IONIC EQUILIBRIA IN MICROHETEROGENEOUS SYSTEMS - PROTOLYTIC BEHAVIOR OF INDICATOR DYES IN MIXED PHOSPHATIDYLCHOLINE-DIPHOSPHATIDYLGLYCEROL LIPOSOMES

Protolytic and partition equilibria of a series of indicator dyes (Neu tral Red, Acridine Orange, Bromothymol Blue, Bromophenol Blue, Methyl Yellow, Methyl Orange, Hexamethoxy Red, Quinaldin Red) were studied in a suspension of liposomes composed of phosphatidylcholine and diphosp hatidylglycerol (18:1, mol:mol). The 'apparent' pK(a)(a) values were d etermined spectrophotometrically for various phospholipids (up to 3 mm ol dm(-3)) and supporting electrolyte (up to 2 mol dm(-3)) concentrati ons. The calculated binding constants (K) of various dyes allowed extr apolation of pK(a)(a) to pK(a)(ac), characterizing the acid-base prope rties of completely bound dyes. This revealed some solvating and elect rostatic properties of the lipid-water interface, such as that the mic roenvironments of Neutral Red, Acridine Orange and Bromothymol Blue in the lipid bilayer are intermediate between those of non-ionic and ani onic micelles, while the pK(a)(ac) values of lipophilic indicators are similar to those observed in micelles of anionic surfactants. In the acidic region (pH < 3) the medium effects (Delta pK(a)(ac) = pK(a)(ac) - pK(a)(w)) are similar to those observed with non-ionic micelles, as seen for Methyl Yellow and some other dyes. This is in agreement with the fact of protonation of the lipid's phosphate groups.