C(Th)-C4(n-hexyl)2S-(Th)CO2) are soluble in THF and form thin films with spin-coating (Th = thiophene). The reactions between Mo2(TiPB)4 and 2-thienylcarboxylic acid (Th-H), 2,2′-bithiophene-5-carboxylic acid (BTh-H), and (2,2′:5′,2″-terthiophene)-5-carboxylic acid (TTh-H) yield compounds of formula trans-Mo2(TiPB)2L2, where L = Th, BTh, and TTh (the corresponding thienylcarboxylate), and these compounds are considered as models for the aforementioned oligomers. In all cases, the thienyl groups are substituted or coupled at the 2,5 positions. Based on the x-ray analysis, the molecular structure of trans-Mo2(TiPB)2(BTh)2 reveals an extended Lπ-M2δ-Lπ conjugation. Calculations of the electronic structures on model compounds, in which the TiPB are substituted by formate ligands, reveal that the HOMO is mainly attributed to the M2δ orbital, which is stabilized by back-bonding to one of the thienylcarboxylate π* combinations, and the LUMO is an in-phase combination of the thienylcarboxylate π* orbitals. The compounds and the oligomers are intensely colored due to M2δ–thienyl carboxylate π* charge transfer transitions that fall in the visible region of the spectrum. For the molybdenum complexes and their oligomers, the photophysical properties have been studied by steady-state absorption spectroscopy and emission spectroscopy, together with time-resolved emission and transient absorption for the determination of relaxation dynamics. Remarkably, THF solutions the molybdenum complexes show room-temperature dual emission, fluorescence and phosphorescence, originating mainly from 1MLCT and 3MM(δδ*) states, respectively. With increasing number of thienyl rings from 1 to 3, the observed lifetimes of the 1MLCT state increase from 4 to 12 ps, while the phosphorescence lifetimes are ≈80 μs. The oligomers show similar photophysical properties as the corresponding monomers in THF but have notably longer-lived triplet states, ≈200 μs in thin films. These results, when compared with metallated oligothiophenes of the later transition elements, reveal that M2δ–thienyl π conjugation leads to a very small energy gap between the 1MLCT and 3MLCT states of <0.6 eV.

Inhibition of cathepsin activity in a cell-based assay by a light-activated ruthenium compound.

Light‐activated inhibition of cathepsin activity was demonstrated in a cell‐based assay. Inhibitors of cathepsin K, Cbz‐Leu‐NHCH2CN (2) and Cbz‐Leu‐Ser(OBn)‐CN (3), were caged within the complexes cis‐[Ru(bpy)2(2)2]Cl2 (4) and cis‐[Ru(bpy)2(3)2](BF4)2 (5) (bpy=2,2′‐bipyridine) as 1:1 mixtures of Δ and Λ stereoisomers. Complexes 4 and 5 were characterized by 1H NMR, IR, and UV/Vis spectroscopies and electrospray mass spectrometry. Photochemical experiments confirm that 4 releases two molecules of 2 upon exposure to visible light for 15 min, whereas release of 3 by 5 requires longer irradiation times. IC50 determinations against purified cathepsin K under light and dark conditions with 4 and 5 confirm that inhibition is enhanced from 35‐ to 88‐fold, respectively, upon irradiation with visible light. No apparent toxicity was observed for 4 in the absence or presence of irradiation in bone marrow macrophage (BMM) or PC3 cells, as determined by MTT assays, at concentrations up to 10 μM. Compound 5 is well tolerated at lower concentrations (<1 μM), but does show growth‐inhibitory effects at higher concentrations. Confocal microscopy experiments show that 4 decreases intracellular cathepsin activity in osteoclasts with light activation. These results support the further development of caged nitrile‐based inhibitors as chemical tools for investigating spatial aspects of proteolysis within living systems.

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.

Ruthenium Tris(2-pyridylmethyl)amine as an Effective Photocaging Group for Nitriles

Ruthenium(II) tris(2-pyridylmethyl)amine (TPA) is an effective caging group for nitriles that provides high levels of control over the enzyme activity with light. Two caged nitriles were prepared, [Ru(TPA)(MeCN)2](PF6)2 (1) and [Ru(TPA)(3)2](PF6)2 (2), where 3 is the cathepsin K inhibitor Cbz-Leu-NHCH2CN, and characterized by various spectroscopic techniques and mass spectrometry. Both 1 and 2 show the release of a single nitrile within 20 min of irradiation with 365 nm light. Complex 2 acts as a potent, photoactivated inhibitor of human cathepsin K. IC50 values were determined for 2 and 3. Enzyme inhibition for 2 was enhanced by a factor of 89 upon exposure to light, with IC50 values of 63 nM (light) and 5.6 μM (dark).

