The sacrificial role of easily oxidizable sites in the protection of DNA from damage

It has been suggested that DNA contains sacrificial nucleobase sequences that protect sensitive regions of the genome from oxidative damage. Oxidation of DNA by loss of an electron generates a radical cation that can migrate long distances by hopping. The radical cation can be trapped irreversibly at certain sites (GG steps) by reaction with H(2)O or O(2) leading to the formation of lesions (oxidative damage). A series of DNA oligomers that contain regularly spaced GG steps and an 8-oxo-7,8-dihydroguanine (8-oxoG), which serves as a proxy for possibly sacrificial protective low oxidation potential sites, was prepared and analyzed. We find that in certain special sequences of DNA nucleobases that 8-oxoG protects remote GG steps from oxidative damage but that this is not a general phenomenon extending to normal mixed sequence DNA. This is a consequence of the change in the relative rate of charge hopping compared with trapping of the radical cation. When hopping is relatively slow, 8-oxoG exerts no protective effect. Thus, it seems unlikely that low oxidation potential sequences play a meaningful part in protecting mixed sequence DNA from damage

A fluorescence emission study of nitro- and nitromethyl-substituted 1,4-diarylbutadienes in solid state

1-(4-Nitrophenyl)-4-phenylbuta-1E,3E-diene 1, 1,4-di (4-nitrophenyl)buta-1E,3E-diene 2, 1-phenyl-2-methyl-4-(4-nitrophenyl) buta-1E,3E-diene 3, and 1-4-di (4-nitrophenyl)-2-methylbuta-1E,3E-diene 4 have been synthesized and their fluorescence emissions in solid state in comparison with their ambient temperature fluorescence emissions in organic solvents of varying relative permittivity and in ethanol-methanol matrix at 77 K have been investigated. It has been found that while the UV-vis absorption of the dienes in solution phase largely remains unaffected by solvent polarity, the fluorescence maximum (lambda (f) max) gets significantly altered and a much red-shifted lambda (f) max is observed in solvents of relatively higher relative permittivity (e.g. polar solvents like acetonitrile and methanol). In ethanol-methanol glass at 77 K, the fluorescence of the dienes gets significantly blue-shifted. In solid state also a red-shifted fluorescence is observed, but the red-shift is not as pronounced as in polar solvents. However, it is significantly red-shifted as compared to in solvents having relatively lower relative permittivity (e.g. the non-polar solvents like 1,4-dioxane and THF). The fluorescence spectra of nitro-dienes, particularly 2 and 4 in solid state are significantly red-shifted as compared to their fluorescence emission in relatively non-polar solvents. Further, for the mono-nitro substituted dienes I and 3, the solid state fluorescence emission is similar to that observed in relatively non-polar solvents. The results have been discussed in terms of the singlet excited states of the dienes having different polarity and geometry. It has been suggested that in solution phase in addition to the fluorescence emission from the initially prepared planar locally excited state, the fluorescence emission in these dienes can occur from intramolecular charge transfer excited state and in some cases even from a non-planar, dipolar conformationally relaxed intramolecular charge transfer excited state. The red-shifted emission of these dienes in solid state when compared to that in relatively less polar solvents, and in another highly condensed media such as ethanol-methanol glass at 77 K indicates that the solid state fluorescence of the dienes does not originate from conformationally relaxed excited states. The solid state fluorescence emission of the dienes has been attributed to excimer and intramolecular charge transfer excited state with planar geometry. Further, the role of energy level re-ordering of the two lowest excited states of the dienes, influencing the fluorescence properties has also been discussed

Effect of microheterogeneous media on the fluorescence and fluorescence probe properties of donor-acceptor diarylbutadienes

