Enzymatic reactions, surfactant

A model analogous to the Hill model (for enzymatic reactions), which describes a more accurate dependence of the observed rate constants on surfactant concentration, was developed by Piszkiewicz. This model is applicable especially at low surfactant concentration and the data may be treated without reference to CMC. According to this model, a substrate (S) and n number of detergent molecules (D), aggregate to form critical micelle (D S), which may react to yield the product... 

RME shows particular promise in the recovery of proteins/enzymes [12-14]. In the past two decades, the potential of RME in the separation of biological macromolecules has been demonstrated [15-20]. RMs have also been used as media for hosting enzymatic reactions [21-23]. Martinek et al. [24] were the first to demonstrate the catalytic activity of a-chymotrypsin in RMs of bis (2-ethyl-hexyl) sodium sulfosuccinate (Aerosol-OT or AOT) in octane. Since then, many enzymes have been solubilized and studied for their activity in RMs. Other important applications of RME include tertiary oil recovery [25], extraction of metals from raw ores [26], and in drug delivery [27]. Application of RMs/mi-croemulsions/surfactant emulsions were recognized as a simple and highly effective method for enzyme immobilization for carrying out several enzymatic transformations [28-31]. Recently, Scheper and coworkers have provided a detailed account on the emulsion immobiUzed enzymes in an exhaustive review [32]. 

Although RMs are thermodynamically stable, they are highly dynamic. The RMs constantly colhde with each other and occasionally a colhsion results in the fusion of two RMs temporarily. During this fusion surfactant molecules and the contents residing inside RMs may be exchanged. In AOT reverse micellar system, this dynamic behavior exhibits second-order kinetics with rate constants in the order of 10 to 10 M s [37]. This dynamic nature not only influences the properties of the bulk system but also affects the enzymatic reaction rates [38]. 

Replicating micelles have been realized by producing within the micelle, through a chemical (or enzymatic) reaction, the same surfactant as that constituting the micelle, which therefore grows and redistributes the surfactant by dividing into new micelles [9.207]. 

Holmes et al. (1998) performed two enzymatic reactions, the lipase-catalyzed hydrolysis of y>-nitrophenol butyrate and lipoxygenase-catalyzed peroxidation of linoleic acid, in w/c microemulsions stabilized by a fluorinated two-chained sulfosuccinate surfactant (di-HCF4). The activity of both enzymes in the w/c microemulsion environment was found to be essentially equivalent to that in a water/heptane microemulsion stabilized by Aerosol OT, a surfactant with the same headgroup as di-HCF4. The buffer 2-(A-morpholino)ethanesulfonic acid (MES) was used to fix the pH in the range 5-6. 

Naturally occurring micellar systems, such as phospholipids and bile salts (e.g. cholic and desoxycholic acids, as well as surfactants affect the rates of numerous chemical reactions in vivo and in vitro (Hanahan, 1960 Kavanau, 1965 Knaak et al., 1966a, b Elworthy et al., 1968 Marriott, 1969). The effects of micellization on enzymatic reactions and other biochemical processes have been discussed by Elworthy et al. (1968), Jencks (1969), and Mysels (1969). 

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

Arai et al. [141] described a particular enzymatic reaction for producing a surface-active protein. A highly hydrophobic amino acid was covalently bound to a hydrophilic protein in an enzyme-catalyzed process for this purpose. The covalent attachment of L-Leu n-alkyl ester to gelatin in the presence of papain as catalyst resulted in a proteinaceous surfactant [141,142] with very good emulsifying properties. 

The choice of surfactant is of importance for the rate of many enzymatic reactions in microemulsions. For instance, it has been found that whereas lipase-catalyzed hydrolysis of triglycerides is rapid in microemulsions based on AOT, it is extremely sluggish when... 

Free or immobilized enzymes have been exploited already in a number of systems. Here, biocatalysis may take place in reversed micelles or in an aqueous phase in contact with an organic solvent. In a powdered state some enzymes are able to function in pure organic solvents. Furthermore, modified enzymes such as polymer bound enzymes or surfactant-coated enzymes have been developed so that they can solubilize in organic solvents to overcome diffusion limitation. The advantages of enzymatic reactions using organic solvents can be briefly summarized as follows ... 

