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91.
Grald Lelais Dieter Seebach Bernhard Jaun RaveendraI. Mathad Oliver Flgel Francesco Rossi Marino Campo Arno Wortmann 《Helvetica chimica acta》2006,89(3):361-403
The correlation between β2‐, β3‐, and β2,3‐amino acid‐residue configuration and stability of helix and hairpin‐turn secondary structures of peptides consisting of homologated proteinogenic amino acids is analyzed (Figs. 1–3). To test the power of Zn2+ ions in fortifying and/or enforcing secondary structures of β‐peptides, a β‐decapeptide, 1 , four β‐octapeptides, 2 – 5 , and a β‐hexadecapeptide, 10 , have been devised and synthesized. The design was such that the peptides would a) fold to a 14‐helix ( 1 and 3 ) or a hairpin turn ( 2 and 4 ), or form neither of these two secondary structures (i.e., 5 ), and b) carry the side chains of cysteine and histidine in positions, which will allow Zn2+ ions to use their extraordinary affinity for RS? and the imidazole N‐atoms for stabilizing or destabilizing the intrinsic secondary structures of the peptides. The β‐hexadecapeptide 10 was designed to a) fold to a turn, to which a 14‐helical structure is attached through a β‐dipeptide spacer, and b) contain two cysteine and two histidine side chains for Zn complexation, in order to possibly mimic a Zn‐finger motif. While CD spectra (Figs. 6–8 and 17) and ESI mass spectra (Figs. 9 and 18) are compatible with the expected effects of Zn2+ ions in all cases, it was shown by detailed NMR analyses of three of the peptides, i.e., 2, 3, 5 , in the absence and presence of ZnCl2, that i) β‐peptide 2 forms a hairpin turn in H2O, even without Zn complexation to the terminal β3hHis and β3hCys side chains (Fig. 11), ii) β‐peptide 3 , which is present as a 14‐helix in MeOH, is forced to a hairpin‐turn structure by Zn complexation in H2O (Fig. 12), and iii) β‐peptide 5 is poorly ordered in CD3OH (Fig. 13) and in H2O (Fig. 14), with far‐remote β3hCys and β3hHis residues, and has a distorted turn structure in the presence of Zn2+ ions in H2O, with proximate terminal Cys and His side chains (Fig. 15). 相似文献
92.
Gervais C Dupree R Pike KJ Bonhomme C Profeta M Pickard CJ Mauri F 《The journal of physical chemistry. A》2005,109(31):6960-6969
13C, 14N, 15N, 17O, and 35Cl NMR parameters, including chemical shift tensors and quadrupolar tensors for 14N, 17O, and 35Cl, are calculated for the crystalline forms of various amino acids under periodic boundary conditions and complemented by experiment where necessary. The 13C shift tensors and 14N electric field gradient (EFG) tensors are in excellent agreement with experiment. Similarly, static 17O NMR spectra could be precisely simulated using the calculation of the full chemical shift (CS) tensors and their relative orientation with the EFG tensors. This study allows correlations to be found between hydrogen bonding in the crystal structures and the 17O NMR shielding parameters and the 35Cl quadrupolar parameters, respectively. Calculations using the two experimental structures for L-alanine have shown that, while the calculated isotropic chemical shift values of 13C and 15N are relatively insensitive to small differences in the experimental structure, the 17O shift is markedly affected. 相似文献
93.
