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61.
Pintér A Haberhauer G Hyla-Kryspin I Grimme S 《Chemical communications (Cambridge, England)》2007,(36):3711-3713
Configurationally stable, propeller-like triarylphosphine and triarylphosphine oxide can be synthesized; a chiral scaffold based on Lissoclinum-cyclopeptides linked via three peptide bonds with a triphenylphosphine and triphenylphosphine oxide moiety, respectively, prevents effectively epimerization at the chiral phosphorus atom. 相似文献
62.
Rai R Vasudev PG Ananda K Raghothama S Shamala N Karle IL Balaram P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(20):5917-5926
The beta turn segment in designed peptide hairpins has been expanded by the insertion of beta-, gamma- and delta-amino acids at the i+2 position. The model octapeptides Boc-Leu-Phe-Val-DPro-Ac6c-Leu-Phe-Val-OMe (1), Boc-Leu-Phe-Val-DPro-beta3-Ac6c-Leu-Phe-Val-OMe (2), and Boc-Leu-Phe-Val-DPro-Gpn-Leu-Phe-Val-OMe (3) have been shown to adopt beta hairpin conformations in methanol by the observation of key diagnostic nuclear Overhauser effects. Boc-Leu-Val-Val-DPro-delta-Ava-Leu-Val-Val-OMe (4) adopts a beta hairpin conformation in crystals; this is stabilized by three cross-strand hydrogen bonds as demonstrated by X-ray diffraction. The canonical C10 turn in an alpha-alpha segment is expanded to C11, C12, and C13 turns in alpha-beta, alpha-gamma, and alpha-delta segments, respectively. The crystal structures of Piv-LPro-beta3-Ac6c-NHMe (5) and Boc-Ac6c-Gpn-Ac6c-OMe (6) reveal intramolecularly hydrogen-bonded C11 and C12 conformations, respectively. Computer modeling of octapeptide sequences that contain centrally positioned hybrid-turn segments, by using turn parameters derived from the structures of peptides 5 and 6, establishes the stereochemical acceptability of the beta hairpins in the cases of peptides 2 and 3. Accommodation of omega-amino acids into the turn segments is achieved by the adoption of gauche conformations around the backbone C--C bonds. 相似文献
63.
64.
Luigi Coppola Mohamed Youssry Isabella Nicotera Luigi Gentile 《Journal of colloid and interface science》2009,338(2):550-557
The thermal behavior of unsonicated dispersions of a double-chained surfactant, Dioctadecyldimethylammonium bromide (DODAB), has been studied over a wide concentration range using DSC and dynamic rheology. All dispersions are characterized by the pre- and main transition peaks at 35 °C and 43 °C, respectively. But, only above 10 mM DODAB, a third endotherm at 52 °C appears which may correspond to the (ULVs + Lα fragments) → MLVs transition. The thermal-induced MLV’s size is proportionally dependent on the concentration. In addition, and in agreement with DSC data, dynamic rheology has proven to be an indirect way to elucidate the structural transitions in these DODAB vesicular dispersions. 相似文献
65.
Isabella Richter Mark R. Warren Jusaku Minari Souad A. Elfeky Wenbo Chen Mary F. Mahon Paul R. Raithby Prof. Tony D. James Kazuo Sakurai Simon J. Teat Steven D. Bull John S. Fossey Dr. 《化学:亚洲杂志》2009,4(1):194-198
The crystal structures of phenylpropylpyridine‐N‐oxide and N‐methyl‐phenylpropylpyridinium iodide are compared, revealing that hydrogen bonding with the solvent molecule plays an important role in the N‐oxide compound, whilst electrostatic interactions are predominant in controlling the solid‐state orientation of the N‐methylated compound. Fluorescence spectroscopy and NOESY indicate that in contrast to the previously reported pyridinium iodide, the N‐oxide is not subject to intramolecular π‐stacking, as judged by excimer emission and a lack of corresponding cross peaks, respectively. 相似文献
66.
Mustazza C Borioni A Sestili I Sbraccia M Rodomonte A Ferretti R Del Giudice MR 《Chemical & pharmaceutical bulletin》2006,54(5):611-622
Some spiro[piperidine-4,2'(1'H)-quinazolin]-4'(3'H)-ones 3 and spiro[piperidine-4,5'(6'H)-[1,2,4]triazolo[1,5-c]quinazolines] 4 were synthesized and evaluated as ligands of the nociceptin receptor. The examined compounds showed partial agonistic activity, except compounds 3, 4n that proved to be pure antagonists. 相似文献
67.
Isabella L. Karle Y. B. R. D. Rajesh Subramania Ranganathan 《Journal of chemical crystallography》2009,39(3):201-208
Abstract Individually prepared, equivalent amounts of 2,5-bis-(4-pyridyl)-1,3,4-oxadiazole (4-BPO) and benzenehexacarboxylic acid (mellitic
acid, MA) when mixed, deposited long rods, whose X-ray structure showed a unique assembly of 40 membered planar modules composed
from two alternating units each of MA and 4-BPO, rather than the anticipated helical profile. Several novel features of complexes
of MA with organic molecules are seen here. In almost all cases the intermolecular hydrogen bonding between MA anions is so
stable that the MA anions and the complementary cations lie in separate planes. This barrier is overcome in the present case.
The basic module associates to form quartets by very simple hydrogen bonding of the carboxylic groups of MA. The quartet assembly
is characterized by the rotation of the left hand pair compared to the right hand pair. The mode of the macromolecular assembly
is clear, that is, they are layered in alternating planes. Space group: P21/c with a = 9.181(1) ?, b = 9.624(1) ?, c = 29.390(2) ?, and β = 94.626(4)°. The modules are embedded in an extensively hydrogen bonded MA network. The assembly profile of MA and 4-BPO
is unique and should be a harbinger for the design of novel functional assemblies. The 40-membered module mimics the self-assemblies
of peptides and can be important in the design of “Haptens” from simple molecules.
Index Abstract Rod like crystals from water that are formed on mixing MA and 4-BPO are shown to consist of 4-molecule modules (MA–4-BPO–MA–4-BPO–)
that contain a 40-membered ring, MA–BPO.
相似文献
68.
A short new proof of the fact that all shifted complexes are fixed by reverse lexicographic shifting is given. A notion of
lexicographic shifting, Δlex—an operation that transforms a monomial ideal of S = K[xi: i ∈ ℕ] that is finitely generated in each degree into a squarefree strongly stable ideal—is defined and studied. It is proved
that (in contrast to the reverse lexicographic case) a squarefree strongly stable ideal I ⊂ S is fixed by lexicographic shifting if and only if I is a universal squarefree lexsegment ideal (abbreviated USLI) of S. Moreover, in the case when I is finitely generated and is not a USLI, it is verified that all the ideals in the sequence
} are distinct. The limit ideal
is well defined and is a USLI that depends only on a certain analog of the Hilbert function of I.
Research partially supported by NSF grants DMS 0070571 and DMS 0100141. 相似文献
69.
A common generalization of two theorems on the face numbers of Cohen-Macaulay (CM, for short) simplicial complexes is established: the first is the theorem of Stanley (necessity) and Bj?rner-Frankl-Stanley (sufficiency) that characterizes all possible face numbers of a-balanced CM complexes, while the second is the theorem of Novik (necessity) and Browder (sufficiency) that characterizes the face numbers of CM subcomplexes of the join of the boundaries of simplices. 相似文献
70.