Free energy landscapes of prototropic tautomerism in pyridoxal 5′‐phosphate schiff bases at the active site of an enzyme in aqueous medium

We have performed hybrid quantum‐classical metadynamics simulations and quantum chemical calculations to investigate the free energy landscapes of intramolecular proton transfer and associated tautomeric equilibrium in pyridoxal 5 ‐phosphate (PLP) Schiff Bases, namely the internal and external aldimines, at the active site of serine hydroxymethyltransferase (SHMT) enzyme in aqueous medium. It is important to determine the relative stability of the two tautomers (ketoenamine and enolimine) of the PLP aldimines to study the catalytic activity of the concerned enzyme. Both the internal PLP aldimine (PLP‐LYS) and the external PLP aldimine (PLP‐SER) of SHMT are found to have a higher stability for the ketoenamine tautomer over the enolimine form. The higher stability of the ketoenamine tautomer can be attributed to the more number of favorable interactions of the ketoenamine form with its surroundings at the active site of the enzyme. The ketoenamine is found to be stabilized by about 2.5 kcal/mol in the PLP‐LYS internal aldimine, while this stabilization is about 6.7 kcal/mol for the PLP‐SER external aldimine at the active site of the enzyme compared to the corresponding enolimine forms. The interactions faced by the PLP aldimines at the active site pocket determine the relative dominance of the tautomers and could possibly alter the tautomeric shift in different PLP dependent enzymes. © 2018 Wiley Periodicals, Inc.     

Milk-clotting activity of cucumisin,a plant serine protease from melon fruit

Cucumisin (EC 3.4.21.25) isolated from prince melon fruit is a plant serine protease. Its milk-clotting activity was compared with plant cysteine proteases such as papain (EC 3.4.22.2) and ficain (EC 3.4.22.3). Cucumisin was more stable than papain under the condition of pH 7.1, 37‡C for 24 h. The milk-clotting activity of cucumisin was the same to that of papain and was half value of that of ficain.     

Retaining in‐gel zymographic activity of cysteine proteases via a cysteine‐supplemented running buffer

Zymography is a powerful technique to separate and identify different enzymatic activities on a standard acrylamide gel. For oxidation prone enzymes such as cysteine proteases however, the oxidizing species generated by electrolysis of the gel running buffer may result in partial or complete inactivation, thus compromising the final readout. This can be only partially remedied by subsequent treatment of the gel with reducing agents. We demonstrate the generation of reactive oxidizing species during electrophoresis and discovered that supplementation of the gel running buffer with a minimum of 5 mM cysteine prevents enzyme inactivation and allows retention of proteolytic activity as measured by zymography on model substrate N α‐benzoyl‐l ‐arginine p‐nitroanilide, without at the same time altering the mobilities of the gel proteins.     

Facile access to α,β-dehydroalanine and α,β-dehydroamino butyric acid derivatives from DL-serines and threonines

Not only α,β-dehydroamino acids are important constituents for a number of bioactive peptides in nature, but also they are important building blocks for a variety of synthetic amino acids in organic synthesis. Methods to prepare dehydroamino acids have been reported extensively in the literature; however, efficient and convenient protocols are still required. Here we have developed a convenient method to prepare dehydroalanine (ΔAla) and dehydroamino butyric acid (ΔAbu) derivatives derived from DL-serines and DL-threonines, respectively. 4-Toluenesulfonyl chloride (TsCl) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were employed in this procedure, which carried out activation of hydroxyl group and β-elimination in one pot. Because it is convenient and easy to handle, this method will attract the attention of synthetic chemists.     