Calderoni F Demartin F Iapalucci MC Laschi F Longoni G Zanello P 《Inorganic chemistry》1996,35(4):898-905
A reinvestigation of the redox behavior of the [Fe(3)(&mgr;(3)-S)(CO)(9)](2)(-) dianion led to the isolation and characterization of the new [Fe(5)S(2)(CO)(14)](2)(-), as well as the known [Fe(6)S(6)(CO)(12)](2)(-) dianion. As a corollary, new syntheses of the [Fe(3)S(CO)(9)](2)(-) dianion are also reported. The [Fe(5)S(2)(CO)(14)](2)(-) dianion has been obtained by oxidative condensation of [Fe(3)S(CO)(9)](2)(-) induced by tropylium and Ag(I) salts or SCl(2), or more straightforwardly through the reaction of [Fe(4)(CO)(13)](2)(-) with SCl(2). The [Fe(6)S(6)(CO)(12)](2)(-) dianion has been isolated as a byproduct of the synthesis of [Fe(3)S(CO)(9)](2)(-) and [Fe(5)S(2)(CO)(14)](2)(-) or by reaction of [Fe(4)(CO)(13)](2)(-) with elemental sulfur. The structures of [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)] and [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)] were determined by single-crystal X-ray diffraction analyses. Crystal data: for [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)], monoclinic, space group P2(1)/c (No. 14), a = 24.060(5), b = 14.355(6), c = 23.898(13) ?, beta = 90.42(3) degrees, Z = 4; for [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)], monoclinic, space group C2/c (No. 15), a = 34.424(4), b = 14.081(2), c = 19.674(2) ?, beta = 115.72(1) degrees, Z = 4. The new [Fe(5)S(2)(CO)(14)](2)(-) dianion shows a "bow tie" arrangement of the five metal atoms. The two Fe(3) triangles sharing the central Fe atom are not coplanar and show a dihedral angle of 55.08(3) degrees. Each Fe(3) moiety is capped by a triply bridging sulfide ligand. The 14 carbonyl groups are all terminal; two are bonded to the unique central atom and three to each peripheral iron atom. Protonation of the [Fe(5)S(2)(CO)(14)](2)(-) dianion gives reversibly rise to the corresponding [HFe(5)S(2)(CO)(14)](-) monohydride derivative, which shows an (1)H-NMR signal at delta -21.7 ppm. Its further protonation results in decomposition to mixtures of Fe(2)S(2)(CO)(6) and Fe(3)S(2)(CO)(9), rather than formation of the expected H(2)Fe(5)S(2)(CO)(14) dihydride. Exhaustive reduction of [Fe(5)S(2)(CO)(14)](2)(-) with sodium diphenyl ketyl progressively leads to fragmentation into [Fe(3)S(CO)(9)](2)(-) and [Fe(CO)(4)](2)(-), whereas electrochemical, as well as chemical oxidation with silver or tropylium tetrafluoroborate, in dichloromethane, generates the corresponding [Fe(5)S(2)(CO)(14)](-) radical anion which exhibits an ESR signal at g = 2.067 at 200 K. The electrochemical studies also indicated the existence of a subsequent one-electron anodic oxidation which possesses features of chemical reversibility in dichloromethane but not in acetonitrile solution. A reexamination of the electrochemical behavior of the [Fe(3)S(CO)(9)](2)(-) dianion coupled with ESR monitoring enabled the spectroscopic characterization of the [Fe(3)S(CO)(9)](-) radical monoanion and demonstrated its direct involvement in the generation of the [Fe(5)S(2)(CO)(14)](n)()(-) (n = 0, 1, 2) system. 相似文献
94.
Summary White crystalline complexes of general formula Cu2L4X2 (where X = Cl, Br and L = 1, 3-oxazolidine-2-thione, pyrrolidine-2-thione,N-methyl-1,3-imidazolidine-2-thione andN-ethyl-1,3-imidazolidine-2-thione) and CuLX (where L = 1,3-imidazolidine-2-thione) were prepared by reduction of copper(II) halides and studied by i.r. spectroscopy in the 4000–200 cm– range. Evidence for ligand coordination to the metal through sulphur was found in each case. The(CuCI) vibration in all the chloro derivatives falls atca. 240 cm–. 相似文献
95.
Francesca Benedini Giorgio Bertolini Francesco Ferrario Angelo Motti Alberto Sala Flavio Somenzi 《Journal of heterocyclic chemistry》1995,32(1):103-107
The preparation and the physico-chemical characterization of 2H-pyrido[2,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazin-4(3H)-ones, 2H-thieno[2,3-e]-1,3-oxazin-4(3H)-ones and 2H-thieno[3,4-e]-1,3-oxazine-2,4(3H)-diones are reported. 相似文献
96.
The absolute configurations of acetylated bretonin A (= (+}-( R )-1-[(acetoxy)methyl]-2-{[(4E,6E,8E)-dodeca-4,6,8-trienyl]oxy}ethyl 4-acetoxybenzoate; (?)- 1b ) and isobretonin A (= (+)-(S)-3-{[(4E,6E,8E)-do-deca-4,6,8-trienyl]oxy}-2-hydroxypropyl 4-hydroxybenzoate; (+)-2), previously isolated from an undetermined sponge of the North Brittany sea, were established by comparison with synthetic (+)- lb and (+)- 2 , obtained from the condensation of commerical (?)-(R)-2,2-dimethyl-1,3-dioxolan-4-yl p-toluenesuifonate ((?)-(R)- 15 ) with a mixture of (4E,6E,8E)- ( 14e ) and (4E,6Z,8E)-dodeca-4,6,8-trien-1-ol ( 14z ). This also allowed confirming the structure and configuration of bretonin B (= (S)-2-{[(4E,6Z,8E)-dodeca-4,6,8-trienyl]oxy}-1-(hydroxy-methyl)ethyl 4-hydroxybenzoate; 3 ) which was also isolated from the same sponge, albeit in a too small amount for a complete study. As concerns the glyceryl ethers precursors of the bretonins, co-occurrence of the usual (S)-con-figuration (from 1a ) with the unusual (R)-configuration (from (+)- 2 )) poses intriguing biogenetic problems. 相似文献
97.