Photophysical properties of MM quadruply bonded complexes supported by carboxylate ligands, MM = Mo2, MoW, or W2.

While chemists have extensively studied the photophysical properties of d(6), d(8), and d(10) transition metal complexes, their early transition metal counterparts have received less attention. Quadruply bonded complexes of molybdenum and tungsten supported by carboxylate ligands have intense metal-to-ligand charge transfer (MLCT) absorptions that arise from the electronic coupling of the metal-metal (MM) δ orbital with the CO(2) π-system. This coupling may in turn be linked to an extended π-conjugated organic functional group. The major interaction is akin to the so-called back-bonding in metal carbonyl complexes. By the appropriate selection of MM, its attendant ligands, and the organic group, this absorption can be tuned to span the visible and near IR range, from 400 to 1000 nm. Consequently, these complexes offer potential as photon harvesters for photovoltaic devices and photocatalysis. In this Account, we describe recent studies of dinuclear M(II) containing complexes, where M = Mo or W, and show that there are both parallels and disparities to the monomeric transition metal complexes. These early transition metal complexes have relatively long lived excited state singlets when compared to other transition metal complexes. They also often show unusual dual emission (fluorescence and phosphorescence), with singlet (S(1)) lifetimes that range from 1 to 20 ps, and triplet (T(1)) lifetimes from 3 ns to 200 μs. The fluorescent S(1) states are typically (1)MLCT for both M = Mo and W. These extended singlet lifetimes are uncommon for mononuclear transition metal complexes, which typically have very short lived (1)MLCT states due to rapid femto-second intersystem crossing rates. However, the T(1) states differ. This phosphorescence is MLCT in nature when M = W, while this emission comes from the δδ* state for M = Mo. Through time-resolved femtosecond infrared spectroscopy, we can detect the asymmetric stretch of the CO(2) ligand in both the singlet and triplet δδ* states. Through these analytical methods, we can study how the charge distribution in the singlet and triplet excited states changes over time. In addition, we can detect delocalized or localized examples of MLCT states, which represent class III and I excited state mixed valence in the Robin and Day scheme.

Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)4 (MM ) Mo2, MoW, and W2): Preparation and Photophysical Properties

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

Photoinduced ligand exchange and DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2]+ with long wavelength visible light

The complex cis-[Ru(phpy)(phen)(CH3CN)2](+) (phpy=2-phenylpyridine, phen=1,10-phenanthroline) was investigated as a potential photodynamic therapy (PDT) agent. This complex presents desirable photochemical characteristics including a low energy absorption tail extending into the PDT window (600-850nm) and photoinduced exchange of the CH3CN ligands, generating a species analogous to the chemotherapy drug cisplatin. Furthermore, photochemical reactivity can be controlled through selective irradiation into the Ru-phen singlet metal-to-ligand charge transfer ((1)MLCT) band (λirr=500 nm) of [Ru(phpy)(phen)(CH3CN)2](+) in the presence of excess t-butylammonium chloride (TBACl) resulting in efficient photoinduced production of [Ru(phpy)(phen)(CH3CN)Cl] (Φ=0.25). This lower energy irradiation resulted in greater quantum yield of photosubstitution when compared to direct irradiation into the Ru-phpy (1)MLCT peak (λirr=450 nm; Φ=0.08) in CH2Cl2. It was found that the lower quantum yield observed for irradiation into the Ru→phpy(-)(1)MLCT band results from significant orbital mixing of the phpy(-) ligand with the t2g-type filled set in the metal, giving this state significant ligand-centered character. Lastly, this complex produced a decrease in the mobility of linearized ds-DNA when irradiated with λirr≥420nm, indicative of covalent binding by the transition metal complex similar to that observed for cisplatin. No change in mobility was found for the same samples kept in the dark indicating, unlike cisplatin, DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2](+) only occurs with the activation of light. These observations support the use of cis-[Ru(phpy)(phen)(CH3CN)2](+) as a potential PDT agent by the photoinduced generation of a cisplatin analog.