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151.
Detailed spectral simulations based on ab initio density functional theory computations of the amide I and II infrared (IR) and vibrational circular dichroism (VCD) spectra for Ac-(Ala)(4)-NH(2), Ac-(Aib-Ala)(2)-NH(2), and Ac-(Aib)(4)-NH(2) constrained to 3(10)- and alpha-helical conformations are presented. Parameters from these ab initio calculations are transferred onto corresponding larger oligopeptides to simulate the spectra for dodecamers. The differences between conformations and for different Aib substitution patterns within a conformation are reflected in observable spectral patterns where data are available. Simulated IR spectra show small frequency shifts in the amide I maxima between 3(10)- and alpha-helices, but the same magnitude shifts occur within one conformation upon Aib substitution. Thus, from a computational basis, the frequency of the amide I maximum does not discriminate between the 3(10)- or alpha-helical conformations. Calculated VCD band shapes for 3(10)-helices showed more significant changes in amplitude, with change in the fraction of Aib, than those for alpha-helices. Generally, with increasing Aib content, the overall amide I VCD intensity becomes weaker and the amide I couplet becomes more conservative, while the amide II VCD is less affected. Although the detailed band shape is shown to be sensitive to alpha-Me substitution, the basic pattern of amide I and II relative VCD intensities still differs between alpha- and 3(10)-helices and, as a consequence, successfully discriminates between them. These predictions are all borne out in experimental spectra of Aib, mixed Aib-Ala, and Ala-based helical peptides, where available. 相似文献
152.
Amperometric urea biosensor based on urease and electropolymerized toluidine blue dye as a pH-sensitive redox probe 总被引:4,自引:0,他引:4
Vostiar I Tkac J Sturdik E Gemeiner P 《Bioelectrochemistry (Amsterdam, Netherlands)》2002,56(1-2):113-115
The electropolymerized toluidine blue film deposited on the glassy carbon electrode show amperometrically detectable pH sensitivity. This feature of polytoluidine blue (PTOB) film was used for a construction of an amperometric urea biosensor. We have observed a linear shift of the formal redox potential with increasing pH value between 4 and 8 giving the slope of 81 mV(Delta) pH(-1). Polytoluidine blue film has had a significantly increased stability and higher electrochemical activity compared to the adsorbed monomeric dye. The polytoluidine blue urea biosensor has been operating at a working potential of -200 mV vs. SCE. The sensitivity of the biosensor was 980 nA mM(-1) cm(-2). The biosensor showed linearity in concentration range up to 0.8 mM with the detection limit of 0.02 mM (S/N=3). 相似文献
153.
Richard W.M. Ten Hoedt Gerard van Koten Jan G. Noltes 《Journal of organometallic chemistry》1980,201(1):327-342
Thermal decomposition of configurationally pure 1,2-diarylpropenylcopper compounds Z-Vi2CU4Br2 and Z-Vi2Cu4R2 [Vi (2-Me2NC6H4)CC(Me)-(C6H4Me-4), R 2-Me2NC6H4 or 4-MeC6H4CC] predominantly results in the formation of ViH. In contrast, only dimers (ViVi) were formed on thermolysis of (Z-ViCu2OTf)η which is a further illustration of the influence of the counter anion on the reactivity of organocopper cluster compounds. However, in both cases partial inversion of configuration, giving mixtures of isomers, was observed. The thermolysis of (Z-ViCu2OTf)η is discussed in terms of OTf-enhanced intraaggregate electron-transfer processes. The formation of incipient vinyl cations which are η2-coordinated to the copper cluster can explain the observed isomerization.Also in the hydrolysis reaction of Z-Vi2Cu4Br2 mixtures of isomeric ViH compounds were obtained, the E/Z ratio being dependent on the type of reagent used. Mixtures of isomeric ViX compounds (X Br, Cl, I) were formed in the reaction of Z-Vi2Cu4Br2 with I2 and CuCl2. An explanation for the occurrence of isomerization is presented.Dimers (ViVi) were almost absent in the product mixture resulting from the reaction of Z-Vi2Cu4Br2 with CuCl2. In contrast, ViVi is obtained in about 50% yield from the reaction of ViLi with CuCl2 which is in accord with earlier observations for the reaction of aryllithium compounds with cupric halides.Highly selective E-ViBr formation was observed in the reaction of E-ViLi with AgBr. This reaction probably proceeds via a thermally unstable Z-ViAg2Br intermediate. 相似文献
154.
