Development of a cobinamide-based end-of-service-life indicator for detection of hydrogen cyanide gas.

We describe an inexpensive paper-based sensor for rapid detection of low concentrations (ppm) of hydrogen cyanide gas. A piece of filter paper pre-spotted with a dilute monocyanocobinamide [CN(H2O)Cbi] solution was placed on the end of a bifurcated optical fiber and the reflectance spectrum of the CN(H2O)Cbi was monitored during exposure to 1.0-10.0 ppm hydrogen cyanide gas. Formation of dicyanocobinamide yielded a peak at 583 nm with a simultaneous decrease in reflectance from 450-500 nm. Spectral changes were monitored as a function of time at several relative humidity values: 25, 50, and 85% relative humidity. With either cellulose or glass fiber papers, spectral changes occurred within 10 s of exposure to 5.0 ppm hydrogen cyanide gas (NIOSH recommended short-term exposure limit). We conclude that this sensor could provide a real-time end-of-service-life alert to a respirator user.

A light-activated NO donor attenuates anchorage independent growth of cancer cells: Important role of a cross talk between NO and other reactive oxygen species.

It is established that high concentrations of nitric oxide(1) (NO), as released from activated macrophages, induce apoptosis in breast cancer cells. In this study, we assessed the potential of a light-activated NO donor [(Me2bpb)Ru(NO)(Resf)], a recently reported apoptototic agent, in suppressing the anchorage independent growth potentials of an aggressive human breast cancer cell line. Our results demonstrated the down regulation of anchorage independent growth by light activated NO treatment in the aggressive human breast cancer cell line MDA-MB-231 and afforded insight into the associated mechanism(s). The investigation revealed an up-regulation of the bioactivity of catalase with an accompanied reduction in the endogenous levels of H2O2, a direct substrate of catalase and a recently identified endogenous growth modulator in breast cancer cells. An earlier publication reported that endogenous superoxide (O2(-)) in human breast cancer cells constitutively inhibits catalase bioactivity (at the level of its protein), resulting in increased H2O2 levels. Interestingly in this study, O2(-) was also found to be down- regulated following NO treatment providing a basis for the observed increase in catalase bioactivity. Cells silenced for the catalase gene exhibited compromised reduction in anchorage independent growth upon light activated NO treatment. Collectively this study detailed a mechanistic cross talk between exogenous NO and endogenous ROS in attenuating anchorage independent growth.

Photoactive ruthenium nitrosyls as NO donors: how to sensitize them toward visible light.

Nitric oxide (NO) can induce apoptosis (programmed cell death) at micromolar or higher doses. Although cell death via NO-induced apoptosis has been studied quite extensively, the targeted delivery of such doses of NO to infected or malignant tissues has not been achieved. The primary obstacle is indiscriminate NO release from typical systemic donors such as glycerin trinitrate: once administered, the drug travels throughout the body, and NO is released through a variety of enzymatic, redox, and pH-dependent pathways. Photosensitive NO donors have the ability to surmount this difficulty through the use of light as a localized stimulus for NO delivery. The potential of the method has prompted synthetic research efforts toward new NO donors for use as photopharmaceuticals in the treatment of infections and malignancies. Over the past few years, we have designed and synthesized several metal nitrosyls (NO complexes of metals) that rapidly release NO when exposed to low-power (milliwatt or greater) light of various wavelengths. Among them, the ruthenium nitrosyls exhibit exceptional stability in biological media. However, typical ruthenium nitrosyls release NO upon exposure to UV light, which is hardly suitable for phototherapy. By following a few novel synthetic strategies, we have overcome this problem and synthesized a variety of ruthenium nitrosyls that strongly absorb light in the 400-600-nm range and rapidly release NO under such illumination. In this Account, we describe our progress in designing photoactive ruthenium nitrosyls as visible-light-sensitive NO donors. Our research has shown that alteration of the ligands, in terms of (i) donor atoms, (ii) extent of conjugation, and (iii) substituents on the ligand frames, sensitizes the final ruthenium nitrosyls toward visible light in a predictable fashion. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations provide guidance in this "smart design" of ligands. We have also demonstrated that direct attachment of dye molecules as light-harvesting antennas also sensitize ruthenium nitrosyls to visible light, and TDDFT calculations provide insight into the mechanisms of sensitization by this technique. The fluorescence of the dye ligands makes these NO donors "trackable" within cellular matrices. Selected ruthenium nitrosyls have been used to deliver NO to cellular targets to induce apoptosis. Our open-design strategies allow the isolation of a variety of these ruthenium nitrosyls, depending on the choices of the ligand frames and dyes. These designed nitrosyls will thus be valuable in the future endeavor of synthesizing novel pharmaceuticals for phototherapy.

