Partitioning of 1-pyrenesulfonate into zwitterionic and mixed zwitterionic/anionic fluid phospholipid bilayers

Molecular partitioning into biomembranes is of fundamental importance in diverse biochemical processes and reactions. The majority of aqueous/membrane partition data using model membrane systems, is obtained with pure phosphatidylcholine bilayers, being lipid mixtures less used, while studies involving bilayers containing zwitterionic/anionic mixtures of phospholipids are even more scarce. In this study, the solvatochromic effects of 1-pyrenesulfonate observed at 375 nm in aqueous liposome suspensions, and monitored by second derivative absorption spectrophotometry, enabled the determination of its partition constants into defined phospholipid bilayers. We compare, under cautiously settled experimental conditions, the partition of the anionic amphiphile PSA into fluid zwitterionic bilayers of POPC (Kp = 6.7 × 103, at 25 °C), and into fluid mixed zwitterionic/anionic bilayers containing small proportions of anionic phospholipids. At the same temperature, we found increasing Kp values in parallel with the proportion of POPS mixed with POPC (Kp = 3.4 × 104 and Kp = 7.3 × 104, with 5 and 10 mol% of POPS, respectively). Our interpretation is based on the interfacial properties of fluid and flexible mixed zwitterionic/anionic phospholipid bilayers.