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51.
D-Glyceraldehyde acetonide has been used as the starting point for accessing the enantiomeric cyclobutanols 11 in optically pure condition. The dextrorotatory enantiomer has been transformed in five steps into the [3.2.0] bicyclic lactone 22. While the deoxygenation of 22 proved to be problematical, the uncyclized variant 25 underwent the Barton process smoothly. These findings guided the related conversion of (-)-11 into 34. Use was also made of ring-closing metathesis to bring about the conversion of (+)-11 into [4.2.0] bicyclic lactone building blocks. In general, all three pathways are efficient and offer the prospect of practical side-chain appendage for the purpose of installing the nine-membered ring of pestalotiopsin A (1). 相似文献
52.
Maki T Kidoaki S Usui K Suzuki H Ito M Ito F Hayashizaki Y Matsuda T 《Langmuir : the ACS journal of surfaces and colloids》2007,23(5):2668-2673
To characterize the molecular basis of specific interactions of PDZ proteins, dynamic force spectroscopy (DFS) for the PDZ protein Tax-interacting protein-1 (TIP-1) and its recognition peptide (PDZ-pep) derived from beta-catenin was performed using an atomic force microscope (AFM), together with measurement of thermodynamic and kinetic parameters using surface plasmon resonance (SPR). The unbinding force of this pair was measured under different conditions of AFM tip-retraction velocity. The relationship between the unbinding force and the logarithmic force-loading rate, that is, the dynamic force spectrum, exhibited two different rate regimes, for each of which the forces increased linearly with the force-loading rate. On the basis of the theoretical treatment of the Bell-Evans model, the positions of two different activation barriers in the reaction coordinate and dissociation rate constants in each barrier were evaluated from slopes and x-intercepts of the two linear regimes (first barrier: 0.04 nm and 1.10 x 10 s(-1); second barrier: 0.21 nm and 2.77 x 10(-2) s(-1), respectively). Although two-step unbinding kinetics between TIP-1 and PDZ-pep was suggested from the DFS analysis, SPR results showed single-step dissociation kinetics with a rate constant of 2.89 x 10(-1) s(-1). Different shapes of the free energy profile of the unbinding process were deduced from each result of DFS and SPR. The reason for such topographic differences in the energy landscape is discussed in relation to the differences in the pathways of forced unbinding and spontaneous dissociation. 相似文献
53.
Comprehensive Evaluation of the Physicochemical Properties of LnIII Complexes of Aminoethyl‐DO3A as pH‐Responsive T1‐MRI Contrast Agents
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Dr. Zsolt Baranyai Dr. Gabriele A. Rolla Dr. Roberto Negri Attila Forgács Prof. Giovanni B. Giovenzana Dr. Lorenzo Tei 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(10):2933-2944
N‐Substituted aminoethyl groups were attached to 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid (DO3A) with the aim to design pH‐responsive LnIII complexes based on the pH‐dependent on/off ligation of the amine nitrogen to the metal ion. The following ligands were synthesized: AE ‐ DO3A (aminoethyl‐DO3A), MAE ‐ DO3A (N‐methylaminoethyl‐DO3A), DMAE ‐ DO3A (N,N‐dimethylaminoethyl‐DO3A) and MEM ‐ AE ‐ DO3A (N‐methoxyethyl‐N‐methylaminoethyl‐DO3A). The physicochemical properties of the LnIII complexes were investigated for the evaluation of their potential applicability as magnetic resonance imaging (MRI) contrast agents. In particular, a 1H and 17O NMR relaxometric study was carried out for these GdIII complexes at two different pH values: at basic pH (pendant amino group coordinated to the metal centre) and at acidic pH (protonated amine, not interacting with the metal ion). EuIII complexes allow one to estimate the number of inner‐sphere water molecules through luminescence lifetime measurements and obtain some structural information through variable‐temperature (VT) high‐resolution 1H NMR studies. Equilibria between differently hydrated species were found for most of the complexes at both acidic and basic pH. The thermodynamic stability of CaII, ZnII, CuII and LnIII complexes and kinetics of formation and dissociation reactions of LnIII complexes of AE ‐ DO3A and DMAE ‐ DO3A were investigated showing stabilities comparable to currently approved GdIII‐based CAs. In detail, higher total basicity (Σlog KiH) and higher stability constants of LnIII complexes were found for AE ‐ DO3A with respect to DMAE ‐ DO3A (i.e., log KGd‐ AE‐DO3A =22.40 and log KGd‐ DMAE‐DO3A =20.56). The transmetallation reactions of GdIII complexes are very slow (Gd‐ AE ‐ DO3A : t1/2=2.7×104 h; Gd‐ DMAE ‐ DO3A : 1.1×105 h at pH 7.4 and 298 K) and occur through proton‐assisted dissociation. 相似文献
54.
