Fluorescent Small Molecules Are BIG Enough To Sense Biomacromolecule: Synthesis of Aromatic Thioesters and Understanding Their Interactions with ctDNA.

The visible fluorescent chromophoric moiety present in the water-soluble photoactive yellow protein (PYP) of Ectothiorhodospira halophila is p-hydroxycinnamic acid linked to the cysteine residue (Cys-69) by a thioester bond and it controls the key photoinduced biological processes of the host organism. In the present work, we have synthesized and characterized three structurally different thiophenyl esters [viz., p-hydroxycinnamic-thiophenyl ester (1), p-N,N-dimethylaminocinnamic-thiophenyl ester (2), and S-phenyl-3-(4-chlorophenyl)-3-(phenylthio)propanethioate (3)] in addition to a novel (to the best of our knowledge) stilbene-type olefinic compound, N1,N1,N2,N2-tetramethyl-1,2-bis(phenylthio)ethene-1,2-diamine (4), under the same reaction condition. All of these four compounds showed characteristic and distinguishable chromophoric/fluorophoric behavior in ethanol and also at pH 7.4. However, we have observed that the intrinsic chromophoric/fluorophoric activities of (1) and (2) were greatly influenced during their interactions with calf-thymus DNA, studied by a range of spectroscopic and physicochemical measurements. We have also applied density functional theory [B3LYP, 6-311G+(d,p)]-based method to get optimized structures of (1) and (2), which were explored further for molecular docking studies to understand their mode of interaction with DNA. The present study opens up their possible applications as fluorescence probes for biomacromolecules like DNA in future.

Antioxidant flavone analog functionalized fluorescent silica nanoparticles: Synthesis and exploration of their possible use as biomolecule sensor.

For the first time, a synthetic fluorescent antioxidant flavone analog was successfully anchored onto the surface of the APTES-modified mesoporous silica nanoparticles (NPs) through sulfonamide linkage. The surface chemistry and morphology of the flavone modified fluorescent silica (FMFS) NPs were studied in detail. The flavone moiety when attached onto the FMFS NP surface, imparted its characteristic fluorescence and antioxidant activities to these NPs. Moreover, the NPs are highly biocompatible as evidenced from their cytotoxicity assay on normal lung cell (L132). The fluorescence activity of these biocompatible NPs was further utilized to study their interaction with a biomolecule, BSA (Bovine Serum Albumin). It was interesting to note that the fluorescence behavior of FMFS NPs completely changed on their binding with BSA. On the other hand, the intrinsic fluorescence activity of BSA was also significantly modified due to its interaction with FMFS NPs. Thus, the sensing and detection of biomolecules like BSA in presence of FMFS NPs can be accomplished by monitoring changes in the fluorescence behavior of either FMFS NPs or BSA. Furthermore, these FMFS NPs retained their intrinsic fluorescence behavior in the cellular medium which opens up their possible use as biocompatible cell imaging agents in future.

Unusually slow intramolecular proton transfer dynamics of 4'-N,N-dimethylamino-3-hydroxyflavone in high n-alcohols: involvement of solvent relaxation.

Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.

Effect of an Electron-Donating Substituent at the 3',4'-position of 3-Hydroxyflavone: Photophysics in Bulk Solvents.

Introduction of the methylenedioxy substituent group in the 3',4'-position of 3-hydroxyflavone produced a significant impact on its proton-transfer response, much like the well-known 4'-N,N-dialkylamino group. The potential electron-donating property of the substituent helped sustain a high degree of charge separation in the excited enolic form of the molecule, which was stabilized in relatively polar solvents, whereupon the enol → tautomer excited state intramolecular proton-transfer (ESIPT) rate decreased. Hydrogen-bonding solvents caused further retardation by interfering with the intramolecular hydrogen bond that promotes ESIPT. Among these solvents, hydrogen bond donors appear to be more efficient ESIPT inhibitors than hydrogen bond acceptors. Femtosecond fluorescence experiments revealed that even among the latter the ESIPT time-constants become steadily longer as the hydrogen bond basicity of the solvent increases.

Proton Transfer Dynamics of 4'-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles.

Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.

Design, synthesis and exploring the quantitative structure-activity relationship of some antioxidant flavonoid analogues.

A series of flavonoid analogues were synthesized and screened for the in vitro antioxidant activity through their ability to quench 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical. The activity of these compounds, measured in comparison to the well-known standard antioxidants (29-32), their precursors (38-42) and other bioactive moieties (38-42) resembling partially the flavone skeleton was analyzed further to develop Quantitative Structure-Activity Relationship (QSAR) models using the Genetic Function Approximation (GFA) technique. Based on the essential structural requirements predicted by the QSAR models, some analogues were designed, synthesized and tested for activity. The predicted and experimental activities of these compounds were well correlated. Flavone analogue 20 was found to be the most potent antioxidant.