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Multiple binding sites for inhibitory choline esters in spontaneous decarbamoylation of dimethylcarbamoyl-acetylcholinesterase (AChE) were suggested from a wide range of IC50 values, in contrast with a limited range of AC50 values (concentration giving 50% of maximal activation) at a peripheral activatory site. Association of choline esters containing a long acyl chain (C7-C12) with the hydrophobic zone in the active site could be deduced from a linear relationship between the size of the acyl group and the inhibitory potency in either spontaneous decarbamoylation or acetylthiocholine hydrolysis. Direct support for laurylcholine binding to the active site might come from the competitive inhibition (Ki 33 microM) of choline-catalysed decarbamoylation by laurylcholine. Moreover, its inhibitory action was greater for monomethylcarbamoyl-AChE than for dimethylcarbamoyl-AChE, where there is a greater steric hindrance at the active centre. In further support, the inhibition of pentanoylthiocholine-induced decarbamoylation by laurylcholine was suggested to be due to laurylcholine binding to a central site rather than a peripheral site, similar to the inhibition of spontaneous decarbamoylation by laurylcholine. Supportive data for acetylcholine binding to the active site are provided by the results that acetylcholine is a competitive inhibitor (Ki 7.6 mM) of choline-catalysed decarbamoylation, and its inhibitory action was greater for monomethylcarbamoyl-AChE than for dimethylcarbamoyl-AChE. Meanwhile, choline esters with an acyl group of an intermediate size (C4-C6), more subject to steric exclusion at the active centre, and less associable with the hydrophobic zone, appear to bind preferentially to a peripheral activity site. Thus the multiple effects of choline esters may be governed by hydrophobicity and/or a steric effect exerted by the acyl moiety at the binding sites.

a b s t r a c tQuorum sensing inhibition (QSI) has been suggested as a potential solution to suppress the growth of biofilm on solid surfaces using pure enzymes or enzyme producing. In this study, three plant-oriented organic molecules (cinnamaldehyde, CIN; vanillin, VAN; zingerone, ZIN) were applied as QSIs in forward osmosis (FO) membrane system using Pseudomonas aeruginosa PAO1 as a model biofoulant. After 36 h of FO operation, all tested experiments with QSIs exhibited the retarded flux decline, and resulted in the increase in accumulated permeate volume by 5% (CIN), 21% (VAN), and 15% (ZIN) compared with that of control. It was due to the difference in the characteristics of biofilm formed on the membrane surface, that the biomass on the unit area of membrane surface with QSIs was decreased by 68%, 41%, and 15% in the presence of CIN, VAN, and ZIN, respectively. In the absence of QSIs, membrane surface turned more hydrophobic, which hindered the transport of permeate water due to the formation of hydrophobic biofilm, while those in the presence of QSIs possessed similar contact angle compared with that of the virgin membrane. Furthermore, the amount of extracellular polymeric substances per unit area of membrane was reduced significantly in the presence of QSIs. In conclusion, the addition of QSIs can be the economically feasible strategy to mitigate biofouling not only reducing the amount of biofilm on the membrane surface but also modifying properties of biofilm.

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The choline esters potentiated the choline-catalysed decarbamoylation of dimethylcarbamoyl-acetylcholinesterase in proportion to the length of acyl group, although esters containing an acyl chain longer than the hexanoyl group exhibited a corresponding decrease in the potentiation. In structural requirement analysis it was found that both the quaternary ammonium moiety and the ester bond were important for the effective acceleration of choline-catalysed decarbamoylation. In general, the respective thiocholine ester was found to be more effective than the corresponding choline ester. Whereas the binding affinity (Ka) of choline in the decarbamoylation was not significantly altered, the maximum decarbamoylation rate (kr(max.)) of choline was greatly enhanced in the presence of choline esters or thiocholine esters. Along with the above observation, the isotope solvent effect, the effect of ionic strength and the antagonism studies demonstrate that the choline esters or thiocholine esters may interact with one of peripheral anionic sites, and thereby make the choline-catalysed decarbamoylation more favourable.

a b s t r a c tIn this study, the intervention of bacterial communication or quorum quenching (QQ) technique has been investigated to mitigate biofouling in forward osmosis (FO) membrane processes. The 10 mg-C/L lysate of QQ enzyme-producing Rhodococcus sp. BH4 successfully degraded 79% of the bacterial signal molecule (N-acyl homoserine lactone, AHL). In a continuous lab-scale FO experiment using Pseudomonas aeruginosa PAO1 as a model bio-foulant, flux recovery after physical cleaning was higher in the presence of 10 mg/L of Rhodococcus lysate. The retardation of biofouling in the presence of Rhodococcus lysate was largely due to the reduced amount of bio-volume and extracellular polymeric substances (EPS), which were reduced by 68% and 75% compared with the control. In conclusion, the application of bacteria-oriented QQ molecules could be the potential solution not only to mitigate biofouling but also to meet the economic demands.

Hydraulically irreversible membrane fouling is a major problem encountered during membrane-based water purification. Membrane foulants present large hydrophobic fractions, with humic acid (HA) being a prevalent example of hydrophobic natural organic matter. Furthermore, HA contains numerous aromatic rings (π electrons), and its hydrophobic interactions are a major cause of irreversible membrane fouling. To address this issue, in this study, we used the cation−π interaction, which is a strong noncovalent, competitive interaction present in water. Because the strength of cation−π interactions depends on the combination of cations and π molecules, utilizing the appropriate cations will effectively remove irreversible fouling caused by hydrophobic HA. We performed macroscale experiments to determine the cleaning potential of the test cations, nanomechanically analyzed the changes in HA cohesion caused by the test cations using a surface force apparatus and an atomic force microscope, and used molecular dynamics simulations to elucidate the HA removal mechanism of test studied cations. We found that the addition of 1-ethyl-3methylimidazolium, an imidazolium cation with an aromatic moiety, effectively removed the HA layer by weakening its cohesion, and the size, hydrophobicity, and polarity of the HA layer synergistically affected the HA removal mechanism based on the cation−π interactions.

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