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441.
Addition of Zn(n-Bu(3)Sn)(2) to prochiral aldehydes affords anti-alpha,beta-dialkoxy- and anti-alpha-alkoxy-beta-aminostannanes in good yield (up to 77%) and excellent diastereoselectivity (up to 98% de). syn-Isomers are accessed from the initial adducts via Mitsunobu inversion/saponification. The corresponding thionocarbamates undergo mild Cu(I)-mediated cross-coupling with a variety of organic halides, inter alia, allylic, cinnamylic, propargylic, and acetylenic, with retention of configuration. [reaction: see text]  相似文献   
442.
Aristolindiquinone (1) , the first naturally occurring 8-methyl-juglone earlier isolated from Aristolochia indica has been synthesised from o-allyl-p-cresol in 7% overall yield.  相似文献   
443.
The mechanisms of the aging process of tabun-conjugated acetylcholinesterase were explored using density functional theory calculations. The free energy surfaces were calculated for O-dealkylation (C–O bond breaking) and deamination (P–N bond breaking) pathways for the aging process of tabun-conjugated acetylcholinesterase as suggested by mass and crystallographic studies. Initially, the calculations were performed using tabun-conjugated serine (SUN) molecule. O-dealkylation mechanism proceeds via one-step SN2 type process, whereas the deamination process proceeds via two steps addition–elimination reaction at the phosphorus center of SUN molecule. The recent proposal of another deamination mechanism using human butyrylcholinesterase (hBChE) conjugated with N-mono methyl analogue of tabun (TA4) has also been explored (Nachon et al. in Chem Biol Interact 187:44–48, 2010). The rate-determining activation barrier calculated for this deamination mechanism (26.3 kcal/mol) was comparable with O-dealkylation process (26.9 kcal/mol) with B3LYP/6-31+G* level of theory. To examine the influence of catalytic residue His447, additional calculations were performed with imidazole group of His447 residue. The incorporation of imidazole group of catalytic residue His447 showed marked decrease in the free energies of activation for all the studied aging processes of tabun-inhibited serine. The aging mechanisms have been explored with TA4-inhibited serine, and calculated results showed that the deamination with the rearrangement process is markedly preferred in this case, which supports the Nachon et al. proposal based on the crystallographic studies.  相似文献   
444.
Titanium dioxide nanoparticles (NPs) were synthesized by a sol-gel method from hexafluorotitanic acid using poly(ethylene glycol) as a capping agent. The crystal structure and morphology of the NPs were characterized by X-ray diffraction, FESEM, and TEM. The NPs were used to modify a graphite paste electrode for simultaneous determination of uric acid (UA) and guanine (GU). The effect of calcination temperature on crystal structure and electrocatalytic activity was investigated. The electrochemical responses to UA and GU at bare GP, TiO2–350/GP, and TiO2–600/GP electrodes were compared. The DPV oxidation peaks of UA and GU were found to be strongest at around 304 and 673 mV, respectively, against Ag/AgCl reference electrode, and this are well separated for effective simultaneous determination. UA and GU can be simultaneously determined by this method. Response is linear within the range 0.1–500 μM and 0.1–40 μM for UA and GU, respectively. The detection limits are 70 nM for UA and 50 nM for GU (at an S/N? ratio of?3). The TiO2–600/GP electrode showed excellent analytical performance when analyzing spiked urine and serum samples.
Graphical abstract A graphical representation of cubic TiO2 nanoparticle formation during hydrolysis through sol-gel process.
  相似文献   
445.
An atomistic molecular dynamics simulation of the protein villin headpiece subdomain or HP-36 has been carried out with explicit water to explore the microscopic inhomogeneity of local density reorganization of the hydration layers of the three alpha-helical segments of the protein. The density reorganization of the hydration layer of helix-3 is found to occur faster than that for the hydration layers of the other two helices. It is noticed that such inhomogeneous density reorganization at the surface of different secondary structures exhibits excellent correlation with the microscopic dynamics of hydrogen bonds between the protein residues and the hydration water. Further, it is observed that the reorientation of water molecules involved in the formation and breaking of protein-water or water-water hydrogen bonds plays an important role in determining the dynamics of local density of the hydration layer. The faster density reorganization of the hydration layer of helix-3 is also consistent with the functionality of HP-36, as helix-3 contains several active site residues.  相似文献   
446.
Protein function prediction is one of the central problems in computational biology. We present a novel automated protein structure-based function prediction method using libraries of local residue packing patterns that are common to most proteins in a known functional family. Critical to this approach is the representation of a protein structure as a graph where residue vertices (residue name used as a vertex label) are connected by geometrical proximity edges. The approach employs two steps. First, it uses a fast subgraph mining algorithm to find all occurrences of family-specific labeled subgraphs for all well characterized protein structural and functional families. Second, it queries a new structure for occurrences of a set of motifs characteristic of a known family, using a graph index to speed up Ullman’s subgraph isomorphism algorithm. The confidence of function inference from structure depends on the number of family-specific motifs found in the query structure compared with their distribution in a large non-redundant database of proteins. This method can assign a new structure to a specific functional family in cases where sequence alignments, sequence patterns, structural superposition and active site templates fail to provide accurate annotation.  相似文献   
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