Does in-source decay occur independent of the ionization process in matrix-assisted laser desorption?

The influence of the acidic and basic characters of constituent amino acid residues on the peptide fragment ions produced by in-source decay under matrix assisted laser desorption/ionization (MALDI) conditions has been studied using positive- and negative-ion experiments. Whereas the in-source decay spectra of peptides containing basic Arg and/or Lys residues near the N-terminus showed so-called c_n- and a_n-series ions in positive-ion mode, a peptide that has an acidic amino acid cluster near the N-terminus and a basic residue near the C-terminus characteristically formed y_n- and z_n-series ions in the positive-ion in-source decay spectrum. These results indicated that fragment ion series produced by in-source decay depend strongly upon the acidic and basic characters of the constituent amino acid residues and the near N- and C-termini. It was suggested that in-source decay processes occur intrinsically at NH–C^α and CO–NH bonds independent of the formation of molecular-related ions, and that the cleavages at the NH–C^α and CO–NH bonds occurred independently and were dependent on the matrix used.     

Acidic properties of V2O5-WO3/ZrO2 catalysts

The adsorption of pyridine on V₂O₅−WO₃/ZrO₂ has been studied by FTIR. In V₂O₅/ZrO₂ (2 wt.%), the number of both Br?nsted and Lewis acidic sites increased with the addition of WO₃, while in V₂O₅/ZrO₂ (5 wt.%), Br?nsted sites increased and Lewis sites did not change.     

Memory effect in the chain-collapse process in a dilute polymer solution

The effect of temperature perturbation on a single-chain-collapse process was studied for poly(methyl methacrylate) with the molecular weight M(w)=1.05 x 10(7) in the mixed solvent of tert-butyl alcohol+water (2.5 vol %). In the chain-collapse process after a quench from the theta; temperature to a temperature T(1), the temperature was changed from T(1) to T(2) at the time t(1) after the quench and returned to T(1) at the time t(1)+t(2). In the three stages at T(1), T(2), and T(1), measurements of the mean-square radius of gyration of polymer chains were carried out by static light scattering and the chain-collapse process was represented by the expansion factor as a function of time. An effect of chain aggregation on the measurements was negligibly small because of the very slow phase separation. For the negative temperature perturbation (T(1)>T(2)), the chain-collapse processes observed in the first and third stages were connected smoothly and agreed with the collapse process due to a single-stage quench to T(1). A memory of the chain collapse in the first stage at T(1) was found to persist into the third stage at the same temperature T(1) without being affected by the temperature perturbation of T(2) during t(2). The memory effect was observed irrespective of the time period of t(2). The positive temperature perturbation (T(1)相似文献   

Construction of optically active CF3-containing quaternary carbon centers via stereospecific S(N)2' reaction

[reaction: see text] Phosphates from 3-substituted 4,4,4-trifluorobut-2-en-1-ols were found to be effective for construction of CF3-containing quaternary carbon centers by way of Cu(I)-catalyzed Grignard reactions in the presence of catalytic amounts of CuCN and trimethylsilyl chloride (TMSCl) in an anti S(N)2' manner.     

INTRAMOLECULAR TRAPPING OF THE NITRENE GENERATED BY PHOTOLYSIS OF S,S-DIPHENYL N-ACYLSULFILIMINE

Abstract

Various N-acyldiphenylsulfilimines (1a-k) were found to undergo photolysis in argon atmosphere, to afford the corresponding isocyanates, diphenyl sulfide and diphenyl disulfide. Attempts to trap intramolecularly the acylnitrene with olefinic linkage or sulfide afforded small amounts of the trapped products, whereas photolysis of N-mesitoyldiphenyl-sulfilimine (Ia) afforded a C–H inserted product of the nitrene in a substantial yield together with the isocyanate. Benzophenone-sensitized photolysis of Ia indicates that the nitrene generated is a singlet species.     

