PHOTOLESIONS AND BIOLOGICAL INACTIVATION OF PLASMID DNA ON 254 nm IRRADIATION AND COMPARISON WITH 193 nm LASER IRRADIATION

Plasmid pTZ18R and calf thymus DNA in aerated neutral aqueous solution were irradiated by continuous 254 nm light. The quantum yields are φ_ssb= 4.0 × 10^-5 and φ_dsb= 1.4 × 10^-6 for single- and double-strand break formation, respectively, φ_br= 2.3 × 10^-5 for base release, φ_dn= 2.1 × 10^-3 for destruction of nucleotides, and φ_icl×φ_lds× 1 × 10^-6 for interstrand cross-links and locally denatured sites, respectively. The presence of Tris-HCI/ ethylenediaminetetraacetic acid (10:1, pH 7.5) buffer strongly reduces φ_ssb, The corresponding φ values, obtained on employing pulsed 193 nm laser irradiation, are much larger than those using λ_irr, = 254 nm. This is ascribed to a contribution of chemical reactions induced by photoionization, which is absent for 254 nm irradiation. The quantum yields of inactivation of plasmid DNA (λ_irr= 254 nm) were measured by transformation of the Escherichia coli strains AB1157 (wild type), φ_ina(1157) = 1.6 × 10^-4, AB1886 (uvr^-), φ_ina(1886) = 4.2 × 10^-4, AB2463 (rec^-), φ_ina(2463) = 4.1 × 10^-4 and AB2480 (uvr^- rec^-), φ_ina(2480) = 3.1 × 10^-3. The quantum yields of inactivation of plasmid DNA are compared with those of the four E. coli strains (denoted as chromosomal DNA inactivation) obtained from the literature. The results for E. coli strain AB2480 show that the chromosomal DNA and the plasmid DNA are both inactivated by a single pyrimidine photodimer per genome. With the E. coli strain AB2463 inactivation of plasmid and chromosomal DNA is the same for the same total damage per genome and is ～ 10 times smaller than for AB2480. This is explained by photodimer repair in chromosomal and plasmid DNA and by the absence of dsb repair in both cases. In the repair wild-type strain AB1157, inactivation of the plasmid DNA is roughly 100 times higher than that of the chromosomal DNA. We postulate that a portion of this difference is due to repair of dsb by the recA system in chromosomal DNA and that such repair does not take place in the plasmid DNA. The biological results from 254 nm irradiation are compared with those from 193 nm laser irradiation.     

Synthesis of dichloromethyl-substituted salicylates and pyran-4-ones by cyclocondensation of 1,3-bis(silyloxy)-1,3-butadienes with 1,1-dimethoxy-4,4-dichlorobut-1-en-3-one: control of the C,C- and C,O-regioselectivity by the choice of Lewis acid

The TiCl₄-mediated formal [3+3] cyclocondensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with 1,1-dimethoxy-4,4-dichlorobut-1-en-3-one afforded a variety of functionalized 6-dichloromethyl-4-methoxysalicylates with very good regioselectivity. Some of the products were transformed into 6-formyl-4-methoxysalicylates. The employment of Me₃SiOTf instead of TiCl₄ resulted in a change of the regioselectivity and in the formation of functionalized 2-(dichloromethyl)pyran-4-ones.     

Quantum dissipative chaos in the statistics of excitation numbers

A quantum manifestation of chaotic classical dynamics is found in the framework of oscillatory number statistics for the model of a nonlinear dissipative oscillator. It is shown that the probability distributions and variances of oscillatory number states are strongly transformed in the order-to-chaos transition. A nonclassical, sub-Poissonian statistics of oscillatory excitation numbers is established for chaotic dissipative dynamics in the framework of the Fano factor and Wigner functions. It is proposed to use these results in experimental studies of the quantum dissipative chaos.     

