Elucidating the pH Effects on Oleic Acid and Interactions with Lipid Membranes

The interplay between fatty acids (FAs) and lipid membranes under varying pH conditions is important for deciphering fundamental cellular processes and advancing the design of responsive applications. Endowed with membrane incorporation, fatty acids (FAs) can induce curvature on membranes, resulting in alterations in fluidity, permeability, and stability. Despite the wide amount of research on membrane effects induced by FAs, the pH effect on their interaction behaviors remained unclear. Considering the varying pH environments in vivo, the study investigates how pH modulates the aggregation behavior of oleic acid (OA) and its subsequent interactions with lipid membranes. First, the critical aggregation concentration (CAC) was measured to determine the condition when aggregates formed. Less ordered aggregates with a lower zeta potential were observed under elevated pH conditions. These aggregates were further introduced to lipid membranes to evaluate the corresponding membrane responses using a QCM-D and fluorescence microscopy. pH shifts dramatically alter the OA protonation states of their headgroups, driving their insertion, orientation, and aggregation within membranes. Specifically, under lower pH levels, OA demonstrated an incorporation behavior into the membrane structure with increased membrane viscoelasticity, while disturbance of membrane structural integrity was observed under elevated pH levels. A transition from flatter budding protrusions to elongated tubes on the membrane was observed for incorporated OA, primarily owing to the increasing deprotonation degree that resulted in a change in the packing parameter. A stronger impact on cell viability at higher pH levels verified the membrane disruption behaviors. These findings elucidated that the protonation states of OA headgroups critically influence their membrane affinity and packing behavior, offering new perspectives for understanding membrane biophysics and designing pH-responsive delivery systems.