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31.
Samuel González-Mancebo Javier Lacadena Yolanda García-Alonso Jesús Hernández-Benito Emilio Calle Julio Casado 《Monatshefte für Chemie / Chemical Monthly》2002,133(2):157-166
Summary. Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of
the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction
was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO+/NO2H2
+ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron
donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly
by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant.
Received July 13, 2001. Accepted (revised) September 18, 2001 相似文献
32.
Lynch CL Hale JJ Budhu RJ Gentry AL Finke PE Caldwell CG Mills SG MacCoss M Shen DM Chapman KT Malkowitz L Springer MS Gould SL DeMartino JA Siciliano SJ Cascieri MA Carella A Carver G Holmes K Schleif WA Danzeisen R Hazuda D Kessler J Lineberger J Miller M Emini E 《Organic letters》2003,5(14):2473-2475
[reaction: see text] A novel approach to alpha,alpha-disubstituted-beta-amino acids (beta(2,2)-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1. 相似文献
33.
2-Mercapthoethanol undergoes carbonylation in pyridine solution in the presence of oxygen and of [Ni(CO)3Pyl as a catalyst to give cyclic O, S-ethylelnethiorcarbonate. The isolation of thiolatonickel compound [Ni(SCH2CH2OH)2] by the oxidation of a solution containing [Ni(CO)3Py] and 2-mercaptoethanol, and its reaction with carbon monoxide to give the cyclic thiocarbonate and [Ni(CO)3Py] prove that the reaction proceeds in at least two steps. 相似文献
34.
Emilio Santos 《International Journal of Theoretical Physics》1992,31(10):1909-1913
The problem of hidden variables in quantum mechanics is formalized as follows. A general or contextual (noncontextual) hidden-variables theory is defined as a mappingf: Q×M C (f: QC) whereQ is the set of projection operators in the appropriate (quantum) Hilbert space,M is the set of maximal Boolean subalgebras ofQ andC is a (classical) Boolean algebra. It is shown that contextual (noncontextual) hidden-variables always exist (do not exist). 相似文献
35.
Mineo P Vitalini D Scamporrino E Bazzano S Alicata R 《Rapid communications in mass spectrometry : RCM》2005,19(19):2773-2779
The dependence of the calculated average molecular mass of a polyethylene glycol with a large polydispersity on the instrumental parameters adopted in the acquisition of mass spectra using delayed extraction matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (DE MALDI-TOFMS) was investigated. It has been shown that a combined effect of delay times and potential gradients can act on the ion cloud in the source chamber affecting both mass resolution and average molecular mass value of the analyzed polymeric sample. Also examined was a blend of two different polymers (a PEG and a PMMA commercial sample having a similar average molecular mass), which presents an additional problem concerning the discrimination among the different polymer species as a function of the experimental conditions. In this work, the best instrumental conditions to obtain both good resolution and a correct average molecular mass for the examined polydisperse sample are reported. 相似文献
36.
García-Santos Mdel P González-Mancebo S Hernández-Benito J Calle E Casado J 《Journal of the American Chemical Society》2002,124(10):2177-2182
Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process. 相似文献
37.
Elena Junquera Emilio Aicart 《Journal of inclusion phenomena and macrocyclic chemistry》1997,29(2):119-136
The encapsulation of the acid/base conjugated system salicylic acid(HSA)/salicylate (SA-) by hydroxypropyl--cyclodextrin (HPBCD) has beenstudied through fluorescence emission enhancement measurements in aqueoussolutions at 25 °C. With the aim of analyzing the crucial importance ofa proper and cautious choice of the pH of the medium (i.e. choice of thebuffer), the study has been carried out at pH = 1, 2, 4 and 7. Since the pKa of the HSA/SA- system is 2.95 at 25 °C, the presence of the protonated(HSA) and non-protonated (SA-) forms suitable for inclusion by cyclodextrinvary appreciably within the different pH conditions: 99% HSA,1% SA- at pH = 1, 90% HSA, 10% SA- at pH = 2,10% HSA, 90% SA- at pH = 4, and 0% HSA,100% SA- at pH =7. The association constants K CD : HSA and K CD: SA- have been determined in all cases by using a nonlinear regressionanalysis of the experimental data at three different em. The effectof the pH of the medium on all the equilibria involved as well as in the Kvalues is fully discussed. The 8-anilino-1-naphthalene sulfonate (ANS) +-cyclodextrin (-CD) system, widely reported in literature, hasinitially been studied to check the experimental protocol and the numericalmethod. 相似文献
38.
Dr. Emilio Brunetti Dr. Lionel Marcelis Dr. Fedor E. Zhurkin Prof. Michel Luhmer Prof. Ivan Jabin Prof. Olivia Reinaud Prof. Kristin Bartik 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(55):13730-13738
In this study, the ligand exchange mechanism at a biomimetic ZnII centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate ZnII intermediates, and inversion of the pyramid at the ZnII centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of ZnII in proteins. 相似文献
39.
Dr. Manuel Vázquez Sulleiro Ramiro Quirós-Ovies Mariano Vera-Hidalgo Dr. I. Jénnifer Gómez Dr. Víctor Sebastián Prof. Jesús Santamaría Prof. Emilio M. Pérez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(9):2993-2996
The combination of 2D materials opens a wide range of possibilities to create new-generation structures with multiple applications. Covalently cross-linked approaches are a ground-breaking strategy for the formation of homo or heterostructures made by design. However, the covalent assembly of transition metal dichalcogenides flakes is relatively underexplored. Here, a simple covalent cross-linking method to build 2H-MoS2–MoS2 homostructures is described, using commercially available bismaleimides. These assemblies are mainly connected vertically, basal plane to basal plane, creating specific molecular sized spaces between MoS2 sheets. Therefore, this straightforward approach gives access to the controlled connection of sulfide-based 2D materials. 相似文献
40.