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1.
Stephen P. Waters 《Tetrahedron letters》2006,47(30):5409-5413
A mild and efficient [3+2] nitrile oxide/olefin cycloaddition provided a rapid and convergent entry into precursors of bisphenolic spiroketals, a structural type unique to the rubromycin family of natural products. In addition, implementation of the premise that a hydrogen bond from the C4-OH controls the stereochemistry of the purpuromycin core resulted in moderate diastereocontrol in the spiroketalization. Spectroscopic and X-ray data of these systems have provided the first assignment of the relative configuration of purpuromycin. 相似文献
2.
Let
d−1{(x1,…,xd)
d:x21+···+x2d=1} be the unit sphere of the d-dimensional Euclidean space
d. For r>0, we denote by Brp (1p∞) the class of functions f on
d−1 representable in the formwhere dσ(y) denotes the usual Lebesgue measure on
d−1,
and Pλk(t) is the ultraspherical polynomial.For 1p,q∞, the Kolmogorov N-width of Brp in Lq(
d−1) is given bythe left-most infimum being taken over all N-dimensional subspaces XN of Lq(
d−1).The main result in this paper is that for r2(d−1)2,where ANBN means that there exists a positive constant C, independent of N, such that C−1ANBNCAN.This extends the well-known Kashin theorem on the asymptotic order of the Kolmogorov widths of the Sobolev class of the periodic functions. 相似文献
3.
4.
Gavin D. Henry Author Vitae 《Tetrahedron》2004,60(29):6043-6061
5.
We couple a morphological study of an immiscible binary AB mixture with a micromechanical simulation to determine how the spatial distribution of the A and B domains and the interfacial region (interphase) affects the mechanical behavior of the blend. The morphological studies are conducted through a three-dimensional Cahn-Hilliard (CH) simulation. Through the CH calculations, we obtain the size and structure of the domains for different blend compositions. The output of the CH model serves as the input to the Lattice Spring Model (LSM), which consists of a three-dimensional network of springs. In particular, the location of the different phases is mapped onto the LSM lattice and the appropriate force constants are assigned to the LSM sites. A stress is applied to the LSM lattice and we calculate the elastic response of the material. We find that the local stress and strain fields are highly dependent on the morphology of the system. By integrating the morphological and mechanical models, we can isolate how modifications in the composition of the mixture affect the macroscopic behavior. Thus, we can establish how choices made in the components affect the ultimate performance of the material. 相似文献
6.
Ammar R Ball RC Banerjee S Bhat PC Bosetti P Bromberg C Canough GE Coffin T Dershem TO Dixon RL Fenker HC Ganguli SN Gensch U Girtler P Goshaw AT Grard F Gurtu A Hamilton C Henri VP Hernandez JJ Hrubec J Iori M Jones LW Kuhn D Knauss D Leedom ID Legros P Lemonne J Leutz H Liu X Malhotra PK Marraffino JM Mendez GE Miller R Naumann T Nguyen A Nowak H Pilette P Poirier J Poppleton A Raghavan R Rasner K Reucroft S Robertson WJ Roe BP Roth A Senko M Struczinski W Subramanian A Touboul MC Vonck B 《Physical review letters》1988,61(19):2185-2188
7.
Using flow microcalorimetry, the ion association reaction M2+(aq)+Fe(CN)
6
4–
(aq)=MFe(CN)
6
2–
(aq) (M=Ca, Mg) has been studied at 25°C over the ionic strength range 0.02 to 0.08 mol-dm–3. Analyses of the data to obtain Ho, the enthalpy change at infinite dilution, are described. The value obtained for Ho is sensitive to the kind of functions used to correct for non-ideal behavior. 相似文献
8.
A designed 12-residue beta-hairpin peptide with a diagonal tryptophan (Trp) pair was shown to bind ATP in water through a combination of aromatic and electrostatic interactions. The affinity for ATP was 5800 M-1 (DeltaG approximately -5.0 kcal/mol), a remarkable affinity for a short, structured peptide in water, consisting of entirely natural amino acid residues. Proton NMR measurements indicate that the adenine ring of the nucleotide is intercalated between the diagonal tryptophans in the bound state. Delineation of the contributions to ATP binding to the hairpin suggest that aromatic interactions contribute approximately -1.8 kcal/mol, while individual electrostatic interactions involving the ATP phosphates and positively charged side chains of the hairpin contribute approximately -1 kcal/mol each. The designed beta-hairpin receptor presents a novel minimalist system to investigate the energetic contributions to protein-nucleic acid recognition through the surface of a beta-sheet. 相似文献
9.
Heterobinuclear oxometalate anions based upon [CrMoO7]2-, [CrWO7]2-, and [MoWO7]2- were generated and transferred to the gas phase by the electrospray process from acetonitrile solutions containing two of the salts (Bu4N)2[MO4] (M = Cr, Mo, W). Their reactivities were examined and compared with those of the related homobinuclear anions based upon [M2O7]2- (M = Cr, Mo, W). Particular emphasis was placed upon reactions relevant to gas-phase catalytic cycles described previously for oxidation of alcohols by [Mo2O6(OH)]- (Waters, T.; O'Hair, R. A. J.; Wedd, A. G. J. Am. Chem. Soc. 2003, 125, 3384-3396). The protonated anions [MM'O6(OH)]- each reacted with methanol with loss of water to form [MM'O6(OCH3)]- at a rate that was intermediate between those of [M2O6(OH)]- and [M'2O6(OH)]-. The butylated anions [MM'O6(OBu)]- were generated by collisional activation of the ion-pairs {Bu4N+ [MM'O7]2-}-. Collisional activation of [MM'O6(OBu)]- resulted in either the loss of butanal (redox reaction) or the loss of butene (elimination reaction), with the detailed nature of the observations depending on the nature of both M and M'. Selective 18O labeling indicated that the butoxo ligands of [CrMoO6(OBu)]- and [CrWO6(OBu)]- were located on molybdenum and tungsten, respectively. This structural insight allowed a more detailed comparison of reactivity with the homobinuclear species, and highlighted the importance of the neighboring metal center in these reactions. 相似文献
10.
Kayser V Turton DA Aggeli A Beevers A Reid GD Beddard GS 《Journal of the American Chemical Society》2004,126(1):336-343
Energy migration between tryptophan residues has been experimentally demonstrated in self-assembled peptide tapes. Each peptide contains 11 amino acids with a Trp at position 6. The peptide self-assembly is pH-sensitive and forms amphiphilic tapes, which further stack in ribbons (double tapes) and fibrils in water depending on the concentration. Fluorescence spectra, quenching, and anisotropy experiments showed that when the pH is lowered from 9 to 2, the peptide self-assembly buries the tryptophan in a hydrophobic and restricted environment in the interior of stable ribbons as expected on the basis of the peptide design. These fluorescence data support directly and for the first time the presence of such ribbons which are characterized by a highly packed and stable hydrophobic interior. In common with Trp in many proteins, fluorescence lifetimes are nonexponential, but the average lifetime is shorter at low pH, possibly due to quenching with neighboring Phe residues. Unexpectedly, time-resolved fluorescence anisotropy does not change significantly with self-assembly when in water. In highly viscous sucrose-water mixtures, the anisotropy decay at low pH was largely unchanged compared to that in water, whereas at high pH, the anisotropy decay increased significantly. We concluded that depolarization at low pH was not due to rotational diffusion but mainly due to energy migration between adjacent tryptophan residues. This was supported by a master equation kinetic model of Trp-Trp energy migration, which showed that the simulated and experimental results are in good agreement, although on average only three Trp residues were visited before emission. 相似文献