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  • br Ultrasonic pretreatment of substrates Substrate pretreatm

    2019-07-11


    Ultrasonic pretreatment of substrates Substrate pretreatment is widely used in the biofuel, textile and food industries since the substrates are always difficult to degrade. During some enzymatic hydrolysis reactions, the protective layer of the substrate impedes the reaction. Ultrasonic treatment could destroy the aggregation of the substrate and remove the indigestible cuticle, making the substrate more vulnerable to attack by enzymes [65]. Mechanical effects can unfold the substrates\' structures to change the conformation for an easier combination with enzymes [66]. In addition, ultrasound could degrade substrates and decrease the degree of polymerization directly. During depolymerization, homolytic and/or heterolytic cleavage of a Methoxy-X04 weight may occur, and the breakage of a CC bond in the macromolecule is the most common mechanism [67]. Ultrasound-induced degradation always occurs preferentially near the middle of the chain. Furthermore, the decrease in the particle size can enhance the mass transfer and accelerate the reaction [68]. Therefore, the efficiency and rate of enzymatic reactions as well as the product yield are significantly stimulated.
    Ultrasound assisted enzymatic reactions The enzymatic reaction is always the critical step during many processes, but it is relatively cost- and rate-limited [75]. Conventional enzymatic methods require a significantly long time, and the degree of the hydrolysis always has limitations. Using other techniques to assist enzymatic reactions has attracted much interest. Ultrasound has been known to be used for the intensification of several physical, chemical and biological processes, including enzymatic reactions [76]. Studies on ultrasound assisted enzymatic reactions always aim at accelerating the reaction rate. Ultrasound assisted enzymatic reactions are credible for both solid-liquid-phase systems and liquid-liquid-phase systems (as shown in Table 3).
    Conclusions
    Acknowledgements This work was financially supported by the National Key Research and Development Program of China (grant 2016YFD0400301) and the Key Research and Development Program of Zhejiang Province (grant 2017C02015).
    Introduction Synthetic enzyme substrates are utilised extensively in diagnostic clinical microbiology for the purpose of detecting and identifying pathogenic microorganisms.1, 2, 3 These substrates are designed to target microbiological species of interest (or groups of species) based upon their enzyme activity. An important sub-class of synthetic enzyme substrates are the chromogenic sugar-based enzyme substrates in which hydrolytic cleavage of the sugar moiety from the aglycone is mediated by an appropriate enzyme resulting in the liberation of a hydroxyaryl derivative, as shown by the representative examples in Scheme 1. The hydroxyaryl derivative can be coloured, thus allowing direct visualisation of the hydrolytic reaction as illustrated by the transformation of the colourless ortho-nitrophenyl β-d-galactopyranoside 1 into the yellow-coloured ortho-nitrophenol (ONP) (Eqn 1). Alternatively, if the liberated hydroxyaryl derivative is colourless, a subsequent chemical reaction can be employed to produce a coloured product. Thus, the β-galactoside derivative of 5-bromo-4-chloro-3-hydroxyindole (‘X-gal’) 2 is hydrolysed to produce 5-bromo-4-chloro-3-hydroxyindole which then undergoes an oxidative dimerization in air producing the blue-coloured indigo derivative 3 (Eq. 2). The indole-derived substrate, ALDOLTM 455 4, similarly generates a reactive 3-hydroxyindole intermediate which participates in a subsequent non-oxidative intramolecular aldol condensation yielding the yellow chromophore 5 (Eq. 3). Sugar-based chromogenic substrates have also been designed around a glycosidated catechol moiety (Fig. 1). After enzymatic hydrolysis of the substrate, the resulting catechol aglycone undergoes chelation with metal ions that have been incorporated into the medium, therefore producing coloured metal-chelates. The hydrolysis of esculin 6 by a β-glucosidase enzyme yields d-glucose and esculetin (6,7-dihydroxycoumarin) which, in the presence of iron salts, produced a brown/black complex. Cyclohexenoesculetin-β-d-glucoside 78, 9 (and also its β-d-galactoside derivative) similarly generated a black complex in the presence of iron salts. Alizarin β-d-glucoside 89 (and also its β-d-galactoside derivative) yielded a purple-coloured chelate in the presence of iron salts and a pink-coloured chelate with aluminium salts. Hydrolysis of 3′,4′-dihydroxyflavone β-d-ribofuranoside gave black colonies in the presence of iron and yellow colonies in the presence of aluminium. Other sugar-based substrates, which after enzymatic hydrolysis produce aglycones capable of chelation with metal ions, have also been prepared from non-catechol cores including glycosides of 8-hydroxyquinoline and 3-hydroxyflavone.