E. Keskinova

A Picosecond Flash-Photolysis Study of the Biphotonic Ionization of Psoralen Derivatives and Ethidium Bromide

A picosecond flash-photolysis study of the ionization of psoralen derivatives and ethidium bromide, either free or intercalated in DNA, was carried out. A single short high-intensity 355 nm laser pulse with a few tens of J/cm2 dose, was found to be sufficient for achieving close to the ultimate 100% yield of ionization of either free or DNAbound psoralens. Interestingly, the photoionization efficiency of ethidium bromide is considerably reduced upon intercalation.

Photoproducts generated from hematoporphyrin under high-intensity picosecond flash photolysis. Evidences for involvement of biophotonic processes.

SummaryHematoporphyrin (Hp) solutions were subjected to a wide range high intensity (0.2–10.0 GW/cm2) near-UV laser pulse radiation (λexc=355 nm, pulse duration 30 ps). The formation of stable Hp photoproducts was followed by UV-VIS absorption spectroscopy and liquid-gel column chromatography. As judged by the influence of free radial scavengers, a significant part of the products is assigned to arise from the reaction of Hp with OH · (and H ·) radicals. Using nitroxide radicals (TEMPO and TEMPONE) and the spin trap DMPO the generation of primary transient photoproducts, hydrated electrons (eaq−), OH · and H · radicals, was studied varying the pulse intensity at a constant absorbed light energy. The results showed that bi-photonic processes are responsible for the observed product generation (different foreaq− photoejection and OH · (OH ·) formation). A tentative diagram of the Hp excitation routes involved in the present high intensity laser flash photolysis is suggested. According to it, OH · and H · radicals are supposed to be generated in a resonance energy transfer reaction from highly excited Hp** to water molecules (H2O sensitization).

Electroconvective dendrites in nematic phases having short range smectic order exhibited by the 4,n-alkoxybenzoic acids

Electroconvective (EC) dendrites (parabolic type) in nematic (N) liquid crystals with short range smectic order shown by 4,n-octyloxybenzoic acid (8-OBA) were observed. The driving of the dendrites by lateral d.c. and a.c. electric field parameters in the nematic temperature range was carried out. The dynamics of dendrite growth were studied and analysed by the Ivantsov two-dimensional solution for the diffusion equation. Using various combinations of blocked and unblocked electrodes, we studied dependences between the dimensionless electric field parameter and product of the dendrite parabolic radius and the velocity of the dendrite growth and found that the type of electrode significantly modifies these basic dependences. We attribute this effect possibly to the opposing ion propagation, i.e. gradient of electric field in direction opposite to that of the charge injection. A possible mechanism of EC dendrite growth is suggested. A comparison with thermal dendrites in liquid crystals is presented.

Liquid crystal nanocomposites produced by mixtures of hydrogen bonded achiral liquid crystals and functionalized carbon nanotubes

The liquid crystalline (LC) nature of alkyloxybenzoic acids is preserved after adding of any mesogenic or non-mesogenic compound through hydrogen bonding. However, this noncovalent interaction provokes a sizable effect on the physical properties as, e. g. melting point and mesomorphic states. In the present work we investigate nanocomposites, prepared by mixture of the eighth homologue of p-n-alkyloxybenzoic acids (8OBA) with single-walled carbon nanotubes (SWCNT) with the purpose to modify the optical properties of the liquid crystal. We exercise optical control on the LC system by inserting SWCNT specially functionalized by carboxylic groups. Since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level, molecular modification can lead to different macroscopical nanocomposite symmetry. The thermal properties of the functionalized nanocomposite are confirmed by DSC analyses. The mechanism of the interaction between surface-treated nanoparticles (functionalized nanotubes) and the liquid crystal 8OBA bent- dimer molecules is briefly discussed.

Electroconvection in dimeric nematic liquid crystals with short-range smectic C order: dynamical characteristics

We describe the specific features of electroconvection (EC) in the nematic liquid crystal (NLC) phase with short-range smectic C order (the seventh homologue of the p,n-alkyloxybenzoic acid, 7OBA, which is dimeric). These features are revealed below a definite temperature (T*) within this liquid crystal phase. By polarisation analysis of the azimuthal in-plane director deflection in the EC regime we suggest that this instability has a twist-type character. We indicate that the specificity of the temperature variation of the electroconductivity and the EC in 7OBA is determined by the smectic tilt angle magnitude and the dimers compactness within the supramolecular complexes. We find that response times below T* are about two times shorter than those in classical NLCs, as an equalisation of both response and decay times, which is preferable in LC display techniques. A fourth frequency harmonic is detected for the first time in the N state.

