Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization

Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O(2) have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S.+ were observed with a peak around 470-490 nm. The formation quantum yield of S.+ (0.06-0.29) increased with decreasing oxidation potential (E(ox)) and increasing fluorescence lifetime (tau(f)) of S, except for trans-4-methoxystilbene which has the lowest E(ox) and longer tau(f) among S. The considerable low yield and fast decay in a few tens of nanoseconds time scale were observed for trans-4-methoxystilbene.+ in the presence of O(2), but not for other S.+ . It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and O(2) and the distonic character of trans-4-methoxystilbene.+ with separation and localization of the positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the beta-olefinic carbon are responsible for the fast reaction of trans-4-methoxystilbene.+ with O(2) or superoxide anion, leading to the considerable low yield and fast decay of trans-4-methoxystilbene.+ . The mechanism based on the transient absorption measurement of S.+ during the TPI is consistent with the relatively high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyanoanthracene and O(2) in acetonitrile.     

Monitoring of microenvironmental changes in the major and minor grooves of DNA by dan-modified oligonucleotides

[graph: see text] We describe the synthesis of new environmentally sensitive fluorescence probes to elucidate DNA structures. DNA oligonucleotides containing fluorophore dan (6-(dimethylamino)-2-acylnaphthalene)-modified dC or dG were able to monitor the microenvironmental changes in both the major and minor grooves of DNA with a B- to A-DNA conformational transition and RNA hybridization.     

Mechanism of Enzymatic Hydrolysis of Poly(butylene succinate) and Poly(butylene succinate‐co‐L‐lactate) with a Lipase from Pseudomonas cepacia

Enzymatic hydrolysis of poly(butylene succinate) (PBS) and poly(butylene succinate‐co‐L ‐lactate) (PBSL) has been studied by using a lipase originated from Pseudomonas cepacia. It has been found that the drawn fibers of PBSL are readily hydrolyzed by the action of the lipase, while those of PBS undergo little enzymatic hydrolysis. Since the polymer films of PBS and PBSL are readily hydrolyzed under the same conditions, the enzymatic hydrolysis should depend not only on the crystallinity but also on the molecular orientation. The molecular weight of the samples gradually decreases with incubation time, because nonspecific hydrolysis occurs on the main chains of both PBS and PBSL even in the absence of lipase. The enzymatic hydrolysis of PBS and PBSL gives 4‐hydroxybutyl succinate (HBS) as the main product with traces of succinic acid and butane‐1,4‐diol together with L ‐lactic acid in the case of PBSL. In addition, the hydrolysis rate of the carboxyl end‐capped PBS is much slower than that of the original or hydroxyl end‐capped PBS. These results imply a hydrolysis mechanism involving the preferential exo‐type chain scission from the carboxyl terminals.

Mass remaining of various PBS and PBSL samples as a function of time.     

Ultrafast photoinduced intramolecular charge separation and recombination processes in the oligothiophene-substituted benzene dyads with an amide spacer

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.     

Enzyme electrochemical preparation of a 3-keto derivative of 1,5-anhydro-d-glucitol using glucose-3-dehydrogenase

A novel enzymatic organic synthesis was reported, utilizing glucose-3-dehydrogenase (G3DH) and its regeneration via electrochemical methods. We combined the water-soluble G3DH prepared from a marine bacterium, Halomonas sp. α-15, and electron mediator with the electrode system in order to regenerate the enzyme. Using this system, the conversion of 1,5-anhydro-d-glucitol (1,5AG), a diabetes marker in human blood, was investigated. The final yield of the product, 3-keto anhydroglucitol (3-ketoAG), which was identified by ¹³C nuclear magnetic resonance, was 82% based on the initial amount of 1,5AG. The electrochemical yield of the reaction proceeded almost stoichiometrically. The electrochemical conversion rate of 1,5AG was 1.24 mmol/(L·h), and the electrochemical yield of 1,5AG consumption was 80%, whereas that for 3-ketoAG was 60%.     

Reactions of methylenecyclopropanes with a diethylzinc-bromoform system, and the utilization for synthesis of a novel cyclopropylidene-nucleoside

The reactions of substituted methylenecyclopropane with diethylzinc-bromoform gave bromospiro[2.2]pentane derivatives, bromoform-addition compounds, an oxabicyclo-compound and adjacent dibromo-compound, and the last product was derived to a novel ,β-unsaturated nucloside.     

Four-way-branched DNA-porphyrin conjugates for construction of four double-helix-DNA assembled structures

[reaction: see text] DNA-porphyrin conjugates having four DNA strands were designed and synthesized. Four double helices were assembled using two DNA-porphyrin conjugates and their complementary strands, and the formation of the four double-helix assembled structures with the two DNA-porphyrin units was examined by gel electrophoresis and spectroscopic analysis. The interaction between two porphyrin chromophores in the complex was investigated by measurement of fluorescence lifetimes, and the singlet energy transfer between the two different phorphyrin units (Zn-porphyrin and H2-porphyrin) was observed. These results indicate that multiple and different porphyrin chromophores can be integrated into the DNA structures by programming the sequences of the DNA strands.     

Preparation of 9-hydroxy grayanotoxin derivatives and their acute toxicity in mice

Novel 9alpha and 9beta-hydroxy grayanotoxin II derivatives were prepared by photo-sensitized oxygenation of iso-grayanotoxin II and oxidation of grayanotoxin II tetraacetate with selenium dioxide respectively. The lethal dosage of 9alpha and 9beta-hydroxy grayanotoxin II were lower than that of grayanotoxin II. In addition, the lethal dosage of 9beta-hydroxy-dihydro grayanotoxin II was higher than that of dihydro grayanotoxin II.     

Properties and cage lactone synthesis of diels-alder adducts from 2-pyrones

New cage lactones were synthesized by the photocyclization of the tricyclodienolactones 1a, 1b, 2 which were available from the Diels-Alder reactions of 2-pyrones with p-benzoquinones or norbornadiene. Retro-Diels-Alder reactions of cross-adducts between 2-pyrone-mono-adducts and p-benzoquinone were also described.