Nanoengineered Biomembranes 31 Introduction

The research goal in biomembrane science is to study the function of natural biomembranes and to engineer new types of biomembrane materials on the nanometer scale [122124]. Natural biomembranes, in particular lipid bilayer cell membranes, have an important function in biological molecular exchange (metabolism) and signal transduction processes (e.g., immune reactions, hormone detection) of the living cell. Generally, these processes are mediated across the cellular membrane wall by a transmembrane channel and pores for molecular exchange [125-127] and for signal transduction [128, 129].

Most gram-negative bacteria possess an inner and an outer cellular membrane wall. The inner one contains a

Figure 53. Global cost comparison of yeast cell filtration methods. Reprinted with permission from [115], C. J. M. van Rijn et al., Proc. EBC Congress 501 (1997). © 1997,

Figure 55. Flux comparison of yeast cell filtration methods. Reprinted with permission from [115], C. J. M. van Rijn et al., Proc. EBC Congress 501 (1997). © 1997,

Table 7. Results of three long-run experiments.

Average Initial Water flux Permeate

Porosity beer flux beer flux at 20 °C haze

Table 7. Results of three long-run experiments.

Average Initial Water flux Permeate

Porosity beer flux beer flux at 20 °C haze

0.70 fm

31

14.3 ■

103

18 ■

104

18 ■

105

0.46

slits

0.80 fim

22

7.24.

103

5.6

■ 104

5.8

■ 105

0.58

circles

0.55f m

24

2.21 •

103

1.8

■ 104

4.7

■ 105

0.23

circles

"dense" lipid bilayer, which is electrically closed, so that no ions or other hydrophilic substrates can cross the barrier without the help of highly specific membrane proteins. This inner cell wall is separated from the outer one by an aqueous phase filled with water-soluble polymers, the peptido-glycan. In contrast, the outer cell wall is fairly permeable to smaller solutes below a molecular weight of about 400 Da. Such substances can freely permeate under a concentration gradient through general diffusion porins in the outer cell wall. The most prominent of the general diffusion porins is OmpF (outer membrane protein F) [130, 131] which is very stable and does not denaturate, for example, in 4 M GuaHCL, 70 °C or in 2% SDS. In case of lack of nutrition, the pure diffusion process is too slow and the bacteria need to enhance the efficiency of the translocation functions. For those cases, nature has created a series of rather specific and highly sophisticated membrane channels. An extensively studied example is the malto-oligosaccharide specific channel LamB, or maltoporin [132] of Escherichia coli. Nano-engineered porins in artificial lipid or also block copolymer biomembranes may allow observation of the crossing of a single substrate molecule through the porin channel. A well studied example [133] is the a-hemolysin porin in a diphy-tanoylphosphatidylcholine (lipid) bilayer. See Figure 56.

High-resolution measurements of ion currents through single porin channels to probe, for example, neutral solute transport [134] have already been shown. Also the screening porin forming peptide porin forming peptide

lipid bilayer

Figure 56. Schematic illustration of the assembly of a pore channel with («-hemolysin) peptide oligomers in a (diphy-tanoylphosphatidylcholine) lipid bilayer. Intermediate structures of the assembly process are not considered.

lipid bilayer of facilitated transport of a wide variety of molecules [135], for example, to study antibiotic uptake through porins, to test for toxins, etc., is an object of ongoing interest.

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