Dipyrenylphosphatidylcholine as a probe of bilayer pressures

We have used dipyrenylphosphatidylcholines (dipyPCs) to study the pressure in the fluid lamellar phase formed by mixtures of fully hydrated dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE). As we increase the DOPE mole fraction at 25°C we observe a linear increase in the ratio of the excimer-to-monomer signal (E/M1). We argue that this observation can be understood in terms of an increase in the lateral pressure in the chain region, i.e., in the bilayer plane. This change itself is driven by the decrease in lateral pressure between headgroups as we add DOPE. We expect the lateral pressure to vary in magnitude as we probe the bilayer at different depths [1]. We have confirmed this by recording E/M1 using di[10-(pyren-1-yl)decanoyl]phosphatidylcholine (10dipyPC) and di[4-(pyren-1-yl)butanoyl]phosphatidylcholine (4dipyPC). We find that in 100% DOPC the E/M1 for 4dipyPC is 2.5 times greater than that for 10dipyPC. The above observations can all be rationalized in terms of changes in the lateral pressure profile. An inverse hexagonal liquid crystalline phase is found in the range 100-83% DOPE [2]. In this region of the phase diagram we observe a quadratic variation in E/M1, with a minimum at 95% DOPE. We hypothesize that this variation reflects the chain stretching that is necessitated by the geometrical packing constraints of the hexagonal phase [3]. Again, we find that the E/M1 for 4dipyPC is greater than that for 10dipyPC, but in this phase only by a factor of two.