Syntheses, structures, and properties of new manganese carbonyls as photoactive CO-releasing molecules: design strategies that lead to CO photolability in the visible region.

The unusual role of CO as a signaling molecule in several physiological pathways has spurred research in the area of synthesizing new CO-releasing molecules (CORMs) as exogenous CO donors. Auxiliary control on CO delivery can be achieved if CO can be released under the control of light. To synthesize such photoactive CORMs (photoCORMs) with the aid of smart design, a series of manganese carbonyls have been synthesized with ligands that contain extended conjugation and electron-rich donors on their frames. Five such photoCORMs, namely, [Mn(pimq)(CO)(3)(MeCN)]ClO(4) (1, where pimq = (2-phenyliminomethyl)quinoline), [Mn(qmtpm)(CO)(3)(MeCN)]ClO(4) (2, where qmtpm = 2-quinoline-N-(2'-methylthiophenyl) methyleneimine), [Mn(qmtpm)(CO)(3)Br] (3) [Mn(pmtpm)(CO)(3)(MeCN)]ClO(4) (4, where pmtpm = 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine), and [Mn(pmtpm)(CO)(3)Br] (5), have been synthesized and structurally characterized. These designed carbonyls readily release CO upon exposure to light (400-550 nm). The apparent CO release rates and quantum yield values at 509 nm (φ(509)) of the photoCORMs increase steadily with rise in conjugation in the ligand frame and inclusion of a -SMe group. Addition of Br(-) as an ancillary ligand also improves the CO-donating capacity. Results of density functional theory (DFT) and time dependent DFT (TDDFT) studies indicate that low energy metal-to-ligand charge transfer (MLCT) transitions from Mn-CO bonding into ligand-π orbitals lead to reduction of M-CO(π*) back-bonding and loss of CO from these photoCORMs. Inclusion of -SMe and Br(-) in the coordination sphere attenuates the energies of the HOMO and LUMO levels and causes further enhancement of CO photorelease. Collectively, the results of this work demonstrate that new photoCORMs with excellent sensitivity to visible light can be synthesized on the basis of smart design principles.

Triggered dye release via photodissociation of nitric oxide from designed ruthenium nitrosyls: turn-ON fluorescence signaling of nitric oxide delivery.

Two new fluorescein-tethered nitrosyls derived from designed tetradentate ligands with carboxamido-N donors have been synthesized and characterized by spectroscopic techniques. These two diamagnetic {Ru-NO}(6) nitrosyls, namely, [(Me(2)bpb)Ru(NO)(FlEt)] (1-FlEt, Me(2)bpb = 1,2-bis(pyridine-2-carboxamido)5-dimethylbenzene, FlEt = fluorescein ethyl ester) and [((OMe)(2)IQ1)Ru(NO)(FlEt)] (2-FlEt, (OMe)(2)IQ1 = 1,2-bis(isoquinoline-1-carboxamido)-4,5-dimethoxybenzene), display NO stretching frequencies (ν(NO)) at 1846 and 1832 cm(-1) in addition to their FlEt carbonyl stretching frequencies (ν(CO)) at 1715 and 1712 cm(-1), respectively. Coordination of the dye ligand enhances the absorptivity and NO photolability of these two nitrosyls in the visible region (450-600 nm) of light. Exposure to visible light promotes rapid loss of NO from both {Ru-NO}(6) nitrosyls to generate Ru(III) photoproducts in dry aprotic solvents, such as MeCN and DMF. The FlEt(-) moiety remains bound to the paramagnetic Ru(III) center in such cases, and hence, the photoproducts exhibit very weak fluorescence from the dye unit. In the presence of water, the Ru(III) photoproducts undergo further aquation and loss of the FlEt(-) moiety via protonation. These steps lead to turn-ON fluorescence (from the free FlEt unit) and provide a visual signal of the NO photorelease from 1-FlEt and 2-FlEt in aqueous media.

Dye-tethered ruthenium nitrosyls containing planar dicarboxamide tetradentate N4 ligands: effects of in-plane ligand twist on NO photolability.

