Conformation Suitable (conformation + suitable)

Distribution by Scientific Domains

Selected Abstracts

A Gadolinium-Binding Cyclodecapeptide with a Large High-Field Relaxivity Involving Second-Sphere Water

Abstract A new cyclodecapeptide incorporating two prolylglycine sequences as ,-turn inducers and bearing four side chains with acidic carboxyl groups for cation complexation has been prepared. Structural analysis in water by 1H,NMR spectroscopy and CD shows that this template adopts a conformation suitable for the complexation of lanthanide ions Ln3+, with its carboxyl groups oriented on the same face of the peptide scaffold. Luminescence titrations show that mononuclear Ln,PA complexes are formed with apparent stability constants of log,,110,6.5 (pH,7). The high-field water relaxivity values arising from the Gd,PA complex at 200,500,MHz have been interpreted with molecular parameters determined independently. The experimentally determined water relaxivities are undoubtedly 30,% higher than the expected values for this complex with two inner-sphere (IS) water molecules and a medium-range rotational correlation time (,R=386,ps (±10,%)). This led us to propose the existence of a large second-sphere (2S) contribution to the relaxivity caused by the interaction of water molecules with the hydrophilic peptide ligand by hydrogen-bonding. [source]

Allosteric Tuning of the Intra-Cavity Binding Properties of a Calix[6]arene through External Binding to a ZnII Center Coordinated to Amino Side Chains

Ulrich Darbost Dr.
Abstract Molecular recognition by calix[6]arene-based receptors bearing three primary alkylamino side chain arms (1) is described. Complexation of ZnII ion provides the dinuclear ,-hydroxo complex , XRD characterization of which, together with solution studies, provided evidence of its hosting of neutral polar organic guests G. Treatment of this complex with a carboxylic acid or a sulfonamide (XH) results in the formation of mononuclear species , one of which (X = Cl) has been characterized by XRD. A dicationic complex is obtained upon treatment of with a mixture of an alkylamine and a strong acid. Each of these ZnII complexes features a tetrahedral metal ion bound to the three amino arms of ligand 1 and to an exogenous ligand (either HO,, X,, or RNH2) sitting outside of the cavity. As a result, the metal ion structures the calixarene core, constraining it in a cone conformation suitable for guest hosting. The receptor properties of these compounds have been explored in detail and are compared with those of the trisammonium receptor , based on the same calixarene core, as well as those of the trisimidazole-based dicationic Zn funnel complexes. This study reveals very different host properties, in spite of the common hydrophobic, ,-basic, and hydrogen-bonding acceptor properties of the calixarene cores. A harder external ligand produces a less polarized receptor that is consequently particularly sensitive to the hydrogen-bonding ability of its guest. The less electron-rich the apical ligand, and a fortiori the trisammonium host, the more sensitive the receptor to the dipole moment of the guest. All this stands in contrast with the funnel Zn complexes, in which the coordination link plays a dominant role. It is also shown that the asymmetry of an exo -coordinated enantiopure amino ligand is sensed by the guest. This supramolecular system nicely illustrates how the receptor properties of a hydrophobic cavity can be allosterically tuned by the environment. [source]

Guest-dependent conformation of 18-crown-6 tetracarboxylic acid: Relation to chiral separation of racemic amino acids

CHIRALITY, Issue 7 2008
Hiroomi Nagata
Abstract (+)-18-Crown-6 tetracarboxylic acid (18C6H4) has been used as a chiral selector for various amines and amino acids. To further clarify the structural scaffold of 18C6H4 for chiral separation, single crystal X-ray analysis of its glycine+ (1), H3O+ (2), H5O (3), NH (4), and 2CH3NH (5) complexes was performed and the guest-dependent conformation of 18C6H4 was investigated. The crown ether ring of 18C6H4 in 3, 4, and 5 took a symmetrical C2 or C2 -like conformation, whereas that in 1 and 2 took an asymmetric C1 conformation, which is commonly observed in complexes with various optically active amino acids. The overall survey of the present and related complexes suggests that the molecular conformation of 18C6H4 is freely changeable within an allowable range, depending on the molecular shape and interaction mode with the cationic guest. On the basis of the present results, we propose the allowable conformational variation of 18C6H4 and a possible transition pathway from its primary conformation to the conformation suitable for chiral separation of racemic amines and amino acids. Chirality, 2008. © 2008 Wiley-Liss, Inc. [source]

Non-racemic atropisomeric (thio)ureas as neutral enantioselective anion receptors for amino-acid derivatives: Origin of smaller Kass with thiourea than urea derivatives

CHIRALITY, Issue 9 2006
Christian Roussel
Abstract The synthesis of a limited series of non-racemic atropisomeric 1-(2-(4-methyl-2-thioxothiazol-3(2H)-yl)phenyl)-3-(hetero)aryl-(thio)ureas is described. Using NMR titration experiments monitoring the shift of the two NH of the (thio)urea and the C-5 hydrogen of the heterocycle, the binding constants for some optically pure (thio)-ureas with the enantiomers of N-protected amino acid tetrabutylammonium salts were determined in CD3CN. The obtained enantioselectivities were modest. Contrary to what was expected on the basis of the NH acidity in thiourea versus urea group, the association constants were smaller with the thiourea than with the corresponding urea. X-ray data, DFT calculations, and NMR provided the explanation of that unexpected behavior: the urea presents a pre-organized (Z,Z) conformation suitable for a double hydrogen bond with the carboxylate anion, the thiourea presents a (Z,E) conformation, which must be reorganized in a constrained (Z,Z) conformation in the complex. An intramolecular hydrogen bond between one NH and the thiocarbonyl group of the heterocycle, which is present in the thiourea and absent in the urea, might also contribute to the smaller Kass for the thiourea. The possible implication of these observations in the field of bifunctional organocatalysis is briefly discussed. Chirality, 2006. © 2006 Wiley-Liss, Inc. [source]