Enzymes in liquid membranes

Efforts to produce a phosphate-selective ISE have been hindered by its diverse spe-ciation and lability in biological samples a recent review describes various potentiometric and amperometric approaches to this problem [22]. Of the potentiometric approaches, selectivity is most often achieved using inorganic or organometal-lic extracting agents such as organotin compounds in liquid-membrane ISEs, cobalt complexes or metallic cobalt in coated-wire and metallic electrodes, respectively. Nickel phosphate, silver phosphate, and mixtures of lead precipitates have also been used in phosphate ISEs. All of these sensors suffer from limited selectivity. Enzyme-based sensors for phosphate have also been a topic of research as yet, however, no commercial phosphate sensor exists [22]. 

This chapter is concerned with processes that lead to formation of ISE membrane potentials. The membrane potentials of electrodes with liquid membranes containing a dissolved ion-exchanger ion or a dissolved ionophore (ion carrier), and of electrodes with solid or glassy membranes will be considered. More complicated systems, for example ISEs with a gas gap and enzyme electrodes, will be discussed in chapters 4 and 9. 

A liquid membrane bioreactor was developed as a means of encapsulation for a multi-enzyme system incorporating an oxidation and carbohydrate cleavage, demonstrated using a-glucosidase and glucose oxidase in the conversion of maltose to gluconic acid ... 

Efficient extraction of proteins has been reported with reverse micellar liquid membrane systems, where the pores of the membrane are filled with the reverse micellar phase and the enzyme is extracted from the aqueous phase on one side of membrane while the back extraction into a second aqueous phase takes place at the other side. By this, both the forward and back extractions can be performed using one membrane module [132,208]. Armstrong and Li [209] confirmed the general trends observed in phase transfer using a glass diffusion cell with a reverse micellar liquid membrane. Electrostatic interactions and surfactant concentration affected the protein transfer into the organic membrane and... 

Another emerging technique which deserves mention is the use of a supported ionic liquid membrane. This involves two liquid phases that both contain an enzyme and are separated by the membrane. Lipase-catalyzed esterification takes place in the feed phase to afford a mixture of the (R)-acid and the (S)-ester (Figure 10.22). The latter diffuses through the membrane and is hydrolyzed in the receiving phase to afford the (S)-acid [151, 152]. The methodology has been applied, for example, to the resolution of ibuprofen [151]. 

Enzyme micro-encapsulation is another alternative for sensor development, although in most cases preparation of the microcapsules may require extremely well-controlled conditions. Two procedures have usually been applied to microcapsule preparation, namely interfacial polymerization and liquid drying [80]. Polyamide, collodion (cellulose nitrate), ethylcellulose, cellulose acetate butyrate or silicone polymers have been employed for preparation of permanent micro capsules. One advantage of this method is the double specificity attributed to the presence of both the enzyme and the semipermeable membrane. It also allows the simultaneous immobilization of many enzymes in a single step, and the contact area between the substrate and the catalyst is large. However, the need for high protein concentration and the restriction to low molecular weight substrates are the important limitations to this approach. 

A third type of membrane reactor combines the functions of contactor and separator. An example of this combination membrane reactor is shown in Figure 13.16(c), in which the membrane is a multilayer composite. The layer facing the organic feed stream is an immobilized organic liquid membrane the layer facing the aqueous product solution contains an enzyme catalyst for the deesterification reaction... 

ISF can also be accessed without the need to remove the stratum comeum at mucus membranes. The bucca mucosa of the inside of the mouth is an example. A small amount of interstitial fluid traverses the membrane, but the large concentration of active enzymes in the mouth can quickly consume glucose and saliva can dilute the sample. A number of concepts in the form of contact lens have received attention. The inner eyelid may supply a significant amount of the glucose to the surface liquid on the eye however, concentration due to evaporation and dilution due to tearing present practical challenges. 

Solvent extraction of penicillin from fermentation broths has been well documented in the literature. Penicillin G and penicillin V can be efficiently extracted with amyl acetate or butyl acetate at pH 2.5-3.0 and at 0° to 3°C.33 Schiigerl1 systematically reviewed solvent extraction of different forms of penicillin from fermentation broths. Figure 1 shows an integrated process for the extraction of penicillin G from clarified broth of Penicillium chryso-genurn fermentation.1 Penicillin G is converted to 6-amino penicillanic acid and phenylacetic acid at pH 8 in a 10 L Kiihni extractor by penicillin G-amidase immobilized in an emulsion liquid membrane. The 6-amino penicillanic acid is subsequently converted to ampicillin at pH 6 and the enzyme is recycled. 

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