Modification of Guanine with Photolabile N‐Hydroxypyridine‐2(1H)‐thione: Monomer Synthesis,Oligonucleotide Elaboration,and Photochemical Studies

The photochemistry of N‐hydroxypyridine‐2(1H)‐thione (NHPT), inserted as a photolabile modifier at the 6‐position of 2′‐deoxyguanosine or guanosine, has been evaluated. In particular, 6‐[(1‐oxidopyridin‐2‐yl)sulfanyl]‐ ( 1a ) and 6‐[(pyridin‐2‐yl)sulfanyl]‐2′,6‐dideoxyguanosine ( 2a ), novel photolabile derivatives of the natural nucleosides, were synthesized and characterized. The observed photolysis products of 1a in organic solvents could only be rationalized by assuming a rapid equilibrium with the corresponding 6‐[(2‐thioxopyridin‐1(2H)‐yl)oxy] analogue 3a (Scheme 5). Transient spectroscopy of 1a indicated a strong triplet‐excited state suitable for triplet → triplet energy transfer or singlet‐oxygen generation. The NHPT function was stable enough for (slightly modified) automated solid‐phase oligonucleotide synthesis. The utility of the above compounds is discussed, as well as their potential use in photosensitization of reactive oxygen species in DNA.     

Photohydrodimerization of 6‐Methoxyflavone to 6,6″‐Dimethoxy‐2,2″‐Biflavanones

6‐Methoxyflavone ( 7 ) easily afforded two hydrodimers of rac‐6,6Prime;‐dimethoxy‐2,2″‐biflavanone ( 8a ) and meso‐6,6″‐dimethoxy‐2,2″‐biflavanone ( 8b ) and one reductive product of 6‐methoxyflavanone ( 9 ) by using photolysis with the electron‐donating amines including triethylamine, 2‐(N,N‐dimethylamino)ethanol or N,N‐dimethylaniline in solvents of acetonitrile, benzene or methylene dichloride. They were found to give higher yields of rac‐6,6″‐dimethoxy‐2,2″‐biflavanone ( 8a ) and meso‐6,6″‐dimethoxy‐2,2″‐biflavanone ( 8b ) (38.7% and 4.5%, 35.3% and 6.2%, respectively) in the reaction conditions of 1/10 molar ratio of 6‐methoxy‐flavone ( 7 ) to triethylamine in a solvent of acetonitrile with irradiation of twenty‐four hours by using 306 nm and 352 nm lamps.     

Acyl and CO Ligands in the [Fe]-Hydrogenase Cofactor Scramble upon Photolysis

[Fe]-hydrogenase harbors the iron-guanylylpyridinol (FeGP) cofactor, in which the Fe(II) complex contains acyl-carbon, pyridinol-nitrogen, cysteine-thiolate and two CO as ligands. Irradiation with UV-A/blue light decomposes the FeGP cofactor to a 6-carboxymethyl-4-guanylyl-2-pyridone (GP) and other components. Previous in vitro biosynthesis experiments indicated that the acyl- and CO-ligands in the FeGP cofactor can scramble, but whether scrambling occurred during biosynthesis or photolysis was unclear. Here, we demonstrate that the [¹⁸O₁-carboxy]-group of GP is incorporated into the FeGP cofactor by in vitro biosynthesis. MS/MS analysis of the ¹⁸O-labeled FeGP cofactor revealed that the produced [¹⁸O₁]-acyl group is not exchanged with a CO ligand of the cofactor, indicating that the acyl and CO ligands are scrambled during photolysis rather than biosynthesis, which ruled out any biosynthesis mechanisms allowing acyl/CO ligands scrambling. Time-resolved infrared spectroscopy indicated that an acyl-Fe(CO)₃ intermediate is formed during photolysis, in which scrambling of the CO and acyl ligands can occur. This finding also suggests that the light-excited FeGP cofactor has a higher affinity for external CO. These results contribute to our understanding of the biosynthesis and photosensitive properties of this unique H₂-activating natural complex.     

Electrochemical Studies on the Photolysis of Iron-Arene Complex Photoinitiator

Iron-arene complex was first introduced by researchers in Ciba-Geigy as an effective photoinitiator for the photocuring of epoxy resin. The photolysis of these complexes has been studied by some authors. The electrochemical study reported in this note provides further evidence for the photolysis process which is crucial for the understanding of the photoinitiation mechanism.     

