Interface lipid-water

Hydrolysis of substrates is performed in water, buffered aqueous solutions or biphasic mixtures of water and an organic solvent. Hydrolases tolerate low levels of polar organic solvents such as DMSO, DMF, and acetone in aqueous media. These cosolvents help to dissolve hydrophobic substrates. Although most hydrolases require soluble substrates, lipases display weak activity on soluble compounds in aqueous solutions. Their activity markedly increases when the substrate reaches the critical micellar concentration where it forms a second phase. This interfacial activation at the lipid-water interface has been explained by the presence of a... 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

NCD quenching by FITC-PE Ca site 20A from lipid water interface 411... 

These assumptions were confirmed by the electrophoresis study of the washed creams. Electrophoresis of purified fat globules is a convenient method to characterize and quantify proteins adsorbed at the oil-water interface [35]. Electrophoretic data indicate that no casein, nor whey proteins, were adsorbed at the surface of raw-milk fat globule. Upon homogenization, caseins adsorbed preferentially at the lipid-water interface. In this case, bound a-lactalbumin accounted for 16% of the total interfacial proteins. Heat treatment also induced the interaction of proteins with the fat globules. The amount of bound proteins (per mg of lipids) for heated raw milk was half that for homogenized milk. 

Egreet-Charlier, M., A. Sanson, M. Ptak, and O. Bouloussa. 1978. Ionization of fatty acids at lipid-water interface. FEBSLett. 89 313-316. 

In Figure 26c and 26d some more details are plotted for the PE-modified lipid. On the PE chain there are some features deviating from the parabolic profile near the lipid-water interface. We should realise that in the membrane... 

Sakurai M, Tamagawa H, Inoue Y, Ariga K, Kunitake T (1997) Theoretical study of intermolecular interaction at the lipid-water interface. 1. Quantum chemical analysis using a reaction field theory. J Phys Chem B 101 4810-4816... 

Water molecules are absent from the hydrophobic interior, but both the choline and the phosphate headgroups are fully solvated [41]. Similarly, the first hydration shell of the sulfate headgroup of SDS is formed rather by water molecules than by sodium ions. Because of hydration the charge density due to the lipid headgroups is overcompensated by the water dipoles, thereby reducing the transmembrane potential by 50-100 mV across the lipid water interface and resulting in a negative potential at the aqueous side [42]. 

J. R. Lakowicz and D. Hogen, Dynamic properties of the lipid-water interface of model membranes as revealed by lifetime-resolved fluorescence emission spectra, Biochemistry 20, 1366-1373 (1981). 

J. R. Lakowicz, R. B. Thompson, and H. Cherek, Phase fluorometric studies of spectral relaxation at the lipid-water interface of phospholipid vesicles, Biochim. Biophys. Acta 734, 295-308 (1983). 

Lipases are serine hydrolases that catalyse the hydrolysis of lipids to fatty acids and glycerol [2]. In contrast to esterases, they work at the lipid-water interface and show only little activity in aqueous solutions. Studies of the X-ray structures of human lipase [3,4] and Mucor miehei lipase [5,6] revealed a change in conformation at the lipid-water interface, which explains the increase of activity. 

The presence of bulky, (3-branched side chains can be helix promoting or destabilizing depending on the environment. 100 The role of hydrophobic residues in helix stabilization has been studied in Ala-based peptides 106 as well as through Monte Carlo calculations. 107 The positioning of hydrophobic residues is also important. In amphiphilic helices, hydrophobic residues repeat approximately every three to four residues, such that one side of the helix is hydrophilic and one side hydrophobic. The amphiphilicity makes the peptide susceptible to helix formation in the presence of lipid-water interfaces. 108 109 ... 

Figure 9. Schematic of four mechanisms describing the interaction of Ca with films of DPL (1) ionr-dipole interaction (2) ion-exchange mechanism (3) ionr-ion interaction with ionized anionic lipid contaminant and (4) penetration of electrolyte, HgO, and derived ions into the air- vater or the lipid-water interface. A highlight of Mechanism 4 (consistent with the surface radioactivity data, Ref. 3) is the adsorption of the ions of HCl resulting from th ehydrolysis of CaCU. The coexistence of Ca(OH) and aqueous HCl at the interface requires the formation of compartments or pools that permit the separation of the acid from the base. Such a coexistence of acidic and basic pools is conceivable in the light of the Ca(OH), film on the HCl solution following the hydrolysis of CaCU in the absence of DLP films and is probably a characteristic of DPL films, since the adsorption of Cl was nil without DPL.

A similar modulating effect of cholesterol on the location of the local anesthetic tetracaine has been observed recently (5). The uncharged form of the tetracaine molecule intercalates deeply into DHPC bilayers in the gel state and is displaced to an environment close to the lipid-water interface by increasing the hydrostatic pressure, in other words, by increasing... 

This penetration is apparent only at short distances from the lipid-water interface. 

Cholesterol can modify both the hydrophobic attraction between lipid hydrocarbon chains and electrostatic interactions between lipid polar groups. The influence it has on the location of 9HP reflects this dual effect At low temperature, the "spacer" effect of cholesterol allows the ketone to gain access directly to the lipid-water interface. At high temperatures, a more disordered hydrocarbon core favors the solubilization of the guest molecule. 

Like pancreatic lipase, this enzyme is clearly dependent on a lipid/water interface for maximal activity, where it also may reach a very high catalytic rale, Abo like pancreatic lipase, it is not inhibited by serine esterase inhibitors like DFP orPMSF. 

Similarly, using UV absorption spectroscopy Gracia and Prello [148] studied the influence of membrane chemical composition and drug structure on the localization of benzodiazepines at the lipid-water interface. Their results revealed that the benzodiazepines can be incorporated as an integral part of the bilayer and are not located only at the core, as reported from fluorescence polarization experiments [149]. 

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