Labeling of Hyaluronic Acids with a Rhenium-tricarbonyl Tag and Percutaneous Penetration Studied by Multimodal Imaging

Rhenium fac-Tricarbonyl Bisimine Chalcogenide Complexes: Synthesis, Photophysical Studies, and Confocal and Time-Resolved Cell Microscopy

We describe the preparation, characterization, and imaging studies of rhenium carbonyl complexes with a pyta (4-(2-pyridyl)-1,2,3-triazole) or tapy (1-(2-pyridyl)-1,2,3-triazole)-based heteroaromatic N∧N ligand and thiolate or selenoate X ligand. The stability and photophysical properties of the selenolate complexes are compared with parent chloride complexes and previously described analogues with benzenethiolate ligands. Two complexes were imaged in A549 cells upon excitation at 405 nm. Colocalization studies suggest a lysosomal accumulation, while one parent chloride complex was described to localize at the Golgi apparatus. Preliminary fluorescence lifetime measurements and imaging demonstrate potential for application in time-resolved microscopy techniques due to the long and variable lifetimes observed in cellular environments, including an increase in lifetime between the solution and solid state many times larger than previously reported.

Towards multimodal cellular imaging: optical and X-ray fluorescence

Imaging techniques permit the study of the molecular interactions that underlie health and disease. Each imaging technique collects unique chemical information about the cellular environment. Multimodal imaging, using a single probe that can be detected by multiple imaging modalities, can maximise the information extracted from a single cellular sample by combining the results of different imaging techniques. Of particular interest in biological imaging is the combination of the specificity and sensitivity of optical fluorescence microscopy (OFM) with the quantitative and element-specific nature of X-ray fluorescence microscopy (XFM). Together, these techniques give a greater understanding of how native elements or therapeutics affect the cellular environment. This review focuses on recent studies where both techniques were used in conjunction to study cellular systems, demonstrating the breadth of biological models to which this combination of techniques can be applied and the potential for these techniques to unlock untapped knowledge of disease states.

Recent Advances of Hyaluronan for Skin Delivery: From Structure to Fabrication Strategies and Applications

Hyaluronan (HA) plays a fundamental role in maintaining the homeostasis on skin health. Furthermore, the effect of HA in skin inflammatory diseases is worth studying in the next future. HA and its conjugates change the solubility of active pharmaceutical ingredients, improve emulsion properties, prolong stability, reduce immunogenicity, and provide targeting. HA penetrates to deeper layers of the skin via several mechanisms, which depend on the macromolecular structure and composition of the formulation. The cellular and molecular mechanisms involved in epidermal dysfunction and skin aging are not well understood. Nevertheless, HA is known to selectively activate CD44-mediated keratinocyte signaling that regulates its proliferation, migration, and differentiation. The molecular size of HA is critical for molecular mechanisms and interactions with receptors. High molecular weight HA is used in emulsions and low molecular weight is used to form nanostructured lipid carriers, polymeric micelles, bioconjugates, and nanoparticles. In the fabrication of microneedles, HA is combined with other polymers to enhance mechanical properties for piercing the skin. Hence, this review aims to provide an overview of the current state of the art and last reported ways of processing, and applications in skin drug delivery, which will advocate for their broadened use in the future.

Rhenium(I) conjugates as tools for tracking cholesterol in cells

Cholesterol is vital to control membrane integrity and fluidity, but is also a precursor to produce steroid hormones, bile acids, and vitamin D. Consequently, altered cholesterol biology has been linked to many diseases, including metabolic syndromes and cancer. Defining the intracellular pools of cholesterol and its trafficking within cells is essential to understand both normal cell physiology and mechanisms of pathogenesis. We have synthesized a new cholesterol mimic (ReTEGCholestanol), comprising a luminescent rhenium metal complex and a cholestanol targeting unit, linked using a tetraethylene glycol (TEG) spacer. ReTEGCholestanol demonstrated favourable imaging properties and improved water solubility when compared to a cholesterol derivative, and structurally related probes lacking the TEG linker. A non-malignant and three malignant prostate cell lines were used to characterize the uptake and intracellular distribution of ReTEGCholestanol. The ReTEGCholestanol complex was effectively internalized and mainly localized to late endosomes/lysosomes in non-malignant PNT1a cells, while in prostate cancer cells it also accumulated in early endosomes and multivesicular bodies, suggesting disturbed cholesterol biology in the malignant cells. The ReTEGCholestanol is a novel imaging agent for visualizing endosomal uptake and trafficking, which may be used to define cholesterol related biology including membrane integration and altered lipid trafficking/processing.

Pd(II)-mediated C-H activation for cysteine bioconjugation

Selective bioconjugation remains a significant challenge for the synthetic chemist due to the stringent reaction conditions required by biomolecules coupled with their high degree of functionality. The current trailblazer of transition‐metal mediated bioconjugation chemistry involves the use of Pd(II) complexes prepared via an oxidative addition process. Herein, the preparation of Pd(II) complexes for cysteine bioconjugation via a facile C−H activation process is reported. These complexes show bioconjugation efficiency competitive with what is seen in the current literature, with a user‐friendly synthesis, common Pd(II) sources, and a more cost‐effective ligand. Furthermore, these complexes need not be isolated, and still achieve high conversion efficiency and selectivity of a model peptide. These complexes also demonstrate the ability to selectively arylate a single surface cysteine residue on a model protein substrate, further demonstrating their utility.

