The ionization of the DNA single and double helices (dA)20, (dT)20, (dAdT)10(dAdT)10 and (dA)20(dT)20, induced by nanosecond pulses at 266 nm, is studied by time-resolved absorption spectroscopy. The variation of the hydrated electron concentration with the absorbed laser intensity shows that, in addition to two-photon ionization, one-photon ionization takes place for (dAdT)10(dAdT)10, (dA)20(dT)20 and (dA)20 but not for (dT)20. The spectra of all adenine-containing oligomers at the microsecond time-scale correspond to the adenine deprotonated radical formed in concentrations comparable to that of the hydrated electron. The quantum yield for one-photon ionization of the oligomers (ca. 10(-3)) is higher by at least 1 order of magnitude than that of dAMP, showing clearly that organization of the bases in single and double helices leads to an important lowering of the ionization potential. The propensity of (dAdT)10(dAdT)10, containing alternating adenine-thymine sequences, to undergo one-photon ionization is lower than that of (dA)20(dT)20 and (dA)20, containing adenine runs. Pairing of the (dA)20 with the complementary strand leads to a decrease of quantum yield for one photon ionization by about a factor of 2. 相似文献
The reaction of enone 1, bearing an internal nucleophilic moiety, i.e., furan or pyrrole (X = O, NR'), with isocyanides is presented. The formation of products resulting from the reaction of the zwitterionic intermediate 2 with a second equivalent of isocyanide prior to cyclization to give 3, as well as the direct formation of 4 from 2, is described. 相似文献
We previously demonstrated that the NC1[alpha3(IV)185-191] CNYYSNS peptide inhibited in vivo tumor progression. The YSNS motif formed a beta turn crucial for biological activity. The aim of the present study was to design a YSNSG cyclopeptide with a constrained beta turn on the YSNS residues more stable than CNYYSNS. By nuclear magnetic resonance and molecular modeling, we demonstrated that the YSNSG cyclopeptide actually adopted the expected beta-turn conformation. It promoted melanoma cell adhesion and prevented their adhesion to the native peptide. It inhibited in vitro cell proliferation and migration through Matrigel by downregulating proteolytic cascades. Moreover, intraperitoneal administration of the YSNSG cyclopeptide inhibited melanoma progression far more efficiently than the native peptide. The increased solubility and stability at low pH of the YSNSG cyclopeptide suggest this peptide as a potent antitumor therapeutic agent. 相似文献
Four adjacent open reading frames, cytC1-C4, were cloned from a cytotrienin-producing strain of a Streptomyces sp. by using primers derived from the conserved region of a gene encoding a nonheme iron halogenase, CmaB, in coronamic acid biosynthesis. CytC1-3 were active after expression in Escherichia coli, and CytC4 was active after expression in Pseudomonas putida. CytC1, a relatively promiscuous adenylation enzyme, installs the aminoacyl moieties on the phosphopantetheinyl arm of the holo carrier protein CytC2. CytC3 is a nonheme iron halogenase that will generate both gamma-chloro- and gamma,gamma-dichloroaminobutyryl-S-CytC2 from aminobutyryl-S-CytC2. CytC4, a thioesterase, hydrolytically releases the dichloroaminobutyrate, a known streptomycete antibiotic. Thus, this short four-protein pathway is likely the biosynthetic source of this amino acid antimetabolite. This four-enzyme system analogously converts the proS-methyl group of valine to the dichloromethyl product regio- and stereospecifically. 相似文献
A multistep synthesis for 2-(2-pyridyl-N-oxide) ethylphosphonic acid 6-H2 is described along with its spectroscopic (IR, NMR) data and a single-crystal X-ray diffraction structure analysis. Combination of the ligand with Nd(OH)3 results in the formation of a complex Nd(6-H)3. Single-crystal X-ray diffraction analysis reveals a three-dimensional crystal network generated by hydrogen-bonded chains along the crystallographic c axis. The hydrogen bonds are formed between phosphonic acid anion (6-H)(-1) protons on one chain and pyridyl N-oxide oxygen atoms in neighboring chains. The asymmetric unit contains 1/3[Nd(6-H)3] and there are two unique Nd(III) atoms, each with point symmetry. As a result, each Nd(III) ion is bound to six (6-H)(-1) ligands and the symmetry about the Nd(III) ion is octahedral with each vertex occupied by a phosphonate oxygen atom. The Nd-O bond lengths are essentially identical: Nd(1)-O(3), 2.336 (1) A; Nd(2)-O(4), 2.340 (1) A. The monoanionic ligand (6-H)(-1), therefore, serves to bridge the unique Nd(III) centers. 相似文献
Flat H-terminated Si(111) substrates modified with alkyl monolayers terminated with hydrophobic and hydrophilic functional groups were prepared using known surface functionalization methods and characterized by FTIR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The surfaces were then used for the study of non-specific binding of proteins from complex mixtures (using standard mixture of proteins with average molecular weight approximately 6-66 kDa) by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Protein adsorption on these surfaces (following on-probe fractionation of the mixture) was found to be dependent on the nature of surface functional groups, and nature and pH of rinsing solutions used. The results obtained in this work demonstrate that simple silicon-based surface modifications can be effective for direct analysis of complex mixtures by MALDI-MS. Preliminary results obtained using similarly functionalized porous silicon substrates proved that such substrates are (due to their increased surface areas) better performing than flat silicon. 相似文献
Cross‐coupling carbenes : The coupling of a propargylic ester with a diazoalkane in the presence of [RuCl(cod)Cp*] catalyst leads to the formation of functionalized conjugated dienes with high stereoselectivity. The reaction involves the cross‐coupling of a vinylcarbene fragment, arising from a ruthenium‐catalyzed propargylic ester rearrangement, with a diazoalkane carbene.
The present scanning tunneling microscopy study reports on the growth processes of Co vapor-deposited on a dodecanethiol (DDT) self-assembled monolayer (SAM)/Au(111). We observe strongly modified surface and depth diffusions of Co adatoms depending on the growth temperature. Co deposited at 300 K shows an extremely incomplete regime of condensation on the organic layer. Besides, Co penetrates the DDT monolayer and resides at the DDT/Au(111) interface as 2D clusters. This phenomenon takes place through defects in the SAM which are transient channels. In contrast, Co deposited at 50 K shows a complete condensation and nucleates on defects of the SAM layer as 3D islands sitting most likely on top of the DDTs. These results are of interest in the growing field of organic spintronics where the quality of the organic/ferromagnetic interface is a key issue. 相似文献
Two extremely long lived metastable states (SI and SII) can be accessed by irradiation with light in the blue-green spectral range at temperatures below 200 K in Cs(2)[Ru(ox)(NO)Cl(3)], [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O, and [Ru(terpy)(NO)(OH)Cl][PF(6)]. The crystal structures of the ground states of the oxalate-containing compounds are presented, and the influence of the atomic distances of the cations/anions is discussed with respect to the decay temperatures. The radiationless thermal decay of the metastable states is detected by differential scanning calorimetry (DSC) for the three compounds. Both metastable states decay exponentially in time under isothermal conditions. The excited states are energetically separated from the ground state by potential barriers given by the activation energy of the Arrhenius law. In [Ni(cyclam)][Ru(ox)(NO)Cl(3)].3H(2)O the enthalpy maximum of the thermal decay of SII appears at 182 K, which is a relatively high decay temperature for SII. The reason for this strong temperature shift compared to those of the other compounds could be due to the polarization effect of Ni(2+) on the electron density at the Ru site via the Cl atom. 相似文献