Experimental membrane candidates

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

Our recent work has partially resolved the difficulties of interpretation of potentiometric experiments and provided a quantitative method for deciphering potentiometric data. Using this theory, we have performed quantitative analysis of the data of Belevich et al, in which the experimental amplitudes and rates are related to specific residues that exchange electrons and protons, and generate the observed membrane potential. Using this theory, we have tested proposed candidates for the proton pump site of the enzyme against experimental potentiometric data [37]. 

The presence of two genes, nisB and nisC, encoding 993- and 414-residue proteins without significant homology to other known proteins, but conserved in several lantibiotic operons, has made them strong candidates for post-trans-lational modifications in the maturation pathway of lantibiotics [40]. Limited similarity between NisB and E. coli IlvA, a threonine dehydratase, was reported and hence a dehydratase function for NisB was suggested [40]. Mutation studies of NisB, NisC, EpiB, EpiC, and SpaB indicated that these proteins were essential for nisin, epidermin and subtilin biosynthesis, respectively [40,86,87,190]. As no precursors have been identified and characterized in these mutants, conclusions about the reaction that is catalyzed by these proteins remain speculative [40]. Secondary-structure predictions and experimental evidence confirmed that NisB and SpaB are both membrane-bound [100]. 

For the design of the C-DAK 4000 artificial kidney, and the many similar hemodialysis devices (Daugirdas and Ing, 1988), rates of permeation of the species through the candidate membranes are necessary. Estimates for the permeability of pure species in a microporous membrane can be made from the molecular diffusivity, and pore diameter, porosity, and tortuosity of the membrane (Seader and Henley, 1998), as shown in Example 19.1. For this reason, considerable laboratory experimentation is required when selecting membranes in the molecular structure design step. 

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