Search results for "Urea"
showing 10 items of 888 documents
Structure and stereochemistry of stenophyllolide, a germacrolide from Centaurea aspera var. Stenophylla
1984
Abstract A crystalline compound, named stenophyllolide, obtained from an extract of Centaurea aspera var. stenophylla was shown to be 9β,15-dihydroxygermacra-1(10),4,11-trien-6α,12-olide by X-ray analysis. The molecular structure of stenophyllolide was solved with orthorhombic space group P2 1 2 1 2 1 , a = 11.719 (5), b = 13.389 (5), c = 8.646 (5) A for Z = 4, by direct methods and refined to a final R of 0.06 for 1198 observed reflections.
The purification and properties of nucleoside phosphotransferase from mucosa of chicken intestine
1984
Abstract (1) Nucleoside phosphotransferase (nucleotide:3′-deoxynucleoside 5′-phosphotransferase, EC 2.7.1.77) has been purified from chicken intestine mucosa to apparent homogeneity. The enzyme is represented by a multisubunit protein at different degrees of association. It can dissociate into a compoenent with a marked fall in catalytic activity. (2) The associated forms are similar to the enzyme previously purified from chick embryo as regards: substrate specificity both with respect to nucleoside monophosphate donors and to deoxyribonucleoside acceptors; sigmoidicity in the rate curve with a variable phosphate donor; instability to heat, dilution and lowering of pH; the activating and pr…
Topologically Novel Multiple Rotaxanes and Catenanes Based on Tetraurea Calix[4]arenes
2006
Calix[4]arenes bearing at their wide rim four urea residues easily form hydrogen bonded dimeric capsules. This has been used to preorganise alkenyl functions attached to these urea groups for their controlled connection via metathesis reaction. Multimacrocyclic tetraurea derivatives are thus obtained in excellent yields via heterodimers which are formed exclusively with tetratosylurea derivatives. Heterodimerisation of such bis- and tetraloop tetraureas leads analogously to multicatenanes, or to rotaxanes by stoppering. Huge macrocycles are detached from tetraloop derivatives by cleavage of the urea function.
Mono-, di- and tri-nuclear Ni(II) complexes of N-, O-donor ligands: structural diversity and reactivity
2002
A series of mono-, di- and tri-nuclear Ni(II) complexes of N, O-donating molecules possessing ---H2C---NH--- and ---HC=N--- moieties have been synthesized and characterized and the structures have been determined by single crystal X-ray diffraction. All these exhibited interesting molecular packing in their crystal lattices. Di-nuclear complexes were found to be cleaved in pyridine to result in mononuclear ones with additional coordinations being provided by pyridine. Di-nuclear complexes were found to form urea adducts as demonstrated based on absorption and vibrational studies.
Sesquiterpene lactones from Centaurea alba and C. conifera
1995
Abstract The aerial parts of Centaurea alba yielded five know germacranolides: salonitenolide, 11β, 13-dihydrosalonitenolide, salonitenolide 8- O -(4′-acetoxy-5′-hydroxy)-angelate, cnicin 4′-O-acetate and cnicin. The aerial parts of C. conifera yielded loliolide, 1β,6α-dihydroxy-4(15)-eudesmene, chlorojanerin, chlorohyssopifolin A (centaurepensin) and its C-17 epimer. The latter compound is described for the first time.
Kinetic properties of hexameric tyrosinase from the crustacean Palinurus elephas.
2008
Tyrosinases catalyze hydroxylation of monophenols to o-diphenols and their subsequent oxidation to o-quinones, whereas catecholoxidases catalyze only the latter reaction. Both enzymes occur in all organisms and are Type 3 copper proteins that perform the first steps of melanin formation. In arthropods, they play an essential role in the sclerotization of the exoskeleton. Very few phenoloxidases are characterized structurally or kinetically and the existence of an actual tyrosinase activity has not been demonstrated in most cases. Here we present for the first time a complete kinetic characterization of a tyrosinase from a crustacean (Palinurus elephas) including the influence of inhibitors.…
Template synthesis of multi-macrocycles by metathesis reaction.
2004
Selective heterodimerisation of tetraurea calix[4]arenes containing four or eight ω-alkenyl groups with a tetratosyl urea calix[4]arene has been effectively used to synthesize multi-macrocycles via metathesis reaction.
Cytotoxic activity of some natural and syntetic sesquiterpene lactones
2006
Several natural and synthetic sesquiterpene lactones with different skeletons were tested and found to be active against nine cancer cell lines. Elemane (4), heliangolane (5) and their hydroxy analogues 9 and 10, all containing an α,β-unsaturated aldehyde substituent, were the most potent compounds.
endo-Functionalized molecular tubes : selective encapsulation of neutral molecules in non-polar media
2016
Four endo-functionalized molecular tubes with urea/thiourea groups in the deep cavities have been synthesized, and their binding ability to neutral molecules studied. Very high binding affinity and selectivity have been achieved, which are rationalized by invoking the shape and electrostatic complementarity and dipole alignment.
Compensation of steric demand by cation–π interactions, cobaltocenium cations as guests in tetraurea calix[4]arene dimers
2003
The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the vari…