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    • The acid base properties of the

    2020-01-14

    3. The acid-base properties of the IL are important issues to consider because protonation changes caused by Brønsted-acidic or basic ions may result in enzyme deactivation [29]. Thereby, the acid-base properties of the newly designed IL need proper control for enzymes by selection of appropriate cation and anion types. Biocompatible ILs were reported by several groups [13], [30], [31], [32], [33], [34], [35], [36], [37], [38] including our own [16]. Cations of these ILs do have a functional group with high AN and DN values, but their anions do not. Generally, both the cation and anion of an IL function cooperatively to affect the enzyme activity. We speculate that incorporation of a hydroxyl-functionality in both cationic and anionic units with high dielectric constant values may be advantageous for dissolving and ionizing enzymes. In addition, the newly designed ILs need to provide suitable acid-base environment for the enzymes. Enzymes tested in this work are lipases from Candida antarctica (CAL) and Pseudomonas cepacia (PCL) functioning in neutral environment, therefore, the newly designed ILs should be neutral. Following this rationale, we designed and synthesized two series of ILs (Scheme 1), hydroxyalkyl imidazolium hydroxyalkyl sulphonate ([CmOHMIM][HOCnSO3]) and hydroxyalkyl tri(hydroxyethyl) ammonium hydroxylalkyl sulphonate ([CmOHTEA][HOCnSO3]). We then determined their ionizing–dissociating abilities and their compatibility with lipase activity, in order to test the above hypothesis.
    Materials and methods
    Results and discussion
    Conclusions In conclusion, we developed a hypothesis that ionizing–dissociating abilities of an IL determine the activity of enzymes, and proposed basic principles for biocompatible IL design. The new biocompatible ILs need to possess a hydroxyl group with high DN and AN value in both cationic and anionic units, which units have high dielectric constant lead structure. In addition, the newly designed IL is a salt of a strong SBHA mg and a strong base, providing neutral environment for non-aqueous biocatalysis. The ionizing–dissociating abilities of ILs (K value) paralleled the catalytic activity trend of lipases dissolved in the ILs and the activities of lipases in the newly designed ILs were comparable to those in water. We can conclude that ionizing–dissociating abilities of an IL can be used as a basis for predicting the activity of enzymes dissolved in ILs.
    Acknowledgments We acknowledge the financial supports from the Education Department of Fujian Province, the Science Foundation of Jimei University, the NSF of Fujian Province (2015J01167), and the NSFC (41576085).
    Introduction Organophosphorus compounds are a group of highly toxic substances which are extremely applied as pesticides, insecticides and chemical warfare agents [1]. Extensive uses of these compounds enhanced general concerns because of their adverse effects on human health and environment by contamination of soil, sediments and groundwater. Considering the recent progression in bio science, the use of microorganisms for degradation of these toxic substances as an economic and less disruptive way in comparison to other conventional methods would appear to be very interesting [2]. Organophosphorus hydrolase (OPH) (EC 3.1.8.1) is a biocatalyst which is capable to hydrolyze various organophosphorus compounds. However, low thermostability and specification of this enzyme restricted its industrial applications substantially [3]. Therefore, different techniques such as enzyme immobilization are used currently to make the enzyme more stable even under harsh conditions. Immobilization will improve the enzyme characteristics by keeping its natural catalytic surrounding for repeated operations. The immobilized enzymes would have higher endurance in organic solvents and are capable to reuse for several batches of hydrolysis reactions. As well as, in the reactions which are catalyzed by immobilized enzymes in a heterogeneous environment, termination of the reaction is conveniently accessible just by physical isolation of the enzyme from the solution [4], [5]. Numerous methods have been used for enzyme immobilization such as covalent coupling or physical adsorption to a carrier, entrapment in polymeric networks and encapsulation [6], [7]. The immobilization via multipoint tight covalent binding on insoluble matrices will lead to rigidify the tertiary structure of enzymes and has attracted much attention because of its diverse advantages like low enzyme leakage even in severe reaction conditions and efficient usage of bioconjugates in various types of bioreactors. The activity of biocatalyst will decrease during the covalent immobilization, but the consequent remarkable improvement in stability of the enzyme which is immobilized in optimal conditions would overcome this slight enzyme deactivation [8], [9].