Diarylbutadienes, namely 1-(para-cyanophenyl)-4-phenylbuta-1E,3E-diene (1), 1-(para-methoxyphenyl)-4-phenylbuta-1E,3E-diene (2), 1-(para-cyanophenyl)-4-(para-methoxyphenyl)-1E,3E-diene (3) have been prepared and investigated for their absorption and fluorescence properties in homogeneous media of organic solvents, dioxane-water binary mixtures and in the microheterogeneous media of sodium dodecyl sulfate, cetyl trimethyl ammonium bromide and Triton-X-100 micelles. The fluorescence behaviour in various media has been correlated in terms of empirical solvent parameters such as Dimroth E-T(30) and Kamlet-Taft pi* values, and Kirkwood functions. A Linear correlation between solvent polarity and fluorescence properties is observed. In ethanol-methanol (1:1, v/v) matrix at 77 K, a blue-shifted fluorescence maximum with enhanced fluorescence quantum yields is observed for dienes 2 and 3. The singlet excited state of diene 3 is found to be highly polar in nature. The fluorescence peak intensity of dienes 1 and 2 and the fluorescence emission wavelength maximum (lambda(f), max) of diene 3 undergo significant changes upon changing the concentration of surfactants. The fluorescence properties of these dienes have been further used to characterize the micelles in terms of their critical micelle concentration and micropolarity

Rational tuning of AIEE active coumarin based α-Cyanostilbenes towards Far-Red/NIR region using different π-Spacer and acceptor units

A series of cyanostilbene based D−π–A derivatives, comprising 6-(diethylamino)coumarin as a donor and benzene and benzothiazole as acceptors bridged by different π-spacers (benzene, thiophene), have been synthesized and characterized. The influence of π-spacer and acceptor units on the photophysical, electrochemical, and thermal properties of the dyes was investigated in detail. The incorporation of coumarin donor results in a significant increase of the fluorescence quantum yield. In the solution phase, the absorption and emission of the dye TB show a bathochromic shift (TB > BBT) indicating dominant dynamic intramolecular rotations (IMR) behavior of cyano group over the acceptor functionality. On the other hand, the emission is reversed (BBT > TB) in the case of aggregates. The synergistic combination of intramolecular planarization and torsional alleviation around cyanostilbenes induced by strong electron withdrawing benzothiazole unit in BBT enabled pronounced bathochromic shift upon aggregation. Upon varying the spacer/acceptor substitution, the emission wavelength of the aggregates shifts from visible to far red/NIR region with concomitant large Stokes shifts of 158–213 nm. The fluorescence intensity of the aggregates gradually decreases with increased planarity of the backbone. Various experimental techniques were employed to evaluate the structure–emission phenomena relationship. The aggregation induced enhanced emission (AIEE) behavior of the dyes was substantiated from fluorescence lifetime decay studies and scanning electron microscopy (SEM) analysis. Density functional theory (DFT) computed band gap follows a consistent trend with the values estimated from electrochemical-optical studies. All the dyes possess excellent thermal stability. Our study demonstrated far-red/NIR AIEE cyanostilbene derivatives exhibiting large Stokes shift having excellent thermal properties and fluorescence quantum yields could act as potential candidates for optoelectronic or imaging applications.by Mahalingavelar Paramasivam and Sriram Kanva

Styrylisoxazole based fluorescent probes for the detection of hydrogen sulfide

Styrylisoxazoles bearing nitro group linked to bulky aromatic rings have been synthesized and examined for their absorption and emission studies in organic solvents and water. The molecules showed emission in the visible region with significant solvatochromic emission shifts influenced by the extended conjugation of aromatic rings and intramolecular charge transfer. These absorption and emission changes were used for efficient and sensitive detection of trace concentrations of hydrogen sulfide (H2S) through reduction of the nitro group to the amine in the presence of aqueous sodium sulfide. The experimental results indicated that the probes exhibited excellent emission response with large shifts in the emission and sensitivity with micromolar detection limit.by Jagadish Kumar Katla and Sriram Kanva

Photophysical studies of substituted 1,2-diarylethenes: twisted intramolecular charge transfer fluorescence in dimethoxycyano-substituted 1,2-diarylethene