Transfection mediated by gemini surfactants: engineered escape from the endosomal compartment 总被引:8,自引:0,他引:8
Bell PC Bergsma M Dolbnya IP Bras W Stuart MC Rowan AE Feiters MC Engberts JB 《Journal of the American Chemical Society》2003,125(6):1551-1558
The structure of the lipoplex formed from DNA and the sugar-based cationic gemini surfactant 1, which exhibits excellent transfection efficiency, has been investigated in the pH range 8.8-3.0 utilizing small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-TEM). Uniquely, three well-defined morphologies of the lipoplex were observed upon gradual acidification: a lamellar phase, a condensed lamellar phase, and an inverted hexagonal (H(II)) columnar phase. Using molecular modeling, we link the observed lipoplex morphologies and physical behavior to specific structural features in the individual surfactant, illuminating key factors in future surfactant design, viz., a spacer of six methylene groups, the presence of two nitrogens that can be protonated in the physiological pH range, two unsaturated alkyl tails, and hydrophilic sugar headgroups. Assuming that the mechanism of transfection by synthetic cationic surfactants involves endocytosis, we contend that the efficacy of gemini surfactant 1 as a gene delivery vehicle can be explained by the unprecedented observation of a pH-induced formation of the inverted hexagonal phase of the lipoplex in the endosomal pH range. This change in morphology leads to destabilization of the endosome through fusion of the lipoplex with the endosomal wall, resulting in release of DNA into the cytoplasm. 相似文献
155.
156.
Dissociative recombination of the weakly bound NO-dimer cation: cross sections and three-body dynamics 总被引:1,自引:0,他引:1
Petrignani A Andersson PU Pettersson JB Thomas RD Hellberg F Ehlerding A Larsson M van der Zande WJ 《The Journal of chemical physics》2005,123(19):194306
Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer. 相似文献
157.
Jan Najbar 《Chemical physics》1988,120(3):367-373
It is shown that the fluorescence quenching constant is given by the Laplace transform of the time-dependent bimolecular rate constant k(t). The dependence of the quenching rate constant on the transformation parameter can be obtained from measurements of the fluorescence decay functions and a Stern-Volmer-type analysis in the Laplace domain. The advantages of this type of investigation of the quenching rate function are demonstrated by a numerical calculation for the distance-dependentquenching process in 相似文献
158.
Appels NM van Maanen MJ Rosing H Schellens JH Beijnen JH 《Rapid communications in mass spectrometry : RCM》2005,19(15):2187-2192
Lonafarnib is a novel anticancer drug that inhibits farnesyl transferase. To assess its pharmacokinetic properties, we developed a sensitive and quantitative assay using liquid chromatography coupled with tandem mass spectrometry for the determination of lonafarnib levels in human plasma. Sample pretreatment consisted of the addition of an isotopically labeled internal standard and protein precipitation with acetonitrile using 100 microL plasma. Chromatographic separation was performed on an Inertsil ODS-3 analytical column (50 x 2.1 mm i.d., particle size 5 microm) with acetonitrile/water/formic acid (50:50:0.05, v/v/v) as the mobile phase, at a flow rate of 0.2 mL/min. The analytical run time was 8 min. An API365 triple quadrupole mass spectrometer was used for specific and sensitive detection. It was operated in the positive ion mode and multiple reaction monitoring was used for drug quantification. The method was validated using a concentration range of 2.5 to 2500 ng/mL lonafarnib. Validation of the assay was performed according to the most recent FDA guidelines for bioanalytical method validation and all results were within the requirements. The described method was successfully applied to support a clinical phase I trial with lonafarnib. 相似文献
159.
Janusz Wasilewski Jan F. Biernat 《Journal of inclusion phenomena and macrocyclic chemistry》1991,10(1):109-118
Lipophilic bis-substituted ester and ether derivatives of benzo-15-crown-5 have been synthesised. The correlation between the structure and potentiometric ion-selectivity has been studied in PVC membrane ion-selective electrodes. An ion-selective potassium sensitive electrode based on 4,5-bis (biphenyloxymethyl)benzo-15-crown-5 exhibited the best electrode properties. The detection limit was loga
K = -5.4; logK
K,Na
ppot = -3.5. The effect of the lipophilicity of neutral carriers upon electrode performance has been also discussed. 相似文献
160.
Homoallylic ethers react with Cp2ZrCl2/2 n-BuLi reagent to afford cyclopropane derivatives. Cyclopropylcarbinyl-homoallyl rearrangements involving zirconium species are observed depending on the substrate structure. 相似文献