Optical properties of dimeric liquid crystals doped with single-walled carbon nanotubes

Well oriented distinct local single crystals were grown in smectic C liquid crystal (LC) (heptyloxybenzoic acid – 7OBA) using predefined orientation of the single-walled carbon nanotubes (SWCNT). The SWCNT/SiOx/ITO/glass surface is found to cause a substantial enlargement of the smectic C distinct local single crystals and to reinforce both surface memory effect and surface anchoring. We synthesized and examined a set of nanocomposites, built of LC with hydrogen-bonded in dimers molecules (7OBA) and SWCNT. We found that the surface orientation strength and the interaction of the dimeric molecules with the carbon nanotubes play a decisive role for the type (symmetry), thermal stability and chirality of the LC states induced in the nanocomposites. As a result, a set of phase transitions and new phases (chiral smectic C, reentrant nematic, reentrant chiral nematic and a unique low-temperature chiral biaxial fluid smectic phase CG) not typical for pristine 7OBA were found.

The electroconvection in nematic liquid crystals with short range smectic C order and negative electroconductivity anisotropy

The electroconvective instability (ECI) in nematic liquid crystals (NLCs), appearing beyond smectic C at cooling known as N with short range smecic C order, presents an unique electrooptical phenomenon due to generation and frustration of the smectic layering, which manifests inside the N temperature range. We found that the response time in this material is about an order shorter than those in the classical nematics. For the first time in NLCs were detected harmonics higher than fourth order. A twist type instability is suggested for this material.

Hematoporphyrin-sensitized degradation of deoxyribose and DNA in high intensity near-UV picosecond pulsed laser photolysis

Abstract The photosensitized degradation of deoxyribose and DNA, using hematoporphyrin (HP) and picosecond laser pulses at high intensities (pulse duration 30 ps, λexc = 355 nm, light intensity range 108–1010W/cm2) was studied. Aldehyde formation from 2-deoxy- d -ribose and long-chain double-stranded DNA, when analyzed as a function of light intensity, followed a non-linear dependence, suggesting the involvement of multiphoton light absorption by HP. The degradation mechanism was studied by analysis of the yield dependence on excitation intensity and the effect of added radical scavengers. The participation of OH radicals in the degradation process was confirmed by spin trapping techniques. At low light intensities added N2O largely increased product formation, suggesting that HP photoionization predominates under these conditions. At higher intensities (I ≥ 3 GW/cm2) the product yield was not affected by N2O which, combined with spin trapping data, suggested that OH radical formation occurred, but that neither HP photoionization nor peroxy radical formation was involved. Single and double strand breaks in supercoiled plasmid DNA (pBR 322) confirmed the generation of OH or OH-like radicals during high-intensity excitation of HP. A mechanism involving a multistep excitation of HP, followed by resonance energy transfer to H2O resulting in dissociation to yield OH and H atoms, is proposed.

Ferroelectric state induced in mixture of dimer liquid crystal and perfluorooctanoic acid

ABSTRACT Phase transitions and electro-optical effects are studied in a nanocomposites, grown by mixing heptyloxybenzoic acid (7OBA), displaying hydrogen bonded dimeric liquid crystal (LC) state, with perfluorooctanoic acid. Due to the interaction with the dopant structural units the dimer rings of the LC matrix bent. As a result, transitions from achiral to chiral states, including ferroelectric smectic CG with the lowest C1 symmetry take place.

Generation of free radicals in high-intensity laser photolysis of organic microcyclic compounds: time-resolved spectroscopy and EPR study

Photochemistry can be used as a tool to investigate biological processes. Various natural compounds and phototherapeutic agents including llavin , psoralen , hematopor phyrin and phtalocyanine derivatives have been shown to oxydize purine components upon exposure to near - UV and visible light. It is usually assumed that it is the sensitizer in its triplet state which is the reactive species in photooxidation reactions either via an electron or hydragen atom transfer Ctype ID or via singlet oxygen and superoxide anion generation (type II) . It is currently accepted that superoxide radicals can generate hydroxyl COH ) radicals which are known to be the mai n DNA - damagirig speci es . Recenti y the use of photoserisi ti zed reactions as a tool of probing biomolecular structures and processess has been reviewed Cl - 3).