Analysis of oxidation process of cholecystokinin octapeptide with reactive oxygen species by high‐performance liquid chromatography and subsequent electrospray ionization mass spectrometry

The C‐terminal octapeptide of cholecystokinin (CCK8) includes some easily oxidizable amino acids. The oxidation of CCK8 by reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) and hydroxyl radicals (OH^?) was investigated using reversed‐phase high performance liquid chromatography (RP‐HPLC) and subsequent electrospray ionization mass spectrometry. The mechanism of oxidation of CCK8 in the H₂O₂ system differed from that of CCK8 in the Fenton system, in which OH^? are produced. In the H₂O₂ system, ²⁸Met and ³¹Met were oxidized to methionine sulfoxide, and no further oxidation or degradation/hydrolysis occurred. On the other hand, in the Fenton system, ²⁸Met and ³¹Met residues were oxidized to methionine sulfone via the formation of methionine sulfoxide. In addition, the oxidized product was observed at the Trp residue but not at the Tyr residue, and small peptide fragments from CCK8 were observed in the Fenton system. From these results, it was concluded that ²⁸Met and ³¹Met residues of CCK8 are susceptible to oxidation by ROS. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxygen‐sensing properties of 5,10,15,20‐tetraphenylporphinato platinum(II) and palladium(II) covalently bound on poly(isobutyl‐co‐2,2,2‐trifluoroethyl methacrylate)

A novel methacrylate monomer bearing 5,10,15,20‐tetraphenylporphyrinato palladium(II) (PdTPP) (monomer 1a ) was synthesized and copolymerized with isobutyl methacrylate (IBM) and 2,2,2‐trifluoroethyl methacrylate (TFEM) to give poly (IBM‐co‐TFEM) bearing PdTPP (copolymer 2a ) as a dye‐conjugated oxygen‐permeable polymer for pressure‐sensitive paint applications. The introduction of PdTPP into copolymer 2a was confirmed by UV–vis spectroscopy and extended X‐ray absorption fine structure analysis. The Stern–Volmer plots of the copolymer 2a and a mixture of PdTPP and poly(IBM‐co‐TFEM) both showed downward curvature, unlike that of the platinum complex analogue (copolymer 2b ) previously reported. The plots were successfully fitted with a two‐site model to give two distinct Stern–Volmer constants (K_SV1 and K_SV2) and the partition ratio f₁. Interestingly, the f₁ values for the copolymer 2a were almost constant at about 0.98, whereas those of the mixture of PdTPP and poly(IBM‐co‐TFEM) increased from 0.889 to 0.967 as the temperature was increased. This finding suggests that there are two distinct microheterogeneities, one temperature‐dependent and the other temperature‐independent, in the mixture of PdTPP and poly(IBM‐co‐TFEM). The dye‐conjugation approach effectively eliminates the temperature‐dependent, but not the temperature‐independent microheterogeneity. The luminescence decays of copolymers 2a and 2b and the corresponding mixtures in the absence of oxygen indicated that the temperature‐dependent microheterogeneity involves an oxygen diffusion process, whereas the temperature‐independent one appears to be inherent nature in PdTPP. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 663–670, 2010     

Unexpectedly stable artificial duplex from flexible acyclic threoninol

A new foldamer, acyclic threoninol nucleic acid (aTNA), has been synthesized by tethering each of the genetic nucleobases A, G, C, and T to d-threoninol molecules, which were then incorporated as building blocks into a scaffold bearing phosphodiester linkages. We found that with its fully complementary strand in an antiparallel fashion, the aTNA oligomer forms an exceptionally stable duplex that is far more stable than corresponding DNA or RNA duplexes, even though single-stranded aTNA is rather flexible and thus does not take a preorganized structure.     

High‐Density Liquid‐Crystalline Polymer Brushes Formed by Surface Segregation and Self‐Assembly

High‐density polymer brushes on substrates exhibit unique properties and functions stemming from the extended conformations due to the surface constraint. To date, such chain organizations have been mostly attained by synthetic strategies of surface‐initiated living polymerization. We show herein a new method to prepare a high‐density polymer brush architecture using surface segregation and self‐assembly of diblock copolymers containing a side‐chain liquid‐crystalline polymer (SCLCP). The surface segregation is attained from a film of an amorphous base polymer (polystyrene, PS) containing a minor amount of a SCLCP‐PS diblock copolymer upon annealing above the glass‐transition temperature. The polystyrene portion of the diblock copolymer can work as a laterally mobile anchor for the favorable self‐assembly on the polystyrene base film.