Phospholipid ITIES

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

There is a group of substances, in the presence of which significant changes in the surface tension of the ITIES were observed, which are also likely to influence the differential capacity of the ITIES correspondingly. These substances include various ionic and nonionic surfactants (Section IV.B.2) and amphiphilic phospholipids (Section IV.B.3) or affinity dyes. Attention has focused on phospholipids. 

Capacitance measurements of phospholipid monolayers at the ITIES have been proposed as a suitable tool for studying the enzyme activity under the precise control of the electrical state of the monolayer [81]. Kinetics of hydrolysis of phosphatidylcholine... 

This chapter is basically divided in two parts, namely, the study of surface pressure-molecular area (jr — A) isotherms of phospholipids at ITIES and their effect on ion transfer. In the first part, the emphasis is put on topics which have been left out from Ref [5], i.e., Langmuir film techniques and theoretical modeling of jr — isotherms, as well as on the latest progress in the field, especially on experiments that combine Langmuir techni-... 

From a practical point of view, the potential drop across the inner layer A 02 must be determined by fitting the experimental data to the equations derived from this theoretical approach, which led to some controversy about its value [53,54,56,57]. For the sake of simplicity, and also because recent studies of the ITIES structure do not confirm the presence of an inner layer [51,58], we neglect the finite size of the transferring ion and take X2 = X2 = 0 and A 02 = 0. This is equivalent to accepting that the potential difference Afl02 — A 0 is not modified by the presence of the phospholipids. 

The Electrical Potential Distribution at the ITIES in the Presence of Zwitterionic Phospholipids... 

When a monolayer of phospholipids is adsorbed at the ITIES, there must be a modification of the electrical structure of the interface [60]. Since we aim at describing the effect of this monolayer on the rate of ion transfer in a simple way, we assume a sharp interface also in the presence of phospholipids. The hydrophobic tails are located in the organic phase (negative x region), and the hydrophilic headgroups are located in the aqueous phase (positive X region). 

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

K Verleysen, P Sandra. Analysis of phospholipids in lecithins separation according to hydrophobic-ity by lowering the temperature. J High Resol Chromatogr 20 337-339, 1997. 

The affinity of flavonoids for phospholipid membranes has been unequivocally demonstrated by many authors. Many biological functions of these compounds are also believed to be the result of flavonoid interactions with cell membranes. Partition coefficients for a large group of flavonoids between water and olive oil were determined, and it was shown that the hydrophobic-ity of the compounds is inversely proportional to the number of OH groups. 

To check the predictions of the Marcus theory of ET (6), one has to carry out measurements at higher overpotentials. This can be done by separating the electron donor and acceptor by a well-defined spacer (32), which decreases the ET rate. Molecular monolayers of long-chain saturated phospholipids (Fig. 8) have been used as a spacer in ITIES studies (26,27). 

FIG. 8 The ITIES modified with a monolayer of phospholipid. The insert shows the structure of synthetic saturated phosphatidyl choline lipid. (From Ref. 26.)... 

The kf versus [lipid] dependencies for Fe(CN)s are different. Although the ET rate decreases markedly with increasing concentration of lipid, it does not approach zero at higher concentrations, but reaches the limiting value at about 50 /xM. This saturation suggests the formation of a complete phospholipid monolayer at the ITIES. The formation of compact phospholipid monolayers at similar lipid concentrations was observed at water/nitro-benzene (40a) and water/dichloroethane (40b) interfaces. 

The equilibrium adsorption of phospholipids at the ITIES has previously been studied by several groups (38-43) and shown to follow Frumkin... 

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