Silver nanocube-enhanced far-red/near-infrared fluorescence of conjugated polyelectrolyte for cellular imaging

We present the study of silver nanocube (Ag NC)-enhanced fluorescence of a cationic conjugated polyelectrolyte (CPE) for far-red/near-infrared fluorescence cell imaging. Layer-by-layer self-assembly of polyelectrolytes on 78 nm Ag NCs is used to control CPE-metal distance and its effect on CPE fluorescence. The highest fluorescence enhancement factor (FEF) is obtained for Ag NCs with two bilayers, corresponding to a CPE-metal spacer thickness of ～6 nm. At the optimal excitation wavelength, the FEF is 13.8 with respect to the control silica nanoparticles (NPs). The fluorescent NPs are further used for cellular imaging studies. The CPE-loaded Ag NCs with two bilayers exhibit excellent image contrast, superior to the control of CPE-silica NP at a similar uptake efficiency. The viability test indicates low cytotoxicity of the CPE-loaded Ag NCs, rendering them as promising cell imaging agents.     

Observation of low‐wavenumber out‐of‐plane optical phonon in few‐layer graphene

Few‐layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low‐wavenumber Raman peak of ~120 cm⁻¹ and a phonon‐induced oscillation in the kinetic curve of electron–phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low‐wavenumber out‐of‐plane optical mode in the few‐layer graphene. The phonon band shows an asymmetric shape, a consequence of so‐called Breit‐Wigner‐Fano resonance, resulting from the coupling between the low‐wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low‐wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd.     

Tetrahydrofuran-mediated radical processes: stereoselective synthesis of d,l-hexestrol

The highly stereoselective synthesis of d,l-hexestrol (1), an inhibitor of microtubule assembly, is developed by using, as a key step, an intermolecular coupling of Co₂(CO)₆-complexed propargyl radicals. The latter are generated by novel complementary processes involving an interaction of tetrahydrofuran with Co₂(CO)₆-complexed propargyl alcohols and cations. An isomerically pure d,l-μ-η²-[3,4-di(4-methoxyphenyl)-1,5-hexadiyne]-bis-dicobalthexacarbonyl (d,l-6) is isolated in 69-91% yield with intermolecular coupling reactions exhibiting an excellent chemo- (0.5-7%) and d,l-diastereoselectivity (90-94%). The structure of d,l-6 is determined by X-ray diffraction. The subsequent steps include BBr₃-induced demethylation of 4-methoxyaryl groups, demetalation with cerium(IV) ammonium nitrate, and hydrogenation of acetylenic termini affording d,l-hexestrol (1).     

Photocatalytic Metal Hydride Hydrogen Atom Transfer Mediated Allene Functionalization by Cobalt and Titanium Dual Catalysis

Catalytic metal hydride hydrogen atom transfer (MHAT) reactions have proven to be a powerful method for alkene functionalization. This work reports the discovery of Co-porphines as highly efficient MHAT catalysts with a loading of only 0.01 mol % for unprecedented chemoselective allene functionalization under photoirradiation. Moreover, the newly developed bimetallic strategy by the combination of photo Co-MHAT and Ti catalysis enabled the successful carbonyl allylation with a wide range of amino, oxy, thio, aryl, and alkyl-allenes providing expedient access to valuable β-functionalized homoallylic alcohols in over 100 examples with exceptional regio- and diastereoselectivity. Mechanism studies and DFT calculations supported that selectively transferring hydrogen atoms from cobalt hydride to allenes and generating allyl radicals is the key step in the catalytic cycle.     

Synthesis of pyridone-substituted furan-2(5H)-ones and their intramolecular cyclization to afford furo[3,4-f]isoquinolines

Chemistry of Heterocyclic Compounds - Synthetic approach toward pyridone-substituted furan-2(5H)-ones has been described. Intramolecular cyclization of these compounds leads to novel 7-substituted...     

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τ_T=0.1 μs), these new adducts show a longer-lived triplet excited state (τ_T=9.5 μs). Based on the singlet oxygen quantum yield (Φ_Δ=7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D₁↔Q₁) at ∼80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05∼0.10 eV).