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排序方式: 共有276条查询结果,搜索用时 15 毫秒
1.
Rudovsky J Hermann P Botta M Aime S Lukes I 《Chemical communications (Cambridge, England)》2005,(18):2390-2392
A marked increase of relaxivity has been observed upon rigidifying the internal frame of Gd-containing PAMAM dendrimers: the effect has been attained by either protonation of the dendrimer or by forming supramolecular adducts with cationic polyaminoacids. 相似文献
2.
Tafi A van Almsick A Corelli F Crusco M Laumen KE Schneider MP Botta M 《The Journal of organic chemistry》2000,65(12):3659-3665
On the basis of the X-ray crystal structure of the lipase from Pseudomonas cepacia (PcL)-an enzyme representative for a whole family of Pseudomonas lipases (lipase PS, SAM-2, AK 10, and others with a high degree of homology with PcL)-a computational study was performed to rationalize both the enantioselectivity and substrate specificity (tolerance) displayed by this lipase in the enantioselective hydrolysis of racemic esters 1a-12a from various secondary aromatic alcohols. The major goal of this project was the development of a binding model for PcL which is able to rationalize the experimental findings to predict "a priori the enantioselective behavior of PcL toward a wider range of substrates. A two-step modeling procedure, namely, docking experiments followed by construction of tetrahedral intermediates, was used for the simulation of the involved enzyme-substrate recognition/hydrolysis processes. The study of the recognition process (docking experiments) led to unambiguous identification of the binding geometry for the two enantiomeric series of substrates, but did not suggest a definitive interpretation of the behavior of PcL. Taking into consideration the stereoelectronic requirements of the enzymatic hydrolysis reaction, both the enantioselectivity and tolerance of the enzyme were then explained through the study of the tetrahedral intermediates, in turn constructed from the calculated docking geometries of 1a-12a. 相似文献
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[formula: see text] Paclitaxel and epothilone represent the two major classes of antimicrotubule agents that promote tubulin polymerization and, presumably, mitotic arrest during cell division. A common minireceptor binding site model at beta-tubulin has been constructed for these structurally divergent compounds. Utilizing 20 amino acids identified in photoaffinity labeling experiments, the 3-D model correlates measured and predicted Ki's with r = 0.99 and rms(delta Gcalc-delta Gexp) = 0.2 kcal/mol. In addition, the model predicts the affinity of compounds not used in the training set and explains much of the SAR for the paclitaxel and epothilone families. 相似文献
6.
Silvio Aime Mauro Botta Giancarlo Cravotto Luca Frullano GiovanniB. Giovenzana Simonetta GeninattiCrich Giovanni Palmisano Massimo Sisti 《Helvetica chimica acta》2005,88(3):588-603
Four new ligands for lanthanide ions based on the H3do3a (=1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid) structure and bearing one N‐sulfonylacetamide arm were synthesized, i.e., H4dota‐NHSO2R=10‐{2‐[(R)sulfonylamino]‐2‐oxoethyl}‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acids 1a – e . A 15N‐NMR study of the 15N‐labelled Eu3+ complex of one such ligands, 1d , showed that the coordination of the N‐sulfonylacetamide arm involves the carbonyl O‐atom rather than the N‐atom. The relaxometric properties of the corresponding Gd3+ complexes were investigated as a function of pH and temperature. These complexes have relaxivities in the range 4.5–5.3 mM ?1 s?1, at 20 MHz and 25°, and are characterized by a single H2O molecule in their inner coordination sphere. The mean residence lifetime of this molecule is relatively long (500–700 ns) compared to other anionic complexes. The slow rate of H2O exchange can be justified by the extensive delocalization of the negative charge on the N‐sulfonylacetamide arm. The long residence time of the coordinated H2O allowed the observation of the effect of the prototropic exchange on the relaxivity. The study of the interaction between the complex [Gd( 1e )]‐ and HSA revealed a weak affinity constant highlighting the importance of a localized negative charge on the complex to promote a strong interaction with the protein. 相似文献
7.
