Photoisomerization and structural dynamics of two nitrosylruthenium complexes: a joint study by NMR and nonlinear IR spectroscopies

Insight into the anti-proliferation activity and photoinduced NO release of four nitrosylruthenium isomeric complexes and their HSA complex adducts

Four Ru(II)-centered isomeric complexes [RuCl(5cqn)(Val)(NO)] (1-4) were synthesized with 5cqn (5-chloro-8-hydroxyquinoline) and chiral Val (Val = L- or D-valine) as co-ligand, and their structures were confirmed using the X-ray diffraction method. The cytotoxicity and photodynamic activity of the isomeric complexes and their human serum albumin (HSA) complex adducts were evaluated. Both the isomeric complexes and their HSA complex adducts significantly affected HeLa cell proliferation, with an IC50 value in the range of 0.3-0.5 μM. The photo-controlled release of nitric oxide (NO) in solution was confirmed using time-resolved Fourier transform infrared and electron paramagnetic resonance spectroscopy techniques. Furthermore, photoinduced NO release in living cells was observed using a selective fluorescent probe for NO. Moreover, the binding constants (Kb) of the complexes with HSA were calculated to be 0.17-1.98 × 104 M-1 and the average number of binding sites (n) was found to be close to 1, it can serve as a crucial carrier for delivering metal complexes. The crystal structure of the HSA complex adduct revealed that one [RuCl(H2O)(NO)(Val)]+ molecule binds to a pocket in domain I. This study provides insight into possible mechanism of metabolism and potential applications for nitrosylruthenium complexes.

Synthesis, Biomacromolecular Interactions, Photodynamic NO Releasing and Cellular Imaging of Two [RuCl(qn)(Lbpy)(NO)]X Complexes

Two light-activated NO donors [RuCl(qn)(Lbpy)(NO)]X with 8-hydroxyquinoline (qn) and 2,2′-bipyridine derivatives (Lbpy) as co-ligands were synthesized (Lbpy₁ = 4,4′-dicarboxyl-2,2′-dipyridine, X = Cl⁻ and Lbpy₂ = 4,4′-dimethoxycarbonyl-2,2′-dipyridine, X = NO₃⁻), and characterized using ultraviolet–visible (UV-vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (¹H NMR), elemental analysis and electrospray ionization mass spectrometry (ESI-MS) spectra. The [RuCl(qn)(Lbpy₂)(NO)]NO₃ complex was crystallized and exhibited distorted octahedral geometry, in which the Ru–N(O) bond length was 1.752(6) Å and the Ru–N–O angle was 177.6(6)°. Time-resolved FT-IR and electron paramagnetic resonance (EPR) spectra were used to confirm the photoactivated NO release of the complexes. The binding constant (K_b) of two complexes with human serum albumin (HSA) and DNA were quantitatively evaluated using fluorescence spectroscopy, Ru-Lbpy₁ (K_b~10⁶ with HSA and ~10⁴ with DNA) had higher affinity than Ru-Lbpy₂. The interactions between the complexes and HSA were investigated using matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) and EPR spectra. HSA can be used as a carrier to facilitate the release of NO from the complexes upon photoirradiation. The confocal imaging of photo-induced NO release in living cells was successfully observed with a fluorescent NO probe. Moreover, the photocleavage of pBR322 DNA for the complexes and the effect of different Lbpy substituted groups in the complexes on their reactivity were analyzed.

The Structures, Spectroscopic Properties, and Photodynamic Reactions of Three [RuCl(QN)NO]- Complexes (HQN = 8-Hydroxyquinoline and Its Derivatives) as Potential NO-Donating Drugs

The structures and spectral properties of three ruthenium complexes with 8-hydroxyquinoline (Hhqn) and their derivatives 2-methyl-8-quinolinoline (H2mqn) and 2-chloro-8-quiolinoline (H2cqn) as ligands (QN = hqn, 2mqn, or 2cqn) were calculated with density functional theory (DFT) at the B3LYP level. The UV-Vis and IR spectra of the three [RuCl(QN)NO]⁻ complexes were theoretically assigned via DFT calculations. The calculated spectra reasonably correspond to the experimentally measured spectra. Photoinduced NO release was confirmed through spin trapping of the electron paramagnetic resonance spectroscopy (EPR), and the dynamic process of the NO dissociation upon photoirradiation was monitored using time-resolved infrared (IR) spectroscopy. Moreover, the energy levels and related components of frontier orbitals were further analyzed to understand the electronic effects of the substituent groups at the 2nd position of the ligands on their photochemical reactivity. This study provides the basis for the design of NO donors with potential applications in photodynamic therapy.