To examine the steric effects of the in-plane ligands in dye-sensitized {RuNO}(6) nitrosyls on their NO photolability, two new ligands, namely, 1,2-Bis(pyridine-2-carboxamido)-4,5-dimethoxybenzene (H(2)(OMe)(2)bpb) and 1,2-Bis(Isoquinoline-1-carboxamido)-4,5-dimethoxybenzene (H(2)(OMe)(2)IQ1, H's are dissociable carboxamide protons) have been designed and synthesized. The syntheses and spectroscopic properties of {RuNO}(6) nitrosyls derived from these two ligands, namely, [((OMe)(2)bpb)Ru(NO)(Cl)] (4-Cl), [((OMe)(2)IQ1)Ru(NO)(Cl)] (5-Cl), [((OMe)(2)bpb)Ru(NO)(Resf)] (4-Resf), and [((OMe)(2)IQ1)Ru(NO)(Resf)] (5-Resf), are reported. The structures of 5-Cl, 4-Resf, and 5-Resf have been determined by X-ray crystallography. Removal of the in-plane ligand twist in the quinoline-based R(2)bQb(2-) ligand frame (because of steric interactions between the extended quinoline ring systems) in both R(2)bpb(2-) and R(2)IQ1(2-) (pyridine and 1-isoquinoline rings, respectively, instead of quinoline rings in the equatorial plane) results in enhanced solution stability, as well as higher quantum yield values for NO photorelease upon exposure to 500 nm light. Both dye-tethered {RuNO}(6) nitrosyls 4-Resf and 5-Resf exhibit greater sensitivity to visible light compared to the chloro-bound species 4-Cl and 5-Cl. In addition, the dye-tethered nitrosyls are fluorescent and hence can be used as trackable NO donors in cellular studies.

Mechanism of NO photodissociation in photolabile manganese-NO complexes with pentadentate N5 ligands.

The Mn-nitrosyl complexes [Mn(PaPy(3))(NO)](ClO(4)) (1; PaPy(3)(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn(PaPy(2)Q)(NO)](ClO(4)) (2, PaPy(2)Q(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide) show a remarkable photolability of the NO ligand upon irradiation of the complexes with UV-vis-NIR light [Eroy-Reveles, A. A.; Leung, Y.; Beavers, C. M.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2008, 130, 4447]. Here we report detailed spectroscopic and theoretical studies on complexes 1 and 2 that provide key insight into the mechanism of NO photolabilization in these compounds. IR- and FT-Raman spectroscopy show N-O and Mn-NO stretching frequencies in the 1720-1750 and 630-650 cm(-1) range, respectively, for these Mn-nitrosyls. The latter value for ν(Mn-NO) is one of the highest transition-metal-NO stretching frequencies reported to this date, indicating that the Mn-NO bond is very strong in these complexes. The electronic structure of 1 and 2 is best described as Mn(I)-NO(+), where the Mn(I) center is in the diamagnetic low-spin state and the NO(+) ligand forms two very strong π backbonds with the d(xz) and d(yz) orbitals of the metal. This explains the very strong Mn-NO bonds observed in these complexes, which even supersede the strengths of the Fe- and Ru-NO bonds in analogous (isoelectronic) Fe/Ru(II)-NO(+) complexes. Using time-dependent density functional theory (TD-DFT) calculations, we were able to assign the electronic spectra of 1 and 2, and to gain key insight into the mechanism of NO photorelease in these complexes. Upon irradiation in the UV region, NO is released because of the direct excitation of d(π)_π* → π*_d(π) charge transfer (CT) states (direct mechanism), which is similar to analogous NO adducts of Ru(III) and Fe(III) complexes. These are transitions from the Mn-NO bonding (d(π)_π*) into the Mn-NO antibonding (π*_d(π)) orbitals within the Mn-NO π backbond. Since these transitions lead to the population of Mn-NO antibonding orbitals, they promote the photorelease of NO. In the case of 1 and 2, further transitions with distinct d(π)_π* → π*_d(π) CT character are observed in the 450-500 nm spectral range, again promoting photorelease of NO. This is confirmed by resonance Raman spectroscopy, showing strong resonance enhancement of the Mn-NO stretch at 450-500 nm excitation. The extraordinary photolability of the Mn-nitrosyls upon irradiation in the vis-NIR region is due to the presence of low-lying d(xy) → π*_d(π) singlet and triplet excited states. These have zero oscillator strengths, but can be populated by initial excitation into d(xy) → L(Py/Q_π*) CT transitions between Mn and the coligand, followed by interconversion into the d(xy) → π*_d(π) singlet excited states. These show strong spin-orbit coupling with the analogous d(xy) → π*_d(π) triplet excited states, which promotes intersystem crossing. TD-DFT shows that the d(xy) → π*_d(π) triplet excited states are indeed found at very low energy. These states are strongly Mn-NO antibonding in nature, and hence, promote dissociation of the NO ligand (indirect mechanism). The Mn-nitrosyls therefore show the long sought-after potential for easy tunability of the NO photorelease properties by simple changes in the coligand.