1,2-Diarylethenes, namely (E)-1-(4-cyanophenyl)-2-phenylethene (1), (E)-1-(4-methoxyphenyl)-2-phenylethene (2), (E)-1-(3,4-dimethoxyphenyl)-2-phenylethene (3), (E)-1-(4-cyanophenyl)-2-(4-methoxyphenyl)ethene (4) and (E)-1-(4-cyanophenyl)-2-(3,4-dimethoxyphenyl)ethene (5), have been synthesized and their absorption and fluorescence properties at room temperature in different organic solvents and also in 1,4-dioxane-water binary mixtures have been investigated. Additionally, the fluorescence of these compounds has been examined at 77 K in an ethanol-methanol (1:1 v/v) matrix. Photophysical parameters like absorption, excitation and fluorescence spectra, fluorescence quantum yields, excited state dipole moment changes, and correlation of solvatochromic fluorescence with solvent parameters like E-T(30)-values and the pi*-scale have been made. Compound 5, with one cyano and two methoxy substituents, has been found to exhibit solvent polarity-dependent dual fluorescence. The shorter wavelength fluorescence is attributed to an initially excited delocalized planar state, while the longer wavelength fluorescence is attributed to a non-planar twisted intramolecular charge transfer excited state

Diphenylpolyene-Cholesterol Conjugates as Fluorescent Probes for Microheterogeneous Media

Extrinsically conjugated fluorescent diphenylpolyene cholesterol derivatives are synthesized and spectroscopic investigations in homogeneous and aqueous micellar solutions are described. The emission of these cholesterol conjugates reveals characteristic intra-molecular charge transfer (ICT) behaviour in homogeneous solvents with a mono-exponential decay. Spectroscopic evidence in micellar aqueous solutions reveals a bi-exponential decay. This indicates the presence of two preferred locations of the cholesterol conjugated diphenylpolyenes- sites of lower polarity and interfacial sites. The sensitivity of these fluorophores was utilized to determine the critical micelle concentrations.by Veerabhadraiah Palakollu and Sriram Kanvah Gundimed

Influence of π-spacers and acceptors in Far-red/NIR AIEE active coumarin based α-cyanostilbenes: a combined experimental and DFT study

by Mahalingavelar Paramasivam and Sriram Kanva

Stilbene stilbene shining bright: α-Cyanostilbenes as functional organic materials

247-257π-Conjugated organic molecules with suitable donor or acceptor substituents have evolved as an attractive platform for optoelectronic or biological applications. α-cyanostilbenes, due to their characteristic elastic twist, exhibit excellent photophysical properties and form self-assemblies in favorable solvent media. The ease of synthesis and capability to introduce several desired substituents has allowed their extensive use as stimuli-responsive materials. The uniqueness of these scaffolds is the observance of characteristically enhanced emission in aqueous media due to the formation of nanoparticles or aggregates. The enhanced emission is in contrast to the observation of quenched emission for many fluorophores which limits their aqueous media applications. Such a distinct emission behavior of α-cyanostilbene derivatives has led to their extensive photophysical investigations in solution and solid-state emissive properties yielding remarkable structural or morphological changes in response to pressure, heat, light radiation or the presence of a solvent. This article briefly reviews the functional properties of cyanostilbenes

α-Cyanostilbene: A Multifunctional Spectral Engineering Motif

Aggregation-induced emission (AIE) is a unique photophysical phenomenon of organic chromophores, exhibiting a significant emission enhancement in the condensed phase (aggregate/solid/film) than in the solution phase. This remarkable feature offers excellent strategies to obtain molecular materials possessing unique spectral signatures such as high fluorescence intensity, excellent quantum yield, large Stokes shift, and exquisite optoelectronic properties. Unlike a great library of articles with propeller-shaped tetraphenylethene molecular frameworks, reviews based on the mechanistic understandings of α-cyanostilbenes are relatively rare. Considering this, herein, we highlight the structure–property relationship of α-cyanostilbene-based AIE frameworks for tuning the aggregation through molecular displacement with reference to transition dipoles based on the following parameters: (i) positional substitution and orientation of the α-cyano unit, (ii) π-conjugation length (da or db), (iii) molecular size (DAr) of the peripheral substitutions with respect to the α-cyano unit, and (iv) branching effect. In addition, we explain the utility of their unique AIE characteristics for various optoelectronic applications, including self-assembled nanostructures, chemical sensing, organogelation, white light emission, molecular switches, multiphoton absorption, liquid crystals, anion receptors, and biological probes. It is anticipated that organic materials with a cyanostilbene framework will continue to garner attention in the interdisciplinary fields of biology, chemistry, and materials science for diverse applications