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Polypeptides
Published in Stanislaw Penczek, H. R. Kricheldorf, A. Le Borgne, N. Spassky, T. Uryu, P. Klosinski, Models of Biopolymers by Ring-Opening Polymerization, 2018
Finally, the question needs to be discussed whether the stereochemical course of polymerizations of racemic d,l-NCAs is better called stereospecific, stereoselective, or enantioselective. Since the reaction between a peptide chain end and d- or l-NCA is a reaction between two chiral compounds it resembles enzymatic reactions, and this is one reason why the author prefers the term stereospecificity which originates from the classification of enzymatic reactions in biochemistry. Another reason is that most textbooks of organic chemistry use the term stereospecificity for reaction of a stereochemically differentiated substrate to stereochemically differentiated products (e.g., l,d-(A) + l,d-(B)→l-(A)-l-(B) + d-(A)-d-(B)).205-207 In contrast, a stereoselective reaction is a conversion of a prochiral substrate, without stereoisomers (e.g., an α-olefin) into chiral products. In this connection, it is to be emphasized that stereospecificity is not an alternative term for 100% stereoselectivity.
Enzymes—Kinetics of Enzymatic Reactions
Published in Jean-Louis Burgot, Thermodynamics in Bioenergetics, 2019
Recall that if any reaction leads predominantly or exclusively to only a set of stereoiomers, it is named stereoselective. In a stereospecific reaction, a given isomer leads to one product, while another stereoisomer leads to the opposite product. While all stereospecific reactions are stereoselective, the converse is not true. – Enzymes, in principle, react with only a single type of functional group. Hence, they show chemoselectivity;– They also exhibit regioselectivity and diastereoselectivity. This is due to their three-dimensional structure (see under). When a reaction can potentially give rise to two or more structural isomers but actually produces only one, the reaction is said to be regioselective. Any reaction in which only one of a set of stereoisomers is formed, exclusively or predominantly, it is named stereoselective.– Almost all enzymes are chiral. They are chiral catalysts. This is because they are constituted by L-amino acids (see appendix I-4). The consequence is that any kind of chirality present in the substrate molecule is recognized when the intermediary “complex” enzyme-substrate is formed. Hence, as example, a prochiral substrate may be transformed into an optically active product and both enantiomers of a racemic may react at different rates offering the possibility of kinetic resolution. This property is one of the most remarkable features of enzymes. Most of the time, enzymes are enantioselective.
The generation and reactions of sulfenate anions. An update
Published in Journal of Sulfur Chemistry, 2022
Adam B. Riddell, Matthew R. A. Smith, Adrian L. Schwan
The simple alkylation chemistry has advanced from a sulfenate confirmation tool to one that can evaluate, optimize and/or utilize factors that govern stereoselective alkylation reactions. One of the first examples was the diastereomeric alkylation of an arenesulfenate with a pendant ortho-dimethylaminomethyl group [86]. The sulfenate was generated by monoxidation of an arenethiolate using a specifically developed N-sulfonyl oxaziridine and four electrophiles were employed, delivering diastereomeric ratios up to 98:2 for benzylation. It was proposed that amino nitrogen to sulfenate lithium complexation created two different S-alkylation options for the sulfur, and the methyl directed up from the chiral carbon hindered alkylation on the upper face (Scheme 40) [86].
Stabilization of α-ZrP ceramic nanosheets adsorbing quaternary ammonium ions in organic solvents and their application as a stable solid support for lipase catalyzing stereospecific synthetic reactions
Published in Journal of Asian Ceramic Societies, 2022
Akane Yamada, Gen Onodera, Masanari Kimura, Kai Kamada
In general, enzymatic synthetic reactions provide high substrate and reaction route specificity. Moreover, they proceed under moderate reaction conditions such as atmospheric pressure, neutral pH, and moderate temperature [1]. To date, lipases that promote hydrolytic decomposition of lipids in living bodies have been frequently used for enzymatic asymmetric synthesis. Enzymes have also been applied toward the synthesis of various enantiopure compounds from racemic mixtures. For example, there have been so many reporting lipase-catalyzed resolutions from racemic mixture and diastereomeric mixtures via ester hydrolysis and transesterification reactions [1–15]. Needless to say, these enzymatic syntheses contribute to the progress of clean synthetic chemistry, which is highly desirable in modern chemical industry. Although lipases carry out regio- and stereoselective synthetic reactions at moderate temperatures, they show relatively low chemical durability and dispersibility in the solutions frequently used in synthetic chemistry (acids, bases, and organic solvents).
High dye removal capacity of Peniophora laxitexta immobilized in a combined support based on polyurethane foam and lignocellulosic substrates
Published in Environmental Technology, 2022
Leonardo Majul, Sonia Wirth, Laura Levin
The textile dyeing and finishing industry contributes seriously to the pollution of surface and groundwater resources mainly by the release of dyes, heavy metals, detergents and organic solvents. Most synthetic dyes are not only highly toxic, mutagenic and carcinogenic but also increase biochemical and chemical oxygen demand and reduce light penetration in water bodies, affecting photosynthesis [1]. Dye removal can be achieved by biological, chemical, or physical treatments, however enzymatic degradation (biological) and adsorption (physical) are currently among the most promising methods [2,3]. Among the microorganisms with dye removal capacity, ligninolytic white-rot fungi and their enzymes have been successfully applied for the treatment of effluents containing textile dyes [4,5]. These organisms are able to degrade a wide range of recalcitrant pollutants through their extracellular non-specific and non-stereoselective ligninolytic enzyme system composed mainly by laccases and several peroxidases, such as Mn-peroxidase, lignin-peroxidase and versatile peroxidase [6]. On the other hand, various lignocellulosic waste materials such as spent grain and wood sawdust, as well as fungal biosorption were evaluated as low-cost and high binding capacity adsorbents for the removal of synthetic dyes [7,8], and the combination of these physical and biological adsorbents produced outstanding results in dye removal [2,9]. One way to combine these strategies is by immobilizing ligninolytic fungi on supports that act as physical adsorbents.