Designed iron carbonyls as carbon monoxide (CO) releasing molecules: rapid CO release and delivery to myoglobin in aqueous buffer, and vasorelaxation of mouse aorta.

The physiological roles of CO in neurotransmission, vasorelaxation, and cytoprotective activities have raised interest in the design and syntheses of CO-releasing materials (CORMs) that could be employed to modulate such biological pathways. Three iron-based CORMs, namely, [(PaPy(3))Fe(CO)](ClO(4)) (1), [(SBPy(3))Fe(CO)](BF(4))(2) (2), and [(Tpmen)Fe(CO)](ClO(4))(2) (3), derived from designed polypyridyl ligands have been synthesized and characterized by spectroscopy and X-ray crystallography. In these three Fe(II) carbonyls, the CO is trans to a carboxamido-N (in 1), an imine-N (in 2), and a tertiary amine-N (in 3), respectively. This structural feature has been correlated to the strength of the Fe-CO bond. The CO-releasing properties of all three carbonyls have been studied in various solvents under different experimental conditions. Rapid release of CO is observed with 2 and 3 upon dissolution in both aqueous and nonaqueous media in the presence and absence of dioxygen. With 1, CO release is observed only under aerobic conditions, and the final product is an oxo-bridged diiron species while with 2 and 3, the solvent bound [(L)Fe(CO)](2+) (where L = SBPy(3) or Tpmen) results upon loss of CO under both aerobic and anaerobic conditions. The apparent rates of CO loss by these CORMs are comparable to other CORMs such as [Ru(glycine)(CO)(3)Cl] reported recently. Facile delivery of CO to reduced myoglobin has been observed with both 2 and 3. In tissue bath experiments, 2 and 3 exhibit rapid vasorelaxation of mouse aorta muscle rings. Although the relaxation effect is not inhibited by the soluble guanylate cyclase inhibitor ODQ, significant inhibition is observed with the BK(Ca) channel blocker iberiotoxin.

Ruthenium nitrosyls derived from tetradentate ligands containing carboxamido-N and phenolato-o donors: syntheses, structures, photolability, and time dependent density functional theory studies.

In order to examine the role(s) of designed ligands on the NO photolability of {Ru-NO}(6) nitrosyls, a set of three nitrosyls with ligands containing two carboxamide groups along with a varying number of phenolates have been synthesized. The nitrosyls namely, (NEt(4))(2)[(hybeb)Ru(NO)(OEt)] (1), (PPh(4))[(hypyb)Ru(NO)(OEt)] (2), and [(bpb)Ru(NO)(OEt)] (3) have been characterized by X-ray crystallography. Complexes 1-3 are diamagnetic, exhibit nu(NO) in the range 1780-1840 cm(-1) and rapidly release NO in solution upon exposure to low power UV light (7 mW/cm(2)). Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) calculations on 1-3 indicate considerable contribution of ligand orbitals in the MOs involved in transitions leading to NO photolability. The results of the theoretical studies match well with the experimental absorption spectra as well as the parameters for NO photorelease and provide insight into the transition(s) associated with loss of NO.

Sensitization of ruthenium nitrosyls to visible light via direct coordination of the dye resorufin: trackable NO donors for light-triggered NO delivery to cellular targets.

Three nitrosyl-dye conjugates, namely, [(Me 2bpb)Ru(NO)(Resf)] ( 1-Resf), [(Me 2bQb)Ru(NO)(Resf)] ( 2-Resf), and [((OMe) 2bQb)Ru(NO)(Resf)] ( 3-Resf) have been synthesized via direct replacement of the chloride ligand of the parent {Ru-NO} (6) nitrosyls of the type [(R 2byb)Ru(NO)(L)] with the anionic tricyclic dye resorufin (Resf). The structures of 1-Resf- 3-Resf have been determined by X-ray crystallography. The dye is coordinated to the ruthenium centers of these conjugates via the phenolato-O atom and is trans to NO. Systematic red shift of the d pi(Ru) --> pi*(NO) transition of the parent nitrosyls [(R 2byb)Ru(NO)(L)] due to changes in R and y in the equatorial tetradentate ligand R 2byb (2-) results in its eventual merge with the intense absorption band of the dye around 500 nm in 3-Resf. Unlike the UV-sensitive parent [(R 2byb)Ru(NO)(L)] nitrosyls, these dye-sensitized nitrosyls rapidly release NO when exposed to visible light (lambda >/= 465 nm). Comparison of the photochemical parameters reveals that direct coordination of the light-harvesting chromophore to the ruthenium center in the present nitrosyls results in a significantly greater extent of sensitization to visible light compared to nitrosyls with appended chromophore (linked via alkyl chains). 1-Resf has been employed as a "trackable" NO donor to promote NO-induced apoptosis in MDA-MB-231 human breast cancer cells under the control of light. The results of this work demonstrate that (a) the d pi(Ru) --> pi*(NO) transition (photoband) of {Ru-NO} (6) nitrosyls can be tuned into visible range via careful alteration of the ligand frame(s) and (b) such nitrosyls can be significantly sensitized to visible light by directly ligating a light-harvesting chromophore to the ruthenium center. The potential of these photosensitive nitrosyl-dye conjugates as (i) biological tools to study the effects of NO in cellular environments and (ii) "trackable" NO donors in photodynamic therapy of malignancies (such as skin cancer) has been discussed.

