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71.
72.
Jeffrey M. Coles Debby P. Chang Stefan Zauscher 《Current Opinion in Colloid & Interface Science》2010,15(6):406-416
Mucins have long been recognized as instrumental to biolubrication but the molecular details of their lubrication mechanisms have only been explored relatively recently. The glycoprotein PRG4, also known as lubricin, shares many features with mucins and appears to lubricate through similar mechanisms. A number of studies have contributed to a more in-depth understanding of mucin adsorption and layer formation on surfaces and the mechanisms by which these layers lubricate. Although mucinous glycoproteins differ in their aggregation properties, their adsorption behaviors on surfaces, and in their ability to reduce friction, they share important similarities favorable for lubrication. They are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are both sterically and electrostatically repulsive, all attributes thought to contribute to boundary lubrication. They also hydrophilize hydrophobic surfaces, promoting the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. In this paper we briefly review current knowledge of mucin adsorption and lubrication, with a focus on recent advances. 相似文献
73.
苯并硫氮杂是一类具有重要生理活性的七元杂环化合物,近年来,通过亚胺双键(CN)的环加成反应向此类化合物分子中引入新的环系引起了人们的关注[1~4].这项工作对于合成新的杂类化合物从而筛选出具有特殊生理活性的物质有着重要意义.本文通过1,5-苯并... 相似文献
74.
The overall topic of the investigation was the separation of basic proteins by cation exchange displacement chromatography. For this purpose two principal column morphologies were compared for the separation of ribonuclease A and alpha-chymotrypsinogen, two proteins found in the bovine pancreas. These were a column packed with porous particles (Macro-Prep S, 10 microm, 1000 A) and a monolithic column (UNO S1). Both columns are strong cation exchangers, carrying -SO3(-)-groups linked to a hydrophilic polymer support. Poly(diallyl-dimethylammonium chloride) (PDADMAC), a linear cationic polyelectrolyte composed of 100-200 quaternary pyrrolidinium rings, was used as displacer. The steric mass action (SMA) model and, in particular, the operating regime and dynamic affinity plots were used to aid method development. To date the SMA model has been applied primarily to simulate non-linear displacement chromatography of proteins using low molar mass displacers. Here, the model is applied to polyelectrolytes with a molar mass below 20000 g mol(-1), which corresponds to a degree of polymerization below 125 and an average contour length of less than 60 nm. The columns were characterized in terms of the adsorption isotherms (affinity, capacity) of the investigated proteins and the displacer. 相似文献
75.
Zhaofu Fei 《Journal of fluorine chemistry》2003,121(1):79-81
The title compound RFC(O)NC(NMe2)2 (2) was prepared in virtually quantitative yield by reacting RFC(O)Cl with two equivalents of N,N,N′,N′-tetramethylguanidine and structurally characterized by X-ray diffraction. Due to severe steric crowding, the carbonyl group approaches a perpendicular arrangement (torsion angle of 70.1(2)°) with respect to the phenyl ring. 相似文献
76.
Alexander Zeller 《Journal of organometallic chemistry》2005,690(13):3292-3299
Via NMR-spectroscopy the relative reactivity of N-heterocyclic silylenes (NHSi) and carbenes (NHC) was studied. Reaction of sterically crowded bis-N-heterocyclic Pd(0) carbene complexes with free N-heterocyclic silylenes led to complete displacement of the N-heterocyclic carbene, which is unexpected knowing that usually a silylene is a weaker bound ligand compared to a carbene. High-level DFT calculations on a small model system and the experimentally used complexes confirm the experimental findings and indicate that steric interactions play an important role in the substitution reaction. 相似文献
77.
Hideo Sakurai 《Journal of organometallic chemistry》2004,689(13):2236-2241
C-Si reductive elimination from Pt(R)(SiPh3)(PMe3)2 (R=Me, Pr) was theoretically studied with the density functional theory. For comparisons with the experiment, substitution of PMe3 with diphenylacetylene was taken into account. The calculated activation barriers in the C-Si elimination step after the ligand exchange were 22.0 and 28.9 kcal mol−1 for R=Me and Pr, respectively, which explains the reactivity difference reported experimentally. In order to analyze the energy difference, we optimized transition states of several model complexes, and examined the influence of the steric repulsion between R and the other ligands. Comparisons of the geometries and the barrier heights reveal that the steric repulsion and the Si-alkyl bond energy are important factors controlling the reaction rate. 相似文献
78.