Hydrazide functionalized magnetic nanoparticles for specific extraction of N‐terminal serine and threonine peptides

In this study, we present hydrazide functionalized magnetic nanoparticles as a sorbent prepared by a new and facile method. Scanning electron microscope and Fourier transform infrared were used for characterizing the synthesized nanoparticles. The ability of the sorbent to extract N‐terminal serine and threonine peptides was evaluated. The peptides were modified by oxidation of the hydroxyl group in the 1,2‐amino alcohol structure before extraction. These aldehyde‐forms of peptides were specifically bonded to the hydrazide groups of the sorbent. The formed hydrazone bonds were cleaved in the presence of hydroxylamine reagent. Finally, the oximated peptides were released and quantified with a high‐performance liquid chromatography–diode array spectroscopy. The effects of experimental parameters including extraction time, elution time and elution volume on extraction efficiency were also investigated. The required time for the extraction process to reach equilibrium and elution time was only 8 h. The adsorption efficiency of the sorbent was 79 and 77% for peptides with N‐terminal serine and threonine, respectively. The sorbent showed good specificity for extracting the peptides. In addition, the extraction efficiency of the sorbent remained constant in the presence of a non‐N‐terminal serine and threonine peptide as interference.     

丝氨酸-BrO^-~3-Mn^2^+-H2SO4体系振荡反应的研究

首次报道了间歇釜中以丝氨酸(Ser)-BrO^-~3-Mn^2^+-H2SO4为体系(简称Ser-BZ体系)的新型BZ类振荡反应,其特征如下：(1)虽然Ser不能发生溴代反应,但即使在无丙酮或惰性气体流时也能在间歇釜中观察到持续振荡;(2)振荡诱导期极短(～0),振荡次数较少(<11次);(3)振荡反应受到Cl^-,Br^-,丙烯腈等的抑制;但当加入足够量Ag^+使[Br^-]的振荡抑制后,仍可在Pt电极上观察到振荡现象。根据上述特征和反应产物分析,推测Ser-BZ振荡反应可能是自由基-控制模型,而非Br^--控制模型。加入适量丙酮可诱导连续振荡反应,归因于两种控制模型的共存。通过对Mn^3^+-Ser和BrO^-~3-Mn^2^+反应的动力学研究,并结合FKN机理,对Ser-BZ振荡反应机理进行了初步讨论。     

Enantiomeric separation of d‐ and l‐serine using high‐performance liquid chromatography with electrochemical detection following pre‐column diastereomer derivatization

Enantiomeric separation of d ‐ and l ‐serine on an octadecylsilica column was investigated using (2R)‐2,5‐dioxopyrrolidin‐1‐yl‐2,5,7,8‐tetramethyl‐6‐(tetrahydro‐2H‐pyran‐2‐yloxy)chroman‐2‐carboxylate (R‐NPCA), which was developed for a pre‐column derivatization reagent for electrochemical detection. In addition, (2S)‐2,5‐dioxopyrrolidin‐1‐yl‐2,5,7,8‐tetramethyl‐6‐(tetrahydro‐2H‐pyran‐2‐yloxy)chroman‐2‐carboxylate (S‐NPCA) was newly synthesized from (S)‐(?)‐6‐hydroxy‐2,5,7,8‐tetramethylchroman‐2‐carboxylic acid (S‐α‐CA), and the enantiomeric separation of d ‐ and l ‐serine using S‐NPCA was also examined. The enantiomeric separation of d ,l ‐serine was achieved using the R‐ or S‐NPCA as a chiral derivatization reagent, and the elution orders of the enantiomers were reversed between R‐ and S‐NPCA. The elution orders of d ‐ and l ‐serine unexpectedly reversed between the phosphate buffer at pH 4.0 and pH 2.2, both of which were used in the mobile phase. Separation factors obtained using R‐ and S‐NPCA were similar—1.09 and 1.07, respectively. The detection limit was approximately 940 fmol on the column (signal‐to‐noise ratio 3) when the applied voltage was +650 mV. Copyright © 2009 John Wiley & Sons, Ltd.     

屈昔多巴的合成

以3,4-二羟基苯甲醛为原料,经苄基保护、与甘氨酸缩合、苄氧羰基保护得到关键中间体外消旋苏赤式-3-(3,4-二苄氧基苯基)-N-苄氧羰基丝氨酸(3);3经二环己胺和S-2-氨基-1,1-二苯基-1-丙醇拆分得到屈昔多巴,总收率11.3%,其结构经1H NMR,MS和元素分析确证。