Baranyai Z Botta M Fekete M Giovenzana GB Negri R Tei L Platas-Iglesias C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(25):7680-7685
OBETA, OBETA, you bet: Thermodynamic and kinetic measurements show an apparent paradox. The stability of complexes of lanthanide trivalent ions is higher with the heptadentate ligand OBETA (ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) than with its octadentate homologue EGTA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid). The unusual properties of Gd(OBETA)(-) (see structure), combined with the presence of two fast exchanging coordinated water molecules, candidates this complex as an MRI contrast agent. 相似文献
55.
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57.
Ryo Kanzaki Yasuhiro Umebayashi Tei Maki Shin-ichi Ishiguro 《Journal of solution chemistry》2004,33(6-7):699-709
The distribution of 1,10-phenanthroline (phen) in micellar solutions of the nonionic surfactants Triton X and C12E n with varying poly (ethylene oxide) chain lengths has been studied by potentiometry, calorimetry, and fluorometry at 298 K. Micelles accommodate 1,10-phenanthroline according to the reaction, phen + Ym = Ym(phen), where Ym denotes a surfactant molecule aggregated in micelles. The constant K m for the reaction of Triton X increases as a linear function of n*, the number of ethylene oxide (EO) groups, as K m = KEO n* + Kc. Nonzero K EO and K c values suggest a heterogeneous inner structure of the micelle, i.e., the hydrophobic core surrounded by a hydrophilic poly (ethylene oxide) (PEO) shell. On the basis of molar volumes, the intrinsic thermodynamic parameters of transfer of 1,10-phenantholine were extracted. The enthalpy and entropy of transfer of 1,10-phenanthroline from the PEO shell to the core are found to be small and negative. By using K EO and K c values for C12E n obtained by fluorometry, individual fluorescence spectra of 1,10-phenanthroline in the PEO shell and core were extracted. The fluorescence intensity of 1,10-phenathroline accommodated in the core, like in organic solvents, is significantly reduced relative to that in water. These facts indicate that the aromatic rings of 1,10-phenanthroline penetrate into the hydrophobic core, while its hydrophilic N site is still hydrated in the PEO shell. 相似文献
58.
Loredana Leone Mariangela Boccalon Giuseppe Ferrauto Istvn Fbin Zsolt Baranyai Lorenzo Tei 《Chemical science》2020,11(30):7829
A current challenge in medical diagnostics is how to obtain high MRI relaxation enhancement using GdIII-based contrast agents (CAs) containing the minimum concentration of GdIII ions. We report that in GdHPDO3A-like complexes a primary amide group located in close proximity to the coordinated hydroxyl group can provide a strong relaxivity enhancement at slightly acidic pH. A maximum relaxivity of r1 = 9.8 mM−1 s−1 (20 MHz, 298 K) at acidic pH was achieved, which is more than double that of clinically approved MRI contrast agents under identical conditions. This effect was found to strongly depend on the number of amide protons, i.e. it decreases with a secondary amide group and almost completely vanishes with a tertiary amide. This relaxivity enhancement is attributed to an acid-catalyzed proton exchange process between the metal-coordinated OH group, the amide protons and second sphere water molecules. The mechanism and kinetics of the corresponding H+ assisted exchange process are discussed in detail and a novel simultaneous double-site proton exchange mechanism is proposed. Furthermore, 1H and 17O NMR relaxometry, Chemical Exchange Saturation Transfer (CEST) on the corresponding EuIII complexes, and thermodynamic and kinetic studies are reported. These highlight the optimal physico-chemical properties required to achieve high relaxivity with this series of GdIII-complexes. Thus, proton exchange provides an important opportunity to enhance the relaxivity of contrast agents, providing that labile protons close to the paramagnetic center can contribute.A novel GdHPDO3A-like complex featuring primary amide side chain induces extraordinary high relaxivity by virtue of a simultaneous double-site proton exchange mechanism under slight acidic conditions. 相似文献
59.
