THE BEHAVIOR OF NITROBENZ-2-OXA-1,3-DIAZOL AND COUMARIN DYE INDICATORS IN LIPID MONOLAYERS - A STUDY OF INTERFACIAL EFFECTS ON POLARITY ANDPH

The pK of an acid-base indicator is dependent upon the polarity of the environment in which it is measured. This will be true for anisotropi c systems, such as spread monolayers, as well as for isotropic media. It is useful, therefore, to have some reference for medium polarity wh en comparing acid-base behavior in monolayers and other environments. The fluorescence intensity of the coumarin probe, 3-hexadecanoyl-7-oxy -coumarin (HOC), has been deterMined in monolayers of L-alpha-dioleoyl phosphatidylcholine and 1,2-dioleoyl-sn-glycerol as a function of sub phase pH. For reference, parallel measurements were carried out with a short-chain coumarin analog in ethanol water mixtures. As the corresp onding monitor of local polarity, the time-resolved fluorescence of ni trobenz-2-oxa-1,3-diazol has been determined in both the monolayers an d the solvent mixtures involved in acid-base indicator studies. The pK of HOC in the lipid matrix at the gas-water interface was compared to that in ethanol-water mixtures; it is found that pK of this indicator is shifted to higher values in both types of monolayer relative to th e pK in isotropic media of comparable polarity. The dependence of both polarity and indicator pK on molecular organization in these lipid sy stems is discussed. The variations in pK provide a method for estimati ng the difference between pH in the interface and that of the subphase .