Altieri A Gatti FG Kay ER Leigh DA Martel D Paolucci F Slawin AM Wong JK 《Journal of the American Chemical Society》2003,125(28):8644-8654
A series of [2]rotaxanes containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle is described. Electrochemical reduction and oxidation of the naphthalimide group alters its ability to form hydrogen bonds to the macrocycle to such a degree that redox processes can be used to switch the relative macrocycle-binding affinities of the two stations in a rotaxane by over 8 orders of magnitude. The structure of the neutral [2]rotaxane in solution is established by (1)H NMR spectroscopy and shows that the macrocycle exhibits remarkable positional integrity for the succinamide station in a variety of solvents. Cyclic voltammetry experiments allow the simultaneous stimulation and observation of a redox-induced dynamic process in the rotaxane which is both reversible and cyclable. Model compounds in which various conformational and co-conformational changes are prohibited demonstrate unequivocally that the redox response is the result of shuttling of the macrocycle between the two stations. At room temperature in tetrahydrofuran the electrochemically induced movement of the macrocycle between the two stations takes approximately 50 micros. 相似文献
98.
Blake AJ Bencini A Caltagirone C De Filippo G Dolci LS Garau A Isaia F Lippolis V Mariani P Prodi L Montalti M Zaccheroni N Wilson C 《Dalton transactions (Cambridge, England : 2003)》2004,(17):2771-2779
The coordination chemistry of the new pyridine-based, N2S2-donating 12-membered macrocycle 2,8-dithia-5-aza-2,6-pyridinophane (L1) towards Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II) has been investigated both in aqueous solution and in the solid state. The protonation constants for L1 and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those of ligand L2, which contains a N-aminopropyl side arm. The measured values show that Hg(II) in water has the highest affinity for both ligands followed by Cu(II), Cd(II), Pb(II), and Zn(II). For each metal ion considered, 1:1 complexes with L1 have also been isolated in the solid state, those of Cu(II) and Zn(II) having also been characterised by X-ray crystallography. In both complexes L1 adopts a folded conformation and the coordination environments around the two metal centres are very similar: four positions of a distorted octahedral coordination sphere are occupied by the donor atoms of the macrocyclic ligand, and the two mutually cis-positions unoccupied by L1 accommodate monodentate NO3- ligands. The macrocycle L1 has then been functionalised with different fluorogenic subunits. In particular, the N-dansylamidopropyl (L3), N-(9-anthracenyl)methyl (L4), and N-(8-hydroxy-2-quinolinyl)methyl (L5) pendant arm derivatives of L1 have been synthesised and their optical response to the above mentioned metal ions investigated in MeCN/H2O (4:1 v/v) solutions. 相似文献
99.
2-Chlorotropone was obtained from 2-tosyloxytropone in 88% yield in the recyclable ionic medium BMIMBF4/LiCl. That Li+ acts as a Lewis acid was proven by the lack of reactivity of 2-tosyloxytropone, under the above conditions, on replacing LiCl with NaCl or BMIMCl, or using BMIMCl alone, or a BMIMBF4/MeCN/KCl mixture. 2-Bromo- and 2-iodotropone were obtained along similar lines from LiBr or LiI, respectively, whereas LiF proved unreactive. 相似文献
100.
The kinetics of the interaction between Gemcitabine (a new anticancer drug) and phospholipid membrane models was investigated. This kind of study is of particular importance both in hypothesizing the interaction of Gemcitabine with mammalian cell membranes and in evaluating the potentiality of liposomes as a Gemcitabine delivery system. Unilamellar (LUV) and multilamellar (MLV) membrane models were made up of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidic acid sodium salt (DMPA), or a DMPC-DMPA mixture (1:1 molar ratio). Gemcitabine-phospholipid vesicle interaction was studied by differential scanning calorimetry (DSC) measurements performed at different time intervals. The findings showed slower permeation kinetics of Gemcitabine through MLV than LUV which, at the same lipid/water ratio, are characterized by a larger lipid surface in contact with the drug aqueous solution. Another interesting difference between LUV and MLV is the onset of a transient two-peak structure during the DSC scans of MLVs. The effect is due to the unequal distribution of the drug between the outer and inner bilayers of the multilamellar vesicles during the permeation kinetics. At equilibrium the two-peak structure merges into a unique peak. This finding may provide useful information about the lipid bilayer permeability in model membranes. 相似文献