2078-12-8Relevant articles and documents
Identification of an acyl-enzyme intermediate in a meta-cleavage product hydrolase reveals the versatility of the catalytic triad
Ruzzini, Antonio C.,Ghosh, Subhangi,Horsman, Geoff P.,Foster, Leonard J.,Bolin, Jeffrey T.,Eltis, Lindsay D.
, p. 4615 - 4624 (2012)
Meta-cleavage product (MCP) hydrolases are members of the α/β-hydrolase superfamily that utilize a Ser-His-Asp triad to catalyze the hydrolysis of a C-C bond. BphD, the MCP hydrolase from the biphenyl degradation pathway, hydrolyzes 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to 2-hydroxypenta-2,4-dienoic acid (HPD) and benzoate. A 1.6 A resolution crystal structure of BphD H265Q incubated with HOPDA revealed that the enzyme's catalytic serine was benzoylated. The acyl-enzyme is stabilized by hydrogen bonding from the amide backbone of 'oxyanion hole' residues, consistent with formation of a tetrahedral oxyanion during nucleophilic attack by Ser112. Chemical quench and mass spectrometry studies substantiated the formation and decay of a Ser112-benzoyl species in wild-type BphD on a time scale consistent with turnover and incorporation of a single equivalent of 18O into the benzoate produced during hydrolysis in H218O. Rapid-scanning kinetic studies indicated that the catalytic histidine contributes to the rate of acylation by only an order of magnitude, but affects the rate of deacylation by over 5 orders of magnitude. The orange-colored catalytic intermediate, ESred, previously detected in the wild-type enzyme and proposed herein to be a carbanion, was not observed during hydrolysis by H265Q. In the newly proposed mechanism, the carbanion abstracts a proton from Ser112, thereby completing tautomerization and generating a serinate for nucleophilic attack on the C6-carbonyl. Finally, quantification of an observed pre-steady-state kinetic burst suggests that BphD is a half-site reactive enzyme. While the updated catalytic mechanism shares features with the serine proteases, MCP hydrolase-specific chemistry highlights the versatility of the Ser-His-Asp triad.
Trimethylsilyl Esters as Novel Dual-Purpose Protecting Reagents
Chang, Ting-Shuo,Chen, Jyun-Siao,Hsieh, Ya-Chi,Hsu, Hsiao-Lin,Huang, Po-Hsun,Liu, Jen-Wei,Liu, Yu-Hao,Luo, Shun-Yuan,Wu, Hsin-Ru,Wu, Ren-Tsung,Zhang, Kai-Min
supporting information, (2021/12/02)
Trimethylsilyl esters, AcOTMS, BzOTMS, TCAOTMS, etc., are inexpensive and chemically stable reagents that pose a negligible environmental hazard. Such compounds prove to serve as efficient dualpurpose reagents to respectively achieve acylation and trimethylsilylation of alcohols under acidic or basic conditions. Herein, a detailed study on protection of various substrates and new methodological investigations is described.
IMINE-TYPE QUATERNARY AMMONIUM SALT CATALYST, PREPARATION METHOD THEREOF AND USE THEREOF FOR PREPARATION OF POLYISOCYANATE COMPOSITION
-
Paragraph 0022; 0077; 0079, (2020/12/13)
Disclosed is an imine-type quaternary ammonium salt catalyst, wherein the catalyst has a general structure formula shown by formula I below; in the formula, R1 and R2, respectively, are independently selected from a C1-C20 linear alkyl or a branched C3-C20 alkyl, and a C1-C20 hydroxylalkyl, a C3-C8 cycloalkyl, and arylated alkyl; R3 is a linear or branched alkyl, cycloalkyl or aryl; and R4 is hydrogen, aryl, a linear C1-C15 alkyl or branched C3-C15 alkyl. Also disclosed are a method for preparing the catalyst and a polyisocyanate composition prepared therefrom. The catalyst, by introducing an imine structure, on the basis of ensuring high catalytic activity thereof, is allowed to have properties of high temperature decomposition and inactivation, and when applied to the synthesis of polyisocyanate, can effectively prevent the risk of explosive polymerization caused by an uncontrolled reaction.
A simple and efficient room temperature silylation of diverse functional groups with hexamethyldisilazane using CeO2 nanoparticles as solid catalysts
Anbu, Nagaraj,Vijayan, Chellappa,Dhakshinamoorthy, Amarajothi
, (2019/06/08)
In this study, a mild and efficient method is developed for the silylation of diverse functional groups using CeO2 nanoparticles (n-CeO2) as solid catalysts with hexamethyldisilazane (HMDS) as silylating agent at room temperature. Alcohols, phenols and acids are silylated to their respective silyl derivatives with faster reaction rate while amines and thiols required relatively longer reaction time. Moreover, the solid catalyst is easily be separated from the reaction mixture and recycled more than five times without any obvious decay in its activity. Powder X-ray diffraction (XRD), transmission electron microscope (TEM), UV–vis diffuse reflectance spectra (UV-DRS) and Raman analyses revealed identical structural integrity, particle size, absorption edge and valence state for the reused solid compared to the fresh solid catalyst.
