Biomimetic systems trigger a benzothiazole based molecular switch to 'turn on' fluorescence

A simple dual responsive chemosensor for selective sensing of Cs+ for environmental monitoring and mimicking molecular logic gates

Detection of toxic metals is of vital importance to safeguard both public health and the ecosystem. Herein, we investigate the newly designed and synthesised isoxazole-based azo dye, (E)-cyclopentyl(5-((5-(4-fluorophenyl) isoxazole-3-yl) diazenyl)-2-hydroxyphenyl) methanone (FPAZ), as a dual chromogenic and fluorogenic sensor. FPAZ demonstrates high selectivity, reusability and ultra-sensitivity towards Cs+ ions manifested through naked eye detection in aqueous medium by employing simple and economic optical spectroscopy techniques. The color change from colourless to dark yellow and enhancement of fluorescence intensity reveal about FPAZ-Cs+ complexation by UV-Vis and fluorescence spectroscopy respectively. The complexation is also supported by DFT calculations. The LOD is estimated to be 0.476 µM, which by far, is the lowest LOD obtained for Cs+ detection. Further, FPAZ is fabricated with various flexible materials (paper, cotton, non-woven fabric) which provide information about on-site Cs+ ion contamination by means of change in relative RGB values using a handy smart-phone camera. Besides this, the logic gate as IMPLICATION and INHIBIT is designed employing Cs+ and Cl- ions as inputs and absorbance maxima as output. Overall, the developed chemosensor is simple, quick, and more promising than previously reported systems, as it does not need any chemical modification, expensive instruments, or expertise.

Stabilization of a potential anticancer thiosemicarbazone derivative in Sudlow site I of human serum albumin: In vitro spectroscopy coupled with molecular dynamics simulation

Human Serum Albumin (HSA) is the most important protein in human blood plasma and can acts as a major transporting agent for various drug molecules with flexible binding interaction. To elucidate the interaction of a newly designed potential anticancer thiosemicarbazone based luminophore (E)-1-(4-(diethylamino)-2-hydroxybenzylidene)-4,4-dimethyl-thiosemicarbazide (DAHTS) with HSA under physiological condition, in vitro optical spectroscopic experiments viz UV-Vis absorption, steady state fluorescence, fluroscence anisotropy, time resolved fluorscence (TRF) and cicular dichroism (CD) spectroscopy have been scrutinised. The experimental findings have been corroborated with in silico molecular docking analysis and Molecular Dynamics (MD) simulation. The spectroscopic results demonstrated that the conventionally anion-favouring Sudlow site I of HSA copiously adapt neutral DAHTS molecule with moderate binding affinity. The mean fluorescence lifetime of the sole tryptophan (Trp-214) present in the macromolecule experiences an appreciable diminution with an increase in concentration of the synthesized molecule. DAHTS localize itself close to Trp-214 within subdomain IIA (Sudlow site I) and surrounded by multiple hydrophobic amino acid residues (Val-235, Val-231, Ala-229, Phe-228, Val-325, Phe-326, Leu-327, Met-329, Phe-330, Leu-331, Tyr-332, Leu-346, Leu-347, Val-482, Leu-349, Ala-350, Ala-210, Trp-214, Ala- 213 and Val-216) in HSA. The distinct fluorescence lifetime, diverse pathways and changing rate of population indicates that the rotamerisation of Trp-214 residue is controlled by the guest molecule. Sudlow site I of HSA behaves flexibly and induces an allosteric modulation in the macromolecule resulting a minor deformation in the protein secondary structure as observed in CD (observed 11% change of α-helix content) as well as in MD simulation. The integrated multi-spectroscopic research described herein provides several important information about the binding interaction of a thiosemicarbazone Schiff base with HSA, which can be very significant for thiosemicarbazone based drug designing for academia as well as industry.

Entrapment in micellar assemblies switches the excimer population of potential therapeutic luminophore azapodophyllotoxin

Azapodophyllotoxin is a new class of anti-tumor agent with brilliant therapeutic activity and understanding its physicochemical nature in bio-mimetic microenvironments may provide substantial importance in context of its intercellular localization, efficacy as well as delivery. The present work epitomizes environment-sensitive fluorescence modulation of a prodigy, 4-(2-Hydroxyethyl)-10-phenyl-3,4,6,7,8,10- hexahydro-1H-cyclopenta[g]furo[3,4-b]quinoline-1-one (HPFQ) from the class of anti-cancer agent Azapodophyllotoxin, in differently charged model bio-mimetic micellar microenvironment of cationic CTAB, anionic SDS and neutral Triton X-100 using UV-visible absorption, steady state fluorescence, time-resolved fluorescence and fluorescence anisotropy studies. As a distinct phenomenon, anticancer HPFQ exhibits prolific fluorescence in solvents of varying polarity, originating from a mixed contribution of locally excited, charge transfer and excimer emission. A dramatic modulation in the photophysics of HPFQ has been observed in two types of surfactant consortiums: pre-micellar and post-micellar at physiological and anoxic pH. On photo-excitation, anti-cancer HPFQ exists in monomer-excimer equilibrium with varying ratios in different polarity regions. The marked enhancement in fluorescence intensity of HPFQ in post-micellar region of the surfactant under study probably arises due to regeneration of the monomer from its excimer. This reoccurrence reduces the possibility of Förster resonance energy transfer (FRET) from monomer to excimer, which essentially increases the desired emission intensity. Localization of HPFQ in micellar systems highly depends on polarity gradient inside the micelle, electrostatic, hydrophobic and intermolecular hydrogen bonding interactions. Further corroboration with the polarity sensitive experiments in dioxane-water mixture indicates towards spatial localization of the probe molecule in the stern layer of cationic CTAB, sheer surface of neutral TX100 and outer Gouy-Chapman layer in anionic SDS micelles. A molecular binary logic gate correlates the dominance of micellization over the polarity factor, which enhances the fluorescence response of HPFQ. The enhancement of the emissive potential of anti-cancer HPFQ in biomimetic environments by switching its excimer population may have an immense importance to achieve the status of a dual therapeutic and imaging agent altogether in progressive biomedical research.