Synthesis and evaluation of dihydro-[1,2,4]triazolo[4,3-a]pyridin-2-ium carboxylates as fixed charge fluorescent derivatization reagents for MEKC and MS proteomic analyses

Safirinium Fluorescent "Click" Molecular Probes: Synthesis, CuAAC Reactions, and Microscopic Imaging

Fluorescent labeling utilizing Cu(I)-catalyzed azide-alkyne cycloaddition reactions (CuAAC) is among the leading applications of the "click" chemistry strategy. Fluorescent probes for this approach can be constructed by linking an azide or alkyne group to a fluorophore, such as the recently developed Safirinium derivatives. These compounds are water-soluble, highly fluorescent heterocycles based on 1,2,4-triazolium, with significant potential for various labeling applications, although they have not yet been converted to azide or alkyne probes. Herein, we report the synthesis of Safirinium-based azide and alkyne functionalized molecular probes for "click" chemistry labeling. We also describe their CuAAC reactions with model compounds, including a lipid mimetic long-chain azide, an azido sugar derivative, and azidothymidine, as well as two model alkynes. We demonstrate that the Safirinium-based probes and their derivatives are chemically stable, suitable for fluorescent microscopy observations, and safe to use. Most of these probes show no toxic effects on CHO-K1 and NIH-3T3 cells.

Advances in the Synthesis of Biologically Active Quaternary Ammonium Compounds

This review provides a comprehensive overview of recent advancements in the design and synthesis of biologically active quaternary ammonium compounds (QACs). The covered scope extends beyond commonly reviewed antimicrobial derivatives to include synthetic agents with antifungal, anticancer, and antiviral properties. Additionally, this review highlights examples of quaternary ammonium compounds exhibiting activity against protozoa and herbicidal effects, as well as analgesic and anesthetic derivatives. The article also embraces the quaternary-ammonium-containing cholinesterase inhibitors and muscle relaxants. QACs, marked by their inherent permanent charge, also find widespread usage across diverse domains such as fabric softeners, hair conditioners, detergents, and disinfectants. The effectiveness of QACs hinges greatly on finding the right equilibrium between hydrophilicity and lipophilicity. The ideal length of the alkyl chain varies according to the unique structure of each QAC and its biological settings. It is expected that this review will provide comprehensive data for medicinal and industrial chemists to design and develop novel QAC-based products.

Development of Safirinium dyes for new applications: fluorescent staining of bacteria, human kidney cells, and the horny layer of the epidermis

Low-molecular synthetic fluorophores are convenient tools in bioimaging applications. Several derivatives of Safirinium dyes as well as their reactive N-hydroxysuccinimide (NHS) esters bearing diverse substituents were synthesized and evaluated experimentally in terms of their lipophilicity by means of reverse-phase and immobilized artificial membrane high-performance liquid chromatography. Subsequently, the selected compounds were employed as novel cellular imaging agents for staining Gram-positive and Gram-negative bacteria, human kidney cell line, as well as human skin tissue. The analyzed dyes allowed for visualization of cellular structures such as mitochondria, endoplasmic reticulum, and cellular nuclei. They proved to be useful in fluorescent staining of stratum corneum, especially in the aspect of xenobiotic exposure and its penetration into the skin. The best results were obtained with the use of moderately lipophilic NHS esters of Safirinium Q. The development of Safirinium dyes is a promising alternative for commercially available dyes since the reported molecules have low molecular masses and exhibit efficient staining and remarkable water solubility. Moreover, they are relatively simple and low-cost in synthesis.

Recent developments in capillary and microchip electroseparations of peptides (2019-mid 2021)

The review provides a comprehensive overview of developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) for analysis, microscale isolation, and physicochemical characterization of peptides from 2019 up to approximately the middle of 2021. Advances in the investigation of electromigration properties of peptides and in the methodology of their analysis, such as sample preparation, sorption suppression, EOF control, and detection, are presented. New developments in the individual CE and CEC methods are demonstrated and several types of their applications are shown. They include qualitative and quantitative analysis, determination in complex biomatrices, monitoring of chemical and enzymatic reactions and physicochemical changes, amino acid, sequence, and chiral analyses, and peptide mapping of proteins. In addition, micropreparative separations and determination of significant physicochemical parameters of peptides by CE and CEC methods are described.

