Vesicle walls

Fig. 2 PICsomes formed from oppositely charged building blocks, (a) Chemical structures of the hybrid polypeptides for PICsomes and scheme of the PICsome preparation, (b) Cryo-TEM image of 100-nm-sized PICsomes (scale bar 50 run). Arrows indicate vesicle walls. Adapted from [70] with permission. Copyright 2010 American Chemical Society...

In such vesicle systems, the electrons are transported through the membrane. Electron carriers such as quinones or alloxazines in the vesicle wall enhance remarkably the rate of photoinduced charge separation. The vesicle system shown in Fig. 6 contains the surfactant Zn-porphyrine complex (ZnC12TPyP) in the wall 23). 

The incorporation of 1,3-dibutylalloxazine (DBA) as an electron carrier in the vesicle wall increased the accumulation rate of the reduced disodium 9,10-anthraqui-none-2,6-disulfate (AQDSH2) by 7 times. In the system of Fig. 6, the electron is pumped up in two steps at the inside as well as outside of the vesicle wall, however, since the photoreaction centers (surfactant ZnP) are the same for both the walls, the amount of energy acquired by the two steps excitation is equal to that by one step excitation. The incorporation of different photoreaction centers inside and... 

Fig. 7. Two step pumping of electron at inside and outside of vesicle wall. D Donor A Acceptor M2X, M2X Sensitizers...

The formation of skeletonized vesicles was also reported for vesicles composed of IS and DPPC, where the mole fraction of DPPC varied from 5 to 25 mole percent. Takeoka et aL analyzed the release of entrapped water soluble molecules in order to assess the size of the pores formed in the vesicle wall [49]. The ease of release of saccharides, primarily dextrans, of various molecular... 

Once inside a cell the vesicles lose their coats to become endosomes which may then fuse with lysosomes or with Golgi membranes. The removal of a clathrin coat requires ATP as well as the chaperonin Hsp 70 (Chapter 10) and a coat protein called auxilin.568 Triskelion is distorted and displaced from the clathrin cage. The interior of the newly formed endosome is quickly acidified by the action of a proton pump in the vesicle walls.554 569 This sometimes leads to dissociation of enclosed receptors from their ligands and permits recycling of receptors and lipids of the vesicle membranes to the cell surface. This is the case for the low-density lipoprotein receptor.570 571... 

Figure 17. Schematic representation for two-step activation of electron transport across a vesicle wall. ZnP is octadecyl-pyridiniumyltris(4-pyridyl)porphyrinatozinc(II) cation. DBA is 1,3-dibutylalloxazine, AQDS is 9,10-anthroquinone-2,6-disulfonate (reproduced from ref. 328)...

Finally, a number of studies have employed viologens in the bilayer, either added as an amphiphilic reagent to DPPC,340 or as the sole component of the vesicle walls.337,341 342 In the latter cases, double bonds in the hydrocarbon chain of the viologen have been polymerized to give added stability and rigidity to the bilayer structure. 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

The cylindrical deformations shown in Figure 7.5 are nonequilibrium shapes and have a very short lifetime, which is why they have not been observed previously when standard video acquisition speed was accessible only. The flattening of the vesicle walls starts during the applied pulse and is observed throughout a period of about 1 ms. So far, cylindrical deformations have not been observed in studies on cells [121, 122], However, the temporal resolution in those experiments (3.3 ms per image) was not high enough to detect such short-lived deformations. 

Fig. 12 Illustration of the packing of a-helical PELLys-h-PLLeu copolymer chains in the vesicle wall. Reprinted with permission from [52], copyright (2004) Macmillan...

It seems most likely, therefore, that the vesicle membrane behaves similarly to an axonal membrane, in that it is selectively permeable to K+ ions. Since the vesicle is swimming in the cytoplasm, it would be expected to have a high concentration of K+ at the exterior of its membrane and a low concentration of K+ in its internal medium. Thus a vesicle membrane can be considered to be inside out as compared to a normal plasma membrane forming a neuronal wall. We should hardly be surprised, therefore, to find that vesicle walls have an effective positive charge, whereas cell membranes appear to have external negative charges. ... 

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