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1.
One hundred fifty-seven nm photodissociation of singly-charged peptide ions induces the cleavage of alpha-carbon to carbonyl-carbon bonds along the backbone. a(n) + 1 radical ions are observed as the primary photolysis products of peptides with N-terminal arginines in a linear ion trap mass spectrometer. The radical elimination pathways undertaken by the a(n) + 1 radical ions to form more stable even-electron species are studied in hydrogen-deuterium (H/D) exchange experiments. Two types of a(n) ions along with d-type ions are observed as secondary elimination products. The relative abundance of each depends on the C-terminal residue of the radical fragment ion.  相似文献   

2.
Calculated reaction energies, NBO analyses, and natural charge distributions show that a trimethylsilyl group at the β carbon atom stabilizes the positive charge of an iminium cation more effectively than a tert-butyl group in the same position. Different from the classical β-silicon effect, where theoretical studies suggest that the positive charge is stabilized hyperconjugatively by interaction of the Si-C σ-bond with a formally empty 2p-orbital of suitable symmetry at the carbonium carbon, the stabilization in the case of iminium ions occurs by interaction of the Si-C σ-bond with the π orbital of the CN double bond. Stabilization of the silyl-substituted derivatives by hyperconjugation might be enhanced by interaction of the positive charge with a neighboring induced dipole.  相似文献   

3.
Summary We confirmed the inadequacy of the correction for, binding of water by ions for explaining the discrepancy between the electrostatic theory in [2] and experiment.  相似文献   

4.
Field desorption mass spectrometry exhibited novel ions [ Y-n H+n R ]+and [ Y-nH+nX]+as well as cluster ions [nY+(n-1)X-mH+mR]+and [nY+( n-1)X-mH+mX]+(n,m=0-3; X and Y represent the cation and anion moieties of quaternary ammonium halides),whose formation mechanism was discussed.  相似文献   

5.
Electron-capture induced dissociation of protoporphyrin cations and anions has been studied. The cations captured two electrons in two successive collisions and were converted to the corresponding even-electron anions. About one fifth of the ions lost a hydrogen atom to become radical anions but otherwise very little fragmentation was observed. The anions captured an electron to become dianions. No hydrogen loss occurred, and the only fragmentation channel observed was loss of CO2H, to give a doubly charged carbanion. Our results indicate that protoporphyrin ions are very efficient in accommodating one or even two electrons in the lowest unoccupied molecular orbital of the porphyrin macrocycle, and that electron capture induces only limited dissociation.  相似文献   

6.
Summary NMR and potentiometric methods revealed at least two types of interactions of Na+ and K+ ions with -carrageenan, viz., Coulombic interaction with polysaccharide sulfate groups, and a coordination one, leading, in the case of the Na+ ions, to formation of nonstoichiometric complexes. The absence of any correlation between the coordination binding density of the cations and their promoting effect on gelation process was demonstrated.  相似文献   

7.
The slow coagulation of lyophobic sols by electrolytes that may reduce the surface potential by precipitating, chelating or neutralising the adsorbed peptising ion, is described; and the experimental results are explained by an extension of the earlier relations derived from theVerwey-Overbeek theory of stability. Low concentrations of electrolyte—up to ten per cent of the final coagulation value—mainly reduce the stability of dialysed sols by compression of the double layer, while further addition leads to a rapid fall in stability due to reduction of the surface potential. The coagulation of undialysed sols is mainly caused by reduction of surface potential at all concentrations of electrolyte. The coagulation of undialysed sols by precipitating, etc. electrolytes may be equated to the removal of a corresponding amount of the adsorbed peptising ions; and the final coagulation value (for rapid coagulation) then approximately measures the total peptising ion concentration.  相似文献   

8.
The basicity of so-called superbases is ascribed to the presence of free, i.e., naked, hydroxide ions. Infrared spectroscopic studies of various KOH (and KOCH3) crown ether dicyclohexyl-18-crown-6 and dibenzo-18-crown-6) and Sr(OH)2 cryptand(222) solutions in polar (methanol, chloroform) and non-polar solvents (toluene, n-heptane) revealed that unsolvated OH ions are not present in such strong basic solutions. Formation of potassium crown ether complexes shown by the i.r. bands at 1095–1102 and 990–995 cm−1 (dicyclohexyl-18-crown-6) is always accompanied (and only possible if) by that of methoxide ions (i.r. band at 1050–1060 cm−1). The presence of solvated OH ions cannot be excluded.  相似文献   

9.
[structure: see text] Quantum chemical calculations (B3LYP and Moeller-Plesset second-order perturbation theory) on carbonium ions containing hypercoordinated carbon atoms in distorted square-pyramidal geometries are described. The importance of overall charge (in addition to the delocalization of a certain number of electrons) for the stability of such structures is explored through calculations on isoelectronic systems containing boron atoms. The effects of alkyl substitution as well as variations in the structure of the linker attached to the C(5) core substructure are explored systematically for a variety of structures that do not have severe geometric constraints that rigidly enforce unusual geometries. In addition, transition structures for reactions involving the intramolecular attack of tetracoordinate carbons on carbenium ion centers were located; in some cases, such carbenium-to-carbonium rearrangement processes (in which tetracoordinate carbon centers act as nucleophiles) are actually both exothermic and accompanied by low activation barriers.  相似文献   

10.
SR-method has been used for studying reactions of muonium as an analogue of a light hydrogen isotope with impurity atoms and point-like defects. The relaxation rate of muonium polarization Mu was measured for 11 samples with different concentrations of impurity atoms and defects. The rate constants of reactions of muonium with Fe, Al, Li, Ba, Cu, Mn, Ti and Pb atoms in quartz were defined.  相似文献   