The role of substituted pyridine Schiff bases as ancillary ligands in the optical properties of a new series of fac-rhenium(i) tricarbonyl complexes: a theoretical view

Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells. In this sense, incorporating an asymmetrical pyridine Schiff base (PSB) as the ancillary ligand strongly modifies the staining and luminescent properties of Re(i) tricarbonyl complexes. In this work, we analyzed two series of Re(i) tricarbonyl complexes with their respective PSB ligands: (1) fac-[Re(CO)₃(2,2′-bpy)(PSB)]¹⁺ and (2) fac-[Re(CO)₃(4,4′-bis(ethoxycarbonyl)-2,2′-bpy)(PSB)]¹⁺, where the PSB exhibits substitutions at positions 4 or 6 in the phenolic ring with methyl or halogen substituents. Thus, we performed computational relativistic DFT and TDDFT studies to determine their optical properties. The ten complexes analyzed showed absorption in the visible light range. Furthermore, our analyses, including zero-field splitting (ZFS), allowed us to determine that the low-lying excited state locates below the ³LLCT states. Interestingly, seven of the ten analyzed complexes, whose corresponding PSB harbors an intramolecular hydrogen bond (IHB), exhibited luminescent emission that could be suitable for biological purposes: large Stokes shift, emission in the range 600–700 nm and τ in the order of 10⁻² to 10⁻³ s. Conversely, the three complexes lacking the IHB due to two halogen substituents in the corresponding PSB showed a predicted emission with the lowest triplet excited state energy entering the NIR region. The main differences in the complexes' photophysical behavior have been explained by the energy gap law and time-resolved luminescence. These results emphasize the importance of choosing suitable substituents at the 4 and 6 positions in the phenolic ring of the PSB, which determine the presence of the IHB since they modulate the luminescence properties of the Re(i) core. Therefore, this study could predict Re(i) tricarbonyl complexes' properties, considering the desired emission features for biological and other applications.

Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells.

Rhenium carbonyl complexes bearing methylated triphenylphosphonium cations as antibody-free mitochondria trackers for X-ray fluorescence imaging

A convenient rhenium-based multimodal mitochondrial-targeted probe compatible with Synchrotron Radiation X-ray Fluorescence nano-imaging.

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

A series of [Re(N^N)(CO)3(Cl)] (N^N = diimine) complexes based on 4-(pyrid-2-yl)-1,2,3-triazole (1), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (2), and 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (3) diimine ligands were prepared and their photophysical and electrochemical properties were characterized. The ligand-based reduction wave is shown to be highly sensitive to the nature of the triazole-based ligand, with the peak potential shifting by up to 600 mV toward more positive potential from 1 to 3. All three complexes are phosphorescent in solution at room temperature with λmax ranging from 540 nm (1) to 638 nm (3). Interestingly, the complexes appear to show inverted energy-gap law behaviour (τ = 43 ns for 1 versus 92 ns for 3), which is tentatively interpreted as reduced thermal accessibility of metal-centred (3MC) states from photoexcited metal to ligand charge transfer (3MLCT) states upon stabilisation of the N^N-centred lowest unoccupied molecular orbital (LUMO). The photophysical characterisation, supported by computational data, demonstrated a progressive stabilization of the LUMO from complex 1 to 3, which results in a narrowing of the HOMO–LUMO energy gap (HOMO = highest occupied molecular orbital) across the series and, correspondingly, red-shifted electronic absorption and photoluminescence spectra. The two complexes bearing pyridyl (1) and pyrimidyl (2) moieties, respectively, showed a modest ability to catalyse the electroreduction of CO2, with a peak potential at ca. −2.3 V versus Fc/Fc+. The catalytic wave that is observed in the cyclic voltammograms is slightly enhanced by the addition of water as a proton source.

Monitoring the Kinetics of the Cellular Uptake of a Metal Carbonyl Conjugated with a Lipidic Moiety in Living Cells Using Synchrotron Infrared Spectromicroscopy

Presented here is the exploitation of synchrotron infrared spectromicroscopy to evaluate the feasibility of monitoring the cellular uptake of rhenium-tris-carbonyl-tagged (Re(CO)₃) lipophilic chains in living cells. To this aim, an in-house thermostated microfluidic device was used to limit water absorption while keeping cells alive. Indeed, cells showed a high survival rate in the microfluidic device over the course of the experiment, proving the short-term biocompatibility of the device. We recorded spectra of single, living, fully hydrated breast cancer MDA-MB231 cells and could follow the penetration of the rhenium complexes for up to 2 h. Despite the strong variations observed in the uptake kinetics between individual cells, the Re(CO)₃ complex was traced inside the cells at low concentration and shown to enter them on the hour time scale by active transport.

Phosphine oxide-based tricarbonylrhenium(i) complexes from phosphine/phosphine oxide and dihydroxybenzoquinones

Neutral phosphine oxide (P[double bond, length as m-dash]O) donor-based organometallic complexes [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (1), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (2), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (3), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (4), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PCy3}] (5), and [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PPh3}] (6) were assembled from phosphine/phosphine oxide, a dihydroxybenzoquinone donor and Re2(CO)10via a one-pot solvothermal approach. The soft phosphine donor was transformed into a hard phosphine oxide donor during the formation of 1, 3, 4, 5, and 6. The complexes 1-6 were air and moisture stable and were soluble in polar organic solvents. The complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopic methods. The molecular structures of 1, 2, 4, and 6 were analyzed by single-crystal X-ray diffraction analysis. The UV-Visible absorption studies indicated that 1-6 in THF display strong visible light absorption in the range of ∼350-700 nm.