Luminescence decay of broadband emission from CdS quantum dots

The stretched exponential photoluminescence decay of the energy-resolved broadband emission of purified and unpurified CdS quantum dots (QDs) made in reverse micelles is characterized as a function of photolysis time and thiol addition. Photolysis is found to proportionately increase both the lifetime and quantum yield of these QDs. This proportionality is consistent with a simple parallel channel model of the decay of the excited states. The ultimate QY of the purified sample is found to be as high as 24%, which is twice that previously reported for this preparation. At −70 °C both the QY and the lifetime increase by more than a factor of two, indicating that thermal quenching limits the QY at room temperature. Finally, the addition of alkanethiols is shown to red-shift and quench the emission while only modestly altering the lifetime. These thiolated QDs show an extremely large temperature dependence of QY, demonstrating stronger thermal quenching than the unfunctionalized QDs.     

Optimised synthesis and photochemistry of antenna-sensitised 1-acyl-7-nitroindolines

Benzophenone antenna-sensitised 1-acyl-7-nitroindolines show a significantly enhanced extent of photochemical cleavage in aqueous solution over their non-sensitised analogues and release the carboxylate derived from their 1-acyl group. The present work investigates length and functional group effects in the linker between the benzophenone sensitiser and the nitroindoline and concurrently establishes a more efficient synthetic route to an effective conjugate than previously described. An incidental finding is that a TBDMS ether is stable during claycop-mediated nitration.     

带有四个不同芳基的环丁烷的合成、结构及光化学性质

通过不同杂芳基乙烯分子间的交叉光二聚反应在硫酸溶液中和中压汞灯照射下,合成了4种含7个不同取代芳基的环丁烷衍生物.用紫外和红外光谱及¹H和¹³CNMR谱确定其结构为顺式头尾相对型二聚体.     

On the Most Powerful Chemical Traps for Bis(methoxycarbonyl)carbene (=2‐Methoxy‐1‐(methoxycarbonyl)‐2‐oxoethylidene)

The efficiency and validity of different chemical substrates for trapping bis(methoxycarbonyl)carbene (=2‐methoxy‐1‐(methoxycarbonyl)‐2‐oxoethylidene; 1 ) is dependent on the conditions of carbene generation. On conventional photolysis of dimethyldiazomalonate (=2‐diazopropanedioic acid dimethyl ester; 2 ) by long‐wave UV light (through a Pyrex filter, λ>290 nm), the most powerful trap for carbene 1 in the series of substrates Me₂S, MeOH, cyclohexane, and pyridine is Me₂S (with an efficiency ratio of ca. 6 : 4 : 2 : 1, resp.). When short‐wave decomposition of diazomalonate 2 is employed (through a quartz filter, λ>210 nm), more reliable and useful chemical traps for bis(methoxycarbonyl)carbene ( 1 ) are pyridine and cyclohexane, whose adducts with 1 are rather stable under short‐wave‐irradiation conditions. Application of alcohols for the trapping of 1 proves to be preferential when simultaneous monitoring of carbene and oxoketene formation during photolysis is necessary.     

Synthesis of dinitrosyl iron complexes (DNICs) with intramolecular hydrogen bonding

HSCH₂CONHCH₃ and HSCH₂CON(CH₃)₂ containing a peptide bond are prepared for the synthesis of DNICs with/without intra-molecular hydrogen bonding, respectively. The IR ν(NO) bands of [(NO)₂Fe(SCH₂CONHCH₃)₂]⁻ (2) appears at 1751, 1700 cm⁻¹. In complex 2, the presence of intramolecular [NH?S] hydrogen bonding was verified by the observation of IR spectroscopy with N−H stretching frequency 3334 cm⁻¹ (CDCl₃) and subsequently confirmed by single-crystal X-ray diffraction showing N−S distance of 2.94 Å. Complex 2 displays the rhombic EPR spectrum with g₁ = 2.039, g₂ = 2.031 and g₃ = 2.013 at in frozen H₂O. Complexes 2 and 3 rapidly release NO when exposed to light. The time needed for photolysis reactions of 2 is two times faster than that of 3 in less polar solvent. Representative time courses for the photolability of 2 and 3 in THF display the NO-off ability: 2 > 3.     

Photo SN-bond cleavage and related reactions of thianthrene sulfilimine derivatives

Several 1- and 2-substituted thianthrene sulfilimine derivatives were prepared and the selectivity toward oxidation and N-tosylimination under several conditions was studied. In the photolysis of trans-5-(N-p-tosyl)iminothianthrene 10-oxide (trans-10), photo isomerization to cis-10 was observed. Further, photoimino-transfer reaction of sulfilimines and their 10-mono- and -dioxide derivatives to sulfides was intensively studied to make clear the ability as nitrene precursors.