11.
Summary The chelation behaviour ofbisazo--diketone compounds with La, Ce, Nd, Sm, Gd, Er, and Lu ions was investigated. Composition and stability constants of the chelates have been determined by conductometric and potentiometric techniques. Electronic absorption, IR, and1H NMR spectra as well as molar conductance and elemental and thermal analyses were used to characterize the complexes.
Komplexierung einiger Lanthaniden mitBisazophenyl--diketon-Derivaten
Zusammenfassung Das Komplexierungsverhalten einigerBisazophenyl--diketone mit La, Ce, Nd, Sm, Gd, Er und Lu wurde untersucht. Zusammensetzung und Stabilitätskonstanten der Chelate wurden mit konduktometrischen und potentiometrischen Methoden bestimmt. Zur Charakterisierung der Komplexe wurden UV/Vis-, IR- und1H-NMR-Spektroskopie sowie molare Leitfähigkeit, Elementaranalyse und thermische Analyse herangezogen.
  相似文献   

12.
13.
Time_of_flightmassspectrometer(TOFMS)hasbeenfindinganincreasinglywidevarietyofapplications.Itssensitivity,highanalysisspeed,andlargemassrangemakeitoneofthemostfavorablemassspectrometertechnique.However,thekeyfactorfortheTOFMSisthepulsedionizorthatrestri…  相似文献   

14.
A method has been established to study the competing binding of metal ions with protein by a combined technique of microdialysis with high performance liquid chromatography (HPLC). Ni2+, Cd2+, Zn2+, Cu2+ and human serum albumin (HSA) were chosen as model metal ions and protein. The experimental results show that Ni2+ and Cu2+ share a common primary binding site on HSA, and Zn2+ and Cd2+ share a different common primary binding site from them, but there is a common multi-metal binding site for all of those four metal ions. This method show advantages of fast sampling, easily to be operated and especially to be useful when ideal spectroscopic probes are not available for the study of interaction between protein and metal ions.  相似文献   

15.
ICP-MS is used as an element selective detector for chromium species in aquatic samples using a coupled columm system consisting of a cation guard column and an anion column. The effects of large concentrations of disturbing ions, like NO(3)(-), Cl(-), CO(3)(2-) and SO(4)(2-), are discussed and the comparison with the use of different chromium isotopes for data acquisition is given. Quite low concentrations such as 100 mg/l of CO(3)(2-), 10 mg/l Cl(-) and even 5 mg/l SO(4)(2-) can cause an overestimation of chromium if not properly resolved1995 / Accepted: 24 October 1995  相似文献   

16.
A new approach of polarographlc immunoassay based on the catalytic amplification of the labeled metal ions and the polarographlc detection of the catalytic product was developed.In this approach,the copper ions used as the catalyst for substrate conversion instead of natural enzyme were labeled to model antigen diphtheria toxoid (DT) through the bifunctional chelating reagent diethylenetriamine pentaacetic acid (DTPA).After heterogeneous competitive imrnunoreaction,the oxidation of substrate o-phenylenediatnine (OPD) was catalyzed by the labeled copper ions to generate an electroactive product 2,3-diammophenazine (DAP); subsequently,the product DAP was detected with linear-sweep polarography The proposed assay can determine the concentration in the range of 10-1 000 ng/mL of DT,two orders of magnitude more sensitive than those based on the direct detection of the metal ion labels The proposed immunoassay can be applied to detecting various proteins of interest.  相似文献   

17.
A microfluidic device with integrated electrodes for the electrochemically-modulated extraction of ions across immiscible aqueous–organic liquid–liquid interfaces is presented. Using a Y-shaped microfluidic channel with in situ electrodes and co-flowing aqueous and organic immiscible electrolyte solutions, the manipulation of the applied interfacial potential enabled the extraction of ions from the aqueous phase into the organic phase. Data for the extraction of tetraethylammonium cations from aqueous electrolyte into 1,2-dichloroethane electrolyte are presented. The device demonstrates the benefits of combination of microfluidics and liquid–liquid electrochemistry.  相似文献   

18.
The highly crystalline β-chitin from diatom Thalassiosira weissflogii was esterified via intercalation with succinic anhydride followed by simple heating, maintaining the original crystalline order. Due to the introduced free carboxyl groups, the chitin ester crystal showed ion exchange ability for metal cations in aq. solution. Heavy metal cations such as Pb2+ bound to the β-chitin succinate gave characteristic X-ray diffraction patterns, indicating regular alignment of metal ions. Such materials represent a new type of organometallic architecture, possibly leading to novel functionalities.  相似文献   

19.
Four novel organometallic compounds containing tin(Ⅳ),titanium(Ⅳ) and zirco-nium(Ⅳ) ions were synthesized and strong fluorescent emission was observed from two tin(Ⅳ) complexes.  相似文献   

20.
Ettringite was prepared in the presence of various dosages of CrO 4 2? ions at three different temperatures. X-ray diffraction (XRD) and differential scanning calorimetery (DSC) results showed shift in the peaks characterized to ettringite as the result of formation of Cr-substituted ettringite. Substitution of SO 4 2? by CrO 4 2? ions in ettringite crystal resulted in delay in the rate of its formation, as well as reduction in the crystal diameters. However, the crystal diameters and rate of formation of both ettringite and Cr-substituted ettringite were increased by increasing temperature of preparation, and this was confirmed by scanning electron microscope (SEM) micrographs. The thermodynamic parameters: the enthalpy, entropy, and Gibbs free energy changes for substitution process were studied. It is concluded that the removal of chromate ions by ettringite is an endothermic spontaneous process. The positive value of Δ suggests increasing in randomness during the removal process.  相似文献   

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