Oxidation, basic conditions enzymatic

An excellent review on protein hydrolysis for amino acid composition analysis has been published by Eountoulakis and Lahm [190], Hydrolysis can be performed by either chemical (under either acidic or basic conditions) or enzymatic means. The acidic hydrolysis itself can be carried out in a liquid or a gas-phase mode. The conventional acid hydrolysis uses 6M HCl for 20-24 h at 110°C under vacuum [200], In these conditions, asparagine and glutamine are completely hydrolyzed to aspartic acid and glutamic acid, respectively. Tryptophan is completely destroyed (particularly in the presence of high concentrations of carbohydrate), while cysteine and sometimes methionine are partially oxidized. Tyrosine, serine, and threonine are partially destroyed or hydrolyzed and correction factors have to be applied for precise quantification [190,201],... 

Application of dynamic kinetic resolution has also been reported in a whole-cell enzymatic Baeyer-Villiger process. While this was initially done using slightly basic conditions, Furstoss and Alphand have more recently reported the use of a weakly basic anion exchange resin to promote racemization of the slow oxidizing enantiomer 99 to the fast enantiomer 100.53 Baeyer-Villiger oxidation using recombinant E. coli to overexpress the CHMO from A. calcoaceticus provided excellent yield and % ee of lactone 101. 

Several prodrugs of Hourouracil were obtained by acylation or carbamoylation of N-1 and/or N-3 atoms of the pyrimidine ring of 1. In particniar, an oral drug Carmofur (2) which is 1-hexylcarbamoyl derivative of 1 was launched in Japan in 1981 and later - in other countries [35], The carbamate moiety in 2 decomposes gradually in neutral water or in basic conditions, but it is strongly resistant to acidic hydrolysis and hence can survive acid in the stomach. The 1-hexylcarbamoyl moiety also facilitates the rapid uptake of 2 through the cell membrane [36]. The metabolic activation of Carmofur involves oxidation and scission of the side-chain with slow release of 1 [37]. Two main routes of the side chain transformation are (o-oxidation and ((o-l)-oxidation metabolites 40-43 were detected after adnunis-tration of Carmofur (Fig. 3) [38]. Non-enzymatic hydrolytic decomposition of 2 and its metabolites also contributes to release of 1. 

Chitin is a stable compoimd, incompatible with oxidizing agents [59]. In the solid state imder alkaline condition (e.g., NaOH, KOH, heat at about 120 °C) or by enzymatic hydrolysis in the presence of a chitin deacetylase, it hydrolyses to form the deacetylated degradation product chitosan [6,7,10,11]. It was found that the presence of urea in basic media and at low temperature (—20 °C) had little effect on chitin structure and that urea is of benefit to the stability of chitin solution [38]. 

Basic and applied research efforts show that the oxidative transformation of renewable feedstocks represents a promising approach for the production of high-value chemicals under environmentally acceptable conditions. Many laboratory-scale processes are carried out under mild conditions, and the employment of molecular oxygen ensures ecological and economic advantages. In these processes, a crucial role is played by the catalytic step that requires a specific selectivity toward the desired product. In principle, either enzymatic catalysis or chemical catalysis can be effectively employed in a given process and, in some cases, the performance of the different processes is quite similar as presented in the following sections. 

This type of biosensor basically depends on electrocatalytic activity of the enzymes. Although nonenzymatic biosensors have many advantages over enzyme-based biosensors, which have been discussed in the earlier section, some nonenzymatic biosensors need harsh conditions, such as high pH (aqueous solution of NaOH) which some time hinder many real sample analyses. Moreover, many target analytes cannot be oxidized or reduced at low potential for their selective and sensitive determination on the non-enzymatic electrode surfaces, whereas the immobilized enzyme on the suitable electrode surface can generate electroactive molecules which can be electrochemically... 

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