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute-Solvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution

In this work, the solvation of a nitrosyl ruthenium complex, [(CH₃)₄N][RuCl₃(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in the bio-environment, was studied in two bio-friendly solvents, namely deuterated dimethyl sulfoxide (dDMSO) and water (D₂O). A blue-shifted NO stretching frequency was observed in water with respect to that in dDMSO, which was believed to be due to ligand-solvent hydrogen-bonding interactions, one N═O···D and particularly three Ru-Cl···D, that show competing effects on the NO bond length. The dynamic differences of the NO stretch in these two solvents were further revealed by transient pump-probe IR and two-dimensional IR results: faster vibrational relaxation and faster spectral diffusion (SD) were observed in D₂O, confirming stronger solvent-solute interaction and also faster solvent structural dynamics in D₂O than in DMSO. Further, a significant non-decaying residual in the SD dynamics was observed in D₂O but not in DMSO, suggesting the formation of a stable solvation shell in water due to strong multi-site ligand-solvent hydrogen-bonding interactions, which is in agreement with the observed blue-shifted NO stretching frequency. This work demonstrates that small solvent molecules such as water can form a relatively rigid solvation shell for certain transition metal complexes due to cooperative ligand-solvent interactions and show slower dynamics.

Density functionalized [RuII(NO)(Salen)(Cl)] complex: Computational photodynamics and in vitro anticancer facets

Photodynamic therapy (PDT) is a treatment that uses photosensitizing agents to kill cancer cells. Scientific community has been eager for decades to design an efficient PDT drug. Under such purview, the current report deals with the computational photodynamic behavior of ruthenium(II) nitrosyl complex containing N, N'-salicyldehyde-ethylenediimine (SalenH₂), the synthesis and X-ray crystallography of which is already known [Ref. 38,39]. Gaussian 09W software package was employed to carry out the density functional (DFT) studies. DFT calculations with Becke-3-Lee-Yang-Parr (B3LYP)/Los Alamos National Laboratory 2 Double Z (LanL2DZ) specified for Ru atom and B3LYP/6-31G(d,p) combination for all other atoms were used using effective core potential method. Both, the ground and excited states of the complex were evolved. Some known photosensitizers were compared with the target complex. Pthalocyanine and porphyrin derivatives were the compounds selected for the respective comparative study. It is suggested that effective photoactivity was found due to the presence of ruthenium core in the model complex. In addition to the evaluation of theoretical aspects in vitro anticancer aspects against COLO-205 human cancer cells have also been carried out with regard to the complex. More emphasis was laid to extrapolate DFT to depict the chemical power of the target compound to release nitric oxide. A promising visible light triggered nitric oxide releasing power of the compound has been inferred. In vitro antiproliferative studies of [RuCl₃(PPh₃)₃] and [Ru(NO)(Salen)(Cl)] have revealed the model complex as an excellent anticancer agent. From IC₅₀ values of 40.031mg/mL in former and of 9.74mg/mL in latter, it is established that latter bears more anticancer potentiality. From overall study the DFT based structural elucidation and the efficiency of NO, Ru and Salen co-ligands has shown promising drug delivery property and a good candidacy for both chemotherapy as well as light therapy.

Differentiating Two Nitrosylruthenium Isomeric Complexes by Steady-State and Ultrafast Infrared Spectroscopies

The [Ru(II)-NO⁺] group affects the structure and chemical reactivity of nitrosylruthenium(II) complexes. A characteristic infrared absorption band due to the nitrosyl (NO) stretching motion is shown in the frequency region 1800-1900 cm^-1. In this work, linear infrared (IR) and nonlinear IR methods, including pump-probe and two-dimensional (2D) IR, were utilized to study the structures and dynamics of two isomeric nitrosylruthenium complexes [Ru(OAc)(2mqn)₂NO] (H2mqn = 2-methyl-8-quinolinol) in cis and trans isomeric configurations in a weak polar solvent (CDCl₃). Using the NO stretching mode as a vibrational probe, information about local structural dynamics of the Ru complex as well as solvent fluctuation dynamics was obtained. In particular, a "structured" solvent environment is believed to form in the vicinity of the NO group in the case of the cis isomer with the aid of a neighboring OAc ligand, which is the reason for more efficient vibrational relaxation but more inhomogeneously distributed solvent and thus associated slower spectral diffusion. Our results also suggest a more anharmonic potential surface for the NO stretching mode in the less stable trans isomer.

Structural dynamics of nitrosylruthenium isomeric complexes studied with steady-state and transient pump-probe infrared spectroscopies

The characteristic nitrosyl stretching (NO) in the region of 1800-1900cm(-1) was used to study the geometric and ligand effect on two nitrosylruthenium complexes, namely [Ru(OAc)(2QN)2NO] (QN=2-chloro-8-quinolinol (H2cqn) or QN=2-methyl-8-quinolinol (H2mqn)). The NO stretching frequency (νNO) was found in the following order: νcis-1 (2cqn)>νcis-2 (2cqn)>νcis-1 (2mqn)>νtrans (2mqn). The results exhibited a spectral sensitivity of the NO mode to both charge distribution and ligand arrangement, which was supported by ab initio computations and natural bond orbital (NBO) analyses. Further, the vibrational population of the vibrationally excited NO stretching mode was found to relax on the order of 7-10ps, showing less than 30% variation from one isomer to another, which were explained on the basis of NO local structures and solute-solvent interactions in these isomeric nitrosylruthenium complexes.