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61.
Tadahiro Wakui Johannes Smid 《Journal of inclusion phenomena and macrocyclic chemistry》1985,3(2):197-208
The binding constants,K
N, of sodium and potassium 8-anilinonaphthalene-1-sulfonate (ANS) and of sodium 5-dimethylamino-1-naphthalenesulfonate (DNS) to benzo-18-crown-6 bound to a 2% cross-linked polystyrene network (RN18C6) were measured spectrophotometrically in dioxane and the results compared with those obtained for picrate salts. The network RN18C6 was then used to measure in dioxane and toluene by a competition method the equilibrium constant,K, of the reaction A–M+N+CrA–M+Cr+N.A–M+N denotes the ionic solute (ANS, DNS, methyl orange or picrate salt) bound to the network RN18C6 (N) and A–M+Cr is the solute bound to a soluble ligand Cr, where Cr represents a series of 18-crown-6 and 15-crown-5 compounds. Combining theK
N andK values the formation constants,K
L, of the crown ether complexes of the respective salts were obtained in dioxane. The data show a reversal in the complexation strength of the 18-crown-6 compounds in dioxane when sodium picrate is replaced by sodium ANS. The results were rationalized in terms of a synergistic effect exerted by dioxane, with dioxane forming a 1:1 dioxanate with the crown ion pair complex. This effect is especially strong with ANS and with a rigid planar crown ether like dibenzo-18-crown-6. The binding constants,K
N, of NaANS and NaDNS to RN18C6 in dioxane are nearly three times larger than for sodium picrate, and the same holds for the potassium salts. Differences in anion interactions with the network appear to be a plausible cause for the anion dependence ofK
N. 相似文献
62.
The primary all-trans to 13-cis chromophore isomerization of the light driven chloride pump halorhodopsin has been studied by means of transient absorption spectroscopy in the visible and mid-infrared regime at a time resolution of better than 100 and 220 fs, respectively. The picosecond vibrational dynamics are dominated by two time constants, i.e., 2 and 7.7 ps in accordance with the biphasic decay of the retinal excited electronic state and electronic ground state formation with 1.5 and 6.6 ps. The transient vibrational spectra of the participating electronic states strongly suggest the existence of two distinct S1 populations as a result of an early branching reaction. It is shown that the 13-cis product is formed with the fast time constant, whereas the all-trans educt state is repopulated via both time constants. Concomitant protein dynamics are indicated by spectral changes on a similar time scale in the amide region. 相似文献
63.
Yano J Pushkar Y Glatzel P Lewis A Sauer K Messinger J Bergmann U Yachandra V 《Journal of the American Chemical Society》2005,127(43):14974-14975
The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2-3.5 A, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn-Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn-Mn distances between 2.7 and 2.8 A and, hence, the presence of three di-mu-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures. 相似文献
64.
Carstens MG van Nostrum CF Ramzi A Meeldijk JD Verrijk R de Leede LL Crommelin DJ Hennink WE 《Langmuir : the ACS journal of surfaces and colloids》2005,21(24):11446-11454
Methoxypoly(ethylene glycol)-b-oligo-L-lactate (mPEG-b-OLA) diblock oligomers with monodisperse OLA blocks were obtained by fractionation of polydisperse block oligomers using preparative HPLC. The fractionated oligomers were composed of an mPEG block with a molecular weight of 350, 550, or 750 and an OLA block with a degree of polymerization of 4, 6, 8, or 10. The diblock oligomers with a low PEG content were fully amorphous, with glass transition temperatures ranging from -60 to -20 degrees C, indicating that the blocks were miscible. Upon heating aqueous dispersions of the block oligomers, cloud points, depending on the PEG/OLA ratio of the block oligomer, were observed at temperatures above 40 degrees C. The monodispersity of the hydrophobic block enabled the amphiphilic molecules to form nanoparticles in water with a hydrodynamic radius of 130-300 nm, at concentrations above the critical aggregation concentration (0.4-1 mg/mL), whereas polydisperse mPEG-b-OLAs gave formation of large aggregates. Static light scattering measurements showed that the nanoparticles have a low density (0.6-25 mg/mL), indicating that the particles are highly hydrated. In agreement herewith, the (1)H NMR spectra of nanoparticles in D2O closely resembled spectra in a good solvent for both blocks (CDCl3). It is therefore suggested that the nanoparticles contain a hydrated core of mPEG-b-OLA block oligomers, stabilized by a thin outer PEG layer. The particles were stable for two weeks, except for the mPEG350 series and mPEG750-b-OLA4, indicating that both the PEG block size and the PEG weight fraction of the oligomers determine their stability. The evident self-emulsifying properties of mPEG-b-oligo-l-lactates with monodisperse hydrophobic blocks as demonstrated in this study, together with their expected biocompatibility and biodegradability, make these systems well suitable for pharmaceutical applications. 相似文献
65.
N,N-Dimethylformamide () and N,N-dimethylpivalamide () react with carbonyl chloride isocyanate () in the presence of antimony pentachloride to afford the amino substituted 1-oxa-3-azabutatrienium hexachloroantimonates ,. An X-ray diffraction analysis of confirms the proposed structure. The heterocumulenes , react with aldehydes, ketones and tertiary carboxamides to give the amino substituted 2-azaallenium salts - in high yields. 相似文献
66.
By the phase transfer catalyzed reaction of 9(10H)-acridinone with 1-bromo-2-propyne, 10-(2-propynyl)-9(10H)-acridinone is synthesized. As prototropic rearrangement products of this 10-(1,2-propadienyl)-9(10H)-acridinone and 10-(1-propynyl)-9(10H)-acridinone are obtained, Under the given conditins 1-bromo-2butyne leads to 10-(2-butynyl)-9(10H)-acridinone and 2-chloro-3-butyne leads to 10-(1-methyl-1,2-propddienyl)-9(10H)-acridinone, 10-(1-methyl-2-propynyl)-9(10H)-acridinone, 9-(1-methyl-2-propynyloxy)acridine and 10-[1-methyl-3-(3,4-dimethylphenyl-2-propynyl)]-9(10H)-acridinone. The formation of the products is experimentally confirmed and with published work compared. 相似文献
67.
Johannes Sjstrand 《International journal of quantum chemistry》1987,31(5):733-737
In this paper we outline briefly how microlocal analysis can be applied to give a general approach to the mathematical theory of resonances in the semiclassical limit. We also describe recent results about the asymptotic behavior of resonances generated by closed trajectories and stationary points in the classical flow. 相似文献
68.
Johannes Respondek 《Journal of mass spectrometry : JMS》1980,15(10):544-544
69.
Neugebauer J Louwerse MJ Belanzoni P Wesolowski TA Baerends EJ 《The Journal of chemical physics》2005,123(11):114101
In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle. 相似文献
70.