It was early shown that the macrocyclic Ln(DOTA) complexes (DOTA = 1,4,7,10-tetra-azacyclododecane-N,N',N' ',N' "-tetraacetic acid) exists in solution as a mixture of two enantiomeric pairs of diastereoisomers differing in the ligand conformation, namely, square antiprismatic (SA) and twisted square antiprismatic (TSA) geometries, respectively. Later, extensive (1)H NMR investigations suggested that a coordination change may be superimposed on this conformational equilibrium involving two additional structures in which the metal ion possesses a coordination number of eight (CN 8). It was predicted that these two species, lacking the apical coordinated water molecule, would maintain the SA and TSA coordination geometries, and therefore, they have been labeled as SA' and TSA', respectively. In this work we report the X-ray solid-state crystal structure determination of six Ln(DOTA) complexes representative of all four coordination geometry typologies deduced from NMR solution studies. A distinctive structural feature that discriminates SA (and SA') and TSA (and TSA') structures is represented by the twist angle between the two square planes of the antiprism, the basal four nitrogen, and the apical four oxygen planes. [Ce(DOTA)(H(2)O)](-) displays a TSA structural typology with a twist angle of 25 degrees and a Ce-O(water) distance of 2.59 A. The SA-type structure has been found in the case of complexes with Pr(III), Nd(III), and Dy(III), where the twist angle is 39, 39, and 38 degrees, respectively, and the metal-water oxygen distance varies significantly (Pr-O(w) 2.529 A; Nd-O(w) 2.508 A; and Dy-O(w) 2.474 A). [Tm(DOTA)](-) displays a TSA'-type structure with a twist angle of 24 degrees. As compared with the TSA structure of the corresponding Ce(III) complex, the Tm(III) complex shows an overall marked shrinkage of all metal-nitrogen and metal-oxygen distances (ca. 0.2 A), which reflects the contraction of the metal ionic radius across the series but also the effect associated with the decrease of the CN from 9 to 8. In [Sc(DOTA)](-), the even smaller ionic radius of Sc(III) shifts the geometry of the coordination cage to the more compact SA' typology with a twist angle of 41 degrees, a value very similar to that found in the SA structures of lanthanide(III) ions with CN 9. Finally, an investigation was made into the hydration spheres of the complexes with SA and TSA geometries to account for the experimental evidence of a markedly different rate of water exchange for the two isomeric structures. This is of fundamental importance to the understanding of the corresponding Gd(III) complexes as MRI contrast agents. 相似文献
8.
Poly(butylene terephthalate) (PBT) and a sample of polyamide have been melt processed in the presence of two new phosphazene compounds, namely 2,2-dichloro-4,4,6,6-bis[spyro(2′,2″-dioxy-1′,1″-biphenyl)]cyclotriphosphazene (2Cl-CP) and 2,2-bis(2-methoxy-4-methyleneoxy-phenoxy)-4,4,6,6-bis[spyro(2′,2″-dioxy-1′,1″-biphenyl)]cyclophosphazene (CP-2EPOX).The blends were prepared by using polyamide 6 (PA6) and polyamide 6,6 (PA66) in 25/75 and 75/25 w/w compositions by using a co-rotating twin-screw extruder.The materials have been completely characterized from a mechanical, rheological, and morphological point of view. The results indicate that the additives used cause an increase of the rupture properties and of the viscosity, especially in the PA6 rich blends containing CP-2EPOX. This result can be not only attributed to a chain extension effect on the PA phase but also to in situ formation of PA/PBT copolymers promoted by the presence of the CP compound as confirmed by NMR and MALDI-TOF analyses. The compatibilization effect fades in blends containing PA66, probably due to a thermal deactivation of the additives at higher temperature required to process this polymer. 相似文献
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10.
Maurizio Botta Francesco De Angelis Ingeborg Grgurina Mauro Marzi Rosario Nicoletti 《Journal of heterocyclic chemistry》1985,22(4):1001-1007
The reactivity of Δ3- and Δ2-3-bromomethylcephems toward carboxylate nucleophiles has been studied. The Δ3-bromomethylcephem 1 , less reactive than the Δ2-analogue 4 , is converted in high yields into 3-acyl-oxymethyl-3-cephems 2a-d , generally with no isomerization of the double bond, only within a narrow range of conditions. In particular, the Δ3-7-aminocephalosporanic acid (7-ACA) derivative 2a has been obtained as the only product in 91% yield by treatment of 1 with triethylammonium acetate in acetic acid. The Δ2-bromomethyl-cephem 4 is easily converted into the Δ2-acyloxymethyl-cepheras 5a-d without double bond isomerization, in very high yields. 相似文献