Significant efforts were initially devoted to the purification of NA products from enzymatic reactions prior to MALDl-TOF-MS. As will be indicated in later sections that describe the enzymatic assay formats in more detail, several components of enzymatic reactions are detrimental to the MALDI-process. Hence, purification formats must be able to remove these components, including detergents, surfactants, proteins and unincorporated nucleotides, and must provide the means for the efficient removal of high concentrations of salt commonly used in enzymatic reactions (sodium-, potassium-, magnesium- chlorides and sulfates). 

Extending the use of zeolites into larger dimensions, say to catalyse enzymatic reactions and for the purification of colloidal precious metals, was the aim of researchers at Mobil Corporation (USA), who in 1992 discovered a viable and versatile synthetic procedure to prepare mesoporous materials, i.e. materials with ordered porosity in the range between 20 and 500 A (2-50 nm Kresge et al. 1992). Their first material was termed MCM-41 (Mobil Composition of Matter) and the mechanisms involved templating of a silica sol-gel synthesis by an amphiphilic surfactant. 

Proteins, naturally occurring macromolecular surfactants with amphiphilic nature, are adsorbed onto interfaces, thereby affecting the physical states of interfaces. Many enzymes are involved in catalytic reaction at interfaces. For enzymatic reaction at interfaces, different from the reaction in homogeneous systems, interfacial contact and subsequent conformational change of enzymes are important events determining their catalytic activity. In this chapter, I will describe the conformation of proteins and their interaction (protein-protein and protein-surfactant) at interfaces (mainly liquid-liquid interfaces). The characteristics of enzymatic reaction at liquid-liquid and solid-liquid interfaces, especially lipase reaction, wiU also be described. 

The chemical modification of proteins is not desirable, because of the harsh reaction conditions, the nonspecificity, and the difficulty of removing reagents from the final product [2]. Enzymatic reaction has several advantages, such as the mild reaction conditions, high specificity, and fast reaction rates [3]. Moreover, the use of the enzyme, which occurs naturally in living cells, is acceptable by consumers from the viewpoint of safety. We describe examples of the enzyme-catalyzed synthesis of protein-based surfactants in this chapter. 

The addition of alcohol, as cosurfactant, to the [Cgmim][TfjN]/AOT/water system leads to stable w/IL microemulsions. DLS and protein solubilization experiments confirm the existence of an aqueous nanoenvironment in the IL phase of [C mirnTf N]/ AOT/l-hexanol/water microemulsions [67]. The kinetics of the enzymatic reactions were performed in this quaternary system. Specifically, lipase-catalyzed hydrolysis of p-nitrophenyl butyrate (p-NPB) was used as a model reaction [68]. In a similar way, the hpase-catalyzed hydrolysis of p-NPB was investigated to evaluate the catalytic efficiency in water/AOT/Triton X-100/[C mim][PFJ [69]. A large single-phase microemulsion region can be obtained from the combination of two surfactants in IL. 

In another stndy, Monirnzzaman et al. [215] explored the use of w/IL microemulsions comprised anionic surfactant, AOT/hydrophobic IL [C mim] [TfjN] (l-octyl-3-methyl imidazolium bis(trifluoromethylsnlfonyl)amide)/water/l-hexanol as the reaction medinm for the enzymatic oxidation of pyrogallol catalyzed by HRP. The results demonstrated that the rate of HRP-catalyzed reactions in IL microemnlsions increases significantly compared with that obtained in conventional oil microemnlsions. It was concluded that a w/IL microemulsion may be a very promising system for performing enzymatic reactions with HRP in ILs media. According to them, the findings will be of valne for the development of ILs... 

Biodegradation is carried out by bacteria in nature. By enzymatic reactions, a surfactant molecule is ultimately converted into carbon dioxide, water and oxides of the other elements. If the surfactant does not undergo natural biodegradation then it is stable and persists in the environment. For surfactants the rate of biodegradation varies from 1-2 h for fatty acids, 1-2 days for linear alkyl benzene sul-phonates, and several months for branched alkyl benzene sulphonates. The rate of biodegradation depends on the surfactant concentration, pH and temperature. The temperature effect is particularly important, since the rate can vary by as much a factor of five between summer and winter in Northern Europe. 

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