Facile ligand oxidation and ring nitration in ruthenium complexes derived from a ligand with dicarboxamide-N and phosphine-P donors.

The reaction of the tetradentate dicarboxamide ligand 1,2-bis-N-[2'(diphenylphosphanyl)benzoyl]diaminobenzene (dppbH(2)) with RuCl(3) in DMF or ethanol results in metal-mediated ligand oxidation and formation of the diamagnetic Ru(II) complex [(dppQ)Ru(Cl)(2)] (1) with N(2)P(2) chromophore. The o-phenylenedicarboxamide portion of the dppb(2-) ligand is oxidized to a o-benzoquinonediimine (bqdi) moiety in [(dppQ)Ru(Cl)(2)]. Presence of oxygen accelerates the ligand oxidation process. Unlike other tetradentate dicarboxamide ligands with pyridine-N, phenolato-O, or thiolato-S donors, dppb(2-) provides stability to the +2 oxidation state of ruthenium and facilitates oxidation of the coordinated ligand frame. Results of spectroscopic and redox studies strongly support the +2 oxidation state of Ru in 1. Exposure of 1 to NO(g) does not lead to formation of any metal nitrosyl; instead, the bqdi ring is nitrated to afford [(NO(2)dppQ)Ru(Cl)(2)] (2).

Oxidation-Induced Trapping of Drugs in Porous Silicon Microparticles.

An approach for the preparation of an oxidized porous silicon microparticle drug delivery system that can provide efficient trapping and sustained release of various drugs is reported. The method uses the contraction of porous silicon's mesopores, which occurs during oxidation of the silicon matrix, to increase the loading and retention of drugs within the particles. First, a porous Si (pSi) film is prepared by electrochemical etching of p-type silicon with a resistivity of >0.65 Ω cm in a 1:1 (v/v) HF/ethanol electrolyte solution. Under these conditions, the pore walls are sufficiently thin to allow for complete oxidation of the silicon skeleton under mild conditions. The pSi film is then soaked in an aqueous solution containing the drug (cobinamide or rhodamine B test molecules were used in this study) and sodium nitrite. Oxidation of the porous host by nitrite results in a shrinking of the pore openings, which physically traps the drug in the porous matrix. The film is subsequently fractured by ultrasonication into microparticles. Upon comparison with commonly used oxidizing agents for pSi such as water, peroxide, and dimethyl sulfoxide, nitrite is kinetically and thermodynamically sufficient to oxidize the pore walls of the pSi matrix, precluding reductive (by Si) or oxidative (by nitrite) degradation of the drug payload. The drug loading efficiency is significantly increased (by up to 10-fold), and the release rate is significantly prolonged (by 20-fold) relative to control samples in which the drug is loaded by infiltration of pSi particles postoxidation. We find that it is important that the silicon skeleton be completely oxidized to ensure the drug is not reduced or degraded by contact with elemental silicon during the particle dissolution-drug release phase.

Photolability of NO in designed metal nitrosyls with carboxamido-N donors: a theoretical attempt to unravel the mechanism.

During the past few years, photoactive metal nitrosyls (NO complexes of metals) have drawn attention as potential drugs for delivery of nitric oxide (NO) to biological targets under the control of light. Major success in this area has been achieved with designed metal nitrosyls derived from ligands that contain carboxamide group(s). A number of iron, manganese and ruthenium {MNO}(6) nitrosyls of such kind exhibit excellent NO photolability under low-power visible and near-IR light. The results of theoretical studies on these NO-donors have provided insight into (a) the electronic transitions that lead to photorelease of NO and (b) the structural features of the ligands that dictate the sensitivity of the nitrosyls to light of specific wavelengths. In addition, the results have afforded clear understanding of the electronic configurations of the various nitrosyls. This article highlights these results in a coherent manner. Good matches between the predicted and observed spectral features and NO photolability strongly suggest that theoretical studies should be an integral part of the smart design of such NO-donors in the future research.