Aqueous-membrane interface, electrostatic

The arrangement of the proteins within the membrane seems to depend to some extent on the electrostatic surface potential and interface permittivity. It is influenced by electrostatic interaction between the proteins, polar head groups of the phospholipid and ions within the aqueous medium of the membrane surface. This can be affected by exogenous molecules such as drugs. Phospholipid-induced conformational change in intestinal calcium-binding protein in the absence and presence of Ca2+ has been described [37]. There is, however, no doubt that hydrophobic interactions between peptides and membrane interfaces play an important role. A general frame-... 

Since these interfaces are usually constructed of charged detergents a diffuse electrical double layer is produced and the interfacial boundary can be characterized by a surface potential. Consequently, electrostatic as well as hydrophilic and hydrophobic interactions of the interfacial system can be designed. In this report we will review our achievements in organizing photosensitized electron transfer reactions in different microenvironments such as bilayer membranes and water-in-oil microemulsions.In addition, a novel solid-liquid interface, provided by colloidal Si02 particles in an aqueous medium will be discussed as a means of controlling photosensitized electron transfer reactions. 

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

Most of the difficulties outlined above can be avoided by considering a simplified system in which the membrane is replaced by a lamella of an alkane, e.g., hexane or octane, of the same width as the bilayer [80-84]. These membrane-mimetic systems capture the most important characteristic of the water-membrane system — the coexistence of a polar, aqueous phase and a nonpolar medium in close association. The utility of membrane-mimetics is underscored by experimental studies, which have shown that peptides built of L-leucine and L-lysine fold into the same secondary structures at a water-membrane and as at a water-hydrocarbon interface [85-87]. However, such model systems also have important limitations, chief among which are the absence of specific, electrostatic interactions between the protein and the lipid head groups and the effects of membrane ordering on protein behavior. 

A combination of non-covalent and electrostatic interactions influences the partition of dye in a lipid bilayer hydrophobic guests are incorporated into the lipid region, while hydrophilic sensitizers interact mainly with the aqueous interface at the hydrated internal core of liposomes. As an example liposomal porphyrins could induce endocytoplasmatic damage, leading to change of mitochondria shape, while water-soluble haematoporphyrin mostly photosensitized the plasma membrane [55]. 

---