Tei L Arca M Aragoni MC Bencini A Blake AJ Caltagirone C Devillanova FA Fornasari P Garau A Isaia F Lippolis V Schröder M Teat SJ Valtancoli B 《Inorganic chemistry》2003,42(26):8690-8701
We have prepared and characterized a new phenol-based compartmental ligand (H(2)L) incorporating 1,4,7-triazacyclononane ([9]aneN(3)), and we have investigated its coordination behavior with Cu(II), Zn(II), Cd(II), and Pb(II). The protonation constants of the ligand and the thermodynamic stabilities of the 1:1 and 2:1 (metal/ligand) complexes with these metal ions have been investigated by means of potentiometric measurements in aqueous solutions. The mononuclear [M(L)] complexes show remarkably high stability suggesting that, along with the large number of nitrogen donors available for metal binding, deprotonated phenolic functions are also involved in binding the metal ion. The mononuclear complexes [M(L)] show a marked tendency to add a second metal ion to afford binuclear species. The formation of complexes [M(2)(H(2)L)](4+) occurs at neutral or slightly acidic pH and is generally followed by metal-assisted deprotonation of the phenolic groups to give [M(2)(HL)](3+) and [M(2)(L)](2+) in weakly basic solutions. The complexation properties of H(2)L have also been investigated in the solid state. Crystals suitable for X-ray structural analysis were obtained for the binuclear complexes [Cu(2)(L)](BF(4))(2).(1)/(2)MeCN (1), [Zn(2)(HL)](ClO(4))(3).(1)/(2)MeCN (2), and [Pb(2)(L)](ClO(4))(2).2MeCN (4). In 1 and 2, the phenolate O-donors do not bridge the two metal centers, which are, therefore, segregated each within an N(5)O-donor compartment. However, in the case of the binuclear complex [Pb(2)(L)](ClO(4))(2).2MeCN (4), the two Pb(II) centers are bridged by the phenolate oxygen atoms with each metal ion sited within an N(5)O(2)-donor compartment of L(2)(-), with a Pb.Pb distance of 3.9427(5) A. 相似文献
60.
Xi Yang Dr. Tomohiro Yasukawa Dr. Tei Maki Dr. Yasuhiro Yamashita Prof. Dr. Shū Kobayashi 《化学:亚洲杂志》2021,16(3):232-236
Although strong acid-treated metal oxides are useful heterogeneous superacid catalysts for various organic transformations, they usually have a limited density of acidic sites due to their low surface areas. Herein, heterogeneous trifluoromethanesulfonic acid immobilized nitrogen-doped carbon-incarcerated titanium nanoparticle (NP) catalysts have been developed that are composed of well-dispersed, small Ti NPs (ca 7 nm) that are otherwise difficult to achieve using acid-treated metal oxides. The catalysts showed high activity for Friedel–Crafts acylation with low titanium loading (2 mol%, <1 mg of metal for 1 mmol of substrate). A range of microscopic, spectroscopic and physicochemical studies revealed that the nitrogen-doped carbon immobilized the trifluoromethanesulfonic acid and that the addition of metals further changed the nature of the acidic species and enhanced catalytic activity. 相似文献