Ionization of the acidic phospholipid phosphatidylglycerol has been studied by measuring the surface potential of monomolecular films of the lipid as a function of the aqueous subphase pH and the concentration of monovalent cations (Li, Na, Cs). It is shown that theexperimental data can be interpreted by means of the Gouy-Chapman theory in its simplest formulation, provided an adsorption of cations at the membrane surface is accounted for. This allows us to predict the ionization state of the lipid for given ionic conditions in the subphase. Above pH 4, for subphase ion concentration higher than 10 mM, or for ion concentrations above 0.1 mM at pH 5.6, phosphatidylglycerol is fully deprotonated. Within the limits of our theoretical approach, association constants of the cations to the lipid lie around 0.1-0.6 M-I.With the exception of phosphatidylcholine and phosphatidylethanolamine which are zwitterions over a large pH range (3 -8) [l], phospholipids in biomembranes are acidic substances bearing a net negative charge when they are ionized. Ionization or protonation of these acidic lipids as well as their interactions with cations can have considerable consequences on both membrane structure and functions.From a structural point of view, changes in the pH or the ionic strength of the aqueous phase have been shown to trigger phase transition of acidic phospholipids [2-111. The bivalent cations Mg2+ and Ca2+ can bring about phase separation within mixtures of zwitterionic and acidic phospholipids [12-191. Cations can also modify the dynamic properties of lipids, i.e. their lateral diffusion rate [20], as well as being able to promote morphological changes [21]. As an example, cardiolipin, which normally exists in a lamellar phase in the presence of sodium, adopts the hexagonal H,, phase in the presence of calcium [22, 231. Dipalmitoylglycerophosphoglycerol, which normally stays in a lamellar phase in the presence of sodium, has been shown to exist in an interdigitated phase in the presence of the organic cations choline and acetylcholine [24].From a functional point of view, phospholipids can modulate the activity of membrane enzymes by monitoring, at the membrane surface, a concentration of protons, cations and of any charged metabolites which is quite different from that existing in the bulk [25]. Charges at the membrane surface also contribute to the establishment of transmembrane potentials which are involved in energy-linked transmembrane transport processes [26] and which are postulated to monitor the orientation [27 -291 and the activity of certain membrane proteins [28 -341.For all these reasons, it is of great importance to have a full understanding of the ionization process of acidic phospholipids in membrane systems. This would allow a correct prediction of their ionization state and of the surface potential they generate, for given ionic conditions in the aqueous phase. As a first approach, ionization of phosphatidylglycerol was studied in monolayers by measuring changes in film surface pressure again...
Synopsis The purpose of this work was to compare two methods to determine the formulation of a gel: a 'classical'method previously used and a rational method using matrixes based on the Scheffe algorithm which is particularly useful in the study of these type of mixtures. This gel was composed of three products: C12-C15 Alcohols Benzoate, Cyclomethicone and Ethylene Vinyl Acetate Copolymer in water. The ratios of these compounds were studied according to five criteria: compatibility between phases, consistency, whiteness, cost and 'skin spreading'. To determine the incompatibility zone inside the ternary diagram, we carried out our study by a systematic sequential walk strategy (seven experiments). Three parameters (consistency, whiteness, cost) were optimized using the Scheffe algorithm. This method only needs a limited number of experiments on which to base an empirical mathematical model of the studied phenomena as a function of the different experimental factors. The formulation is therefore discussed as a function of the first degree linear, the second degree and a third degree 'reduced cubical'model. The validity of the possible models are also discussed and the 'reduced cubical'best fits our phenomena.
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