Operational stability

In a continuous reaction, enzyme stability is an important factor to determine whether the immobilized enzyme can be applied to industrial production. Only 

If the enzyme is utilized for catalysis of a desired reaction the operational stability is often a much more interesting parameter than the resting stability. Catalyzing a reaction can influence enzyme stability in both positive and negative fashions  

The numerical value of the operational stability is expressed as the number of units of enzyme which have to be expended to generate a defined amount of product. The following conditions have to be specified besides the obvious ones such as T, pH, and substrate concentration, these are the quantity [E] T (see below) and the degree of conversion x. 

The relevant parameter for the study of the operating stability of impure enzyme preparations is E t, the product of active enzyme concentration E = fnE/V [g L-1] and residence time T. The following derivation illustrates the usefulness of this parameter. 

The reaction rate is defined by Eq. (5.79), with the volumetric reaction rate / [mol (I. min ) and the specific (mass-based) reaction rate r [mol (g min)-1]. 

Equation (5.81) results from rearrangement and integration over the degree of conversion from the initial value to residence time t. 

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As with organic solvents, proteins are not soluble in most of the ionic liquids when they are used as pure solvent. As a result, the enzyme is either applied in immobilized form, coupled to a support, or as a suspension in its native form. For production processes, the majority of enzymes are used as immobilized catalysts in order to facilitate handling and to improve their operational stability [24—26]. As support, either inorganic materials such as porous glass or different organic polymers are used [27]. These heterogeneous catalyst particles are subject to internal and external... 

The industrial development of biotransfonmations is hampered currently by lack of commercial availability of biocatalysts at a reasonable price, insufficient operational stability of most biocatalysts and the practical problems associated with the exploitation of cofactor-dependent biocatalysts. 

Clearly, this kind of problem requires more than a quick-fix solution if operational stability is to be attained. 

NOTE Under normal boiler operating conditions, the pH required to ensure relative waterside stability (operational stability pH) for copper and copper alloys is typically 8.8 to 9.2, and for steel alloys it is 9.2 to 11.0. However, for any specific system the precise operational stability pH range is a function of boiler pressure, temperature, and system metallurgy. 

The class I FruA isolated from rabbit muscle aldolase (RAMA) is the aldolase employed for preparative synthesis in the widest sense, owing to its commercial availability and useful specific activity of 20 U mg . Its operative stability in solution is limiting, but the more robust homologous enzyme from Staphylococcus carnosus has been cloned for overexpression [87], which offers unusual stability for synthetic purposes. Recently, it was shown that less polar substrates may be converted as highly concentrated water-in-oil emulsions [88]. 

Estimates of the norm of the transition operator. Stability considerations are connected with the use of a new method based on the estimation of the norm for the operator of transition from one layer to another. This method actually falls within the category of energy methods. 

As a general rule, the optimal immobilization method is found empirically by a process of trial and error, where the selectivity, activity, and operational stability of the enzyme after immobilization are taken into account. The immobilization process is very sensitive to different parameters and is treated as a kind of art [16]. 

A packed-bed nonpermselective membrane reactor (PBNMR) is presented by Diakov et al. [31], who increased the operational stability in the partial oxidation of methanol by feeding oxygen directly and methanol through a macroporous stainless steel membrane to the PB. Al-Juaied et al. [32] used an inert membrane to distribute either oxygen or ethylene in the selective ethylene oxidation. By accounting for the proper kinetics of the reaction, the selectivity and yield of ethylene oxide could be enhanced over the fixed-bed reactor operation. 

Bommarius, A.S., Drauz, K., Klenk, H. and Wandrey, C. (1992) Operational stability of enzymes - acylase catalyzed resolution of /V-acetyl amino acids to enantiomerically pure L-amino acids. Annals of the New York Academy of Sciences, 672, 126-136. 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

The synthesis of vinylsulfonyl dyes is a good example for a method to obtain pH indicator layers with operational stability of weeks and shelf life of years. Commercial transparencies with a cellulose coating are used as the polymer matrix and the reactive vinylsulfonyl dye is bound to the cellulose... 

Data are available for plasticizers and ionophores, and indicate the operational stability (the higher the log P value, the higher the lipophilicity). The minimal lipophilicity log P required for membrane components with a lifetime of 30 x 24 h upon exposure to aqueous solution is estimated to be around 10 whereas it has to be as high as 25 for direct measurement in blood, serum and plasma. 

Poor operational stability due to drift has largely limited the long-term or implantable application of pH sensors. Some oxide-based electrodes present very high initial... 

I.L. Mattos, L. Gorton, T. Ruzgas, and A.A. Karyakin, Sensor for hydrogen peroxide based on Prussian blue modified electrode improvement of the operational stability. Anal. Sci. 16, 1-5 (2000). 

R. Garjonyte and A. Malinauskas, Operational stability of amperometric hydrogen peroxide sensors, based on ferrous and copper hexacyanoferrates. Sens. Actuators, B B56, 93—97 (1999). 

B. Haghighi, S. Varma, F.M. Alizadeh, Y. Yigzaw, and L. Gorton, Prussian blue modified glassy carbon electrodes - study on operational stability and its application as a sucrose biosensor. Talanta 64, 3-12 (2004). 

Tor [7] developed a new method for the preparation of thin, uniform, self-mounted enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme was dissolved in a solution containing synthetic prepolymers. The electrode was dipped in the solution, dried, and drained carefully. The backbone polymer was then cross-linked under controlled conditions to generate a thin enzyme membrane. The method was demonstrated and characterized by the determination of acetylcholine by an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a penicillinase electrode. Linear response in a wide range of substrate concentrations and high storage and operational stability were recorded for all the enzymes tested. 

Among organic constituent measurements, that of aggregate properties (BOD and COD) and specific parameters (TOC for example) has been well developed for more than 20 years. Concerning BOD, a recent review on biosensors [33] has been published. BOD biofilm-based sensors as well as respirometric systems, other measuring principles, and the commercial BOD instruments are discussed and compared regarding their performance characteristics like linearity, response time, precision, agreement between BOD values obtained from the biosensors and the conventional 5-day test, as well as toxic resistance to various compounds and operational stability. 

J.C. Carter, I. Grizzi, S.K. Heeks, D.J. Lacey, S.G. Latham, P.G. May, O. Ruiz de los Panos, K. Pichler, C.R. Towns, and H.F. Wittmann, Operating stability of light-emitting polymer diodes based on poly(p-phenylene vinylene), Appl. Phys. Lett., 71 34-36, 1997. 

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