Wanda Ziemkowska Romana Anulewicz-Ostrowska 《Journal of organometallic chemistry》2004,689(12):2056-2065
The dependence of the structure of complexes of sterically crowded 2,4-dimethylpentane-2,4-diol with group 13 metals trialkyls on the kind of metal, as well as steric bulk of the substituents on the metal atoms is reported. The reaction of tBu3Ga with 2,4-dimethylpentane-2,4-diol leads to the formation of an unstable dimeric product {tBu2Ga[(OC(CH3)2CH2C(CH3)2OH]}2 (1) possessing a four-membered Ga2O2 core and two unreacted hydroxyl groups. Compound 1 undergoes further intramolecular reaction to yield the unusual (monoalkyl)gallane O,O′-chelate complex {tBuGa[OC(CH3)2CH2C(CH3)2O]}2 (2). In contrast to tBu3Ga, tBu3In reacts with 2,4-dimethylpentane-2,4-diol to give the stable dimeric complex tBu4In2[OC(CH3)2CH2C(CH3)2OH]2 (4) stabilised by two intramolecular O-H?O bonds. At higher temperature compound 4 reacts with an excess of tBu3In to form the trinuclear complex tBu5In3[OC(CH3)2CH2C(CH3)2O]2 (5). The reactions of 2,4-dimethylpentane-2,4-diol with trialkylmetallane with small alkyl groups, i.e. Me3Ga and Me3In allow for the isolation of the trinuclear diolates {Me5M3[OC(CH3)2CH2C(CH3)2O]2} [M=Ga (3), M=In (6)]. The crystal structures of 2, 3 and 4 have been determined by single crystal X-ray diffraction. The reactions of tert-butylmetallane diolates with trimethyl metallanes have been studied. The interaction of the allane complex {tBuAl[OC(CH3)2CH2C(CH3)2O]}2 with Me3Al results in the formation of the trialuminium mixed-ligand product {Me3(tBu)2Al3[OC(CH3)2CH2C(CH3)2O]2} (7). Compounds 2 and 4 undergo a total transmetallation reaction in the presence of Me3M to yield [Me5M3(diol-(2H))2] [M=Al, Ga] products. 相似文献
79.
Three urea derivatives of ergoline-based chiral selectors (CSs), differing in the size of the urea side chain, i.e. dimethyl- (CSI), diethyl- (CSII), and diisopropylurea (CSIII), were used to study the effect of steric hindrance on the enantioseparation of dansyl amino acids (Dns-AAs), pesticides, and mandelic acid under condition of capillary electrophoresis (CE) in linear polyacrylamide coated capillaries. A mixture of organic modifiers (MeOH/THF, 4:1 v/v) in a BGE consisting of 100 mM beta-alanine-acetate was used to increase the solubility of CSs up to 25 mM. The capillary was filled with CS (high UV absorption), and the inlet and outlet vials contained buffer solutions only. The best enantioseparation of Dns-AAs was achieved on CSI. Increased steric hindrance of the chiral binding site led to reduction of both enantioselectivity and resolution. The opposite pattern was observed for the separation of mandelic acid enantiomers, where the best enantioseparation and resolution was obtained with CSIII. Most of the pesticides studied reached maximum selectivity on the diethylurea ergoline derivative (CSII). Enantioseparation of fenoxaprop was found to be independent of steric hindrance. 相似文献
80.
The molecular electrostatic potential of yeast tRNAPhe is calculated at sites bridging the anionic oxygens of each of the 76 phosphate groups of the molecule. A quantitative measure of the steric accessibility of the anionic oxygens of the phosphates toward a spherical cation is presented. Both the resulting potentials and accessibilities are discussed in terms of the molecular and electronic structure of tRNA. 相似文献