Application of Safirinium N-Hydroxysuccinimide Esters to Derivatization of Peptides for High-Resolution Mass Spectrometry, Tandem Mass Spectrometry, and Fluorescent Labeling of Bacterial Cells

Mass spectrometry methods are commonly used in the identification of peptides and biomarkers. Due to a relatively low abundance of proteins in biological samples, there is a need for the development of novel derivatization methods that would improve MS detection limits. Hence, novel fluorescent N–hydroxysuccinimide esters of dihydro-[1,2,4]triazolo[4,3-a]pyridin-2-ium carboxylates (Safirinium P dyes) have been synthesized. The obtained compounds, which incorporate quaternary ammonium salt moieties, easily react with aliphatic amine groups of peptides, both in solution and on the solid support; thus, they can be applied for derivatization as ionization enhancers. Safirinium tagging experiments with ubiquitin hydrolysate revealed that the sequence coverage level was high (ca. 80%), and intensities of signals were enhanced up to 8-fold, which proves the applicability of the proposed tags in the bottom–up approach. The obtained results confirmed that the novel compounds enable the detection of trace amounts of peptides, and fixed positive charge within the tags results in high ionization efficiency. Moreover, Safirinium NHS esters have been utilized as imaging agents for fluorescent labeling and the microscopic visualization of living cells such as E. coli Top10 bacterial strain.

Cytotoxic triterpenoid-safirinium conjugates target the endoplasmic reticulum

Safirinium P and Q fluorescence labels were synthesized and conjugated with spacered triterpenoic acids to access hybrid structures. While the parent safirinium compounds were not cytotoxic at all, many triterpenoid safirinium P and Q conjugates showed moderate cytotoxicity. An exception, however, was safirinium P derived compound 30 holding low EC₅₀ = 5.4 μM (for A375 cells) to EC₅₀ = 7.5 μM (for FaDu cells) as well as EC₅₀ = 6.6 μM for non-malignant fibroblasts NIH 3T3. Fluorescence imaging showed that the safirinium core structures cannot enter the cells (not even after a prolonged incubation time of 24 h), while the conjugates (as exemplified for 30) are accumulating in the endoplasmic reticulum but not in the mitochondria. The development of safirinium-hybrids targeting the endoplasmic reticulum can be regarded as a promising strategy in the development of cytotoxic agents.

Affinity of Fluoroquinolone–Safirinium Dye Hybrids to Phospholipids Estimated by IAM-HPLC

Nowadays, fluoroquinolones (FQs) constitute one of the most important classes of antibiotics. FQs are used to treat infections caused by Gram-positive and Gram-negative species. A set of fluoroquinolone–Safirinium dye hybrids has been synthesized in our laboratory as potential new dual-action antibacterial agents. In the present study we have evaluated how such a modification influences the affinity of FQs to phospholipids. The immobilized artificial membrane (IAM) high-performance liquid chromatography (IAM-HPLC) was used as a tool for the determination of phospholipids partitioning. The obtained results indicate that the fluoroquinolone–Safirinium dye hybrids, especially the SafiriniumP conjugates, display significantly lower affinity to phospholipids than the parent FQs. Despite the fact that the hybrid structures comprise a quaternary nitrogen atom and hence are permanently charged, the attractive electrostatic interactions between the solutes and negatively charged phospholipids do not occur or occur at a lesser extent than in the case of the unmodified FQs. Since affinity of FQs to phospholipids involves molecular mechanism, which is not entirely determined by lipophilicity, assessment of phospholipid partitioning should be considered at the early stage of the development of new FQ antibiotics.