Correlating Membrane Morphological Responses with Micellar Aggregation Behavior of Capric Acid and Monocaprin

The interaction of single-chain lipid amphiphiles with phospholipid membranes is relevant to many scientific fields, including molecular evolution, medicine, and biofuels. Two widely studied compounds within this class are the medium-chain saturated fatty acid, capric acid, and its monoglyceride derivative, monocaprin. To date, most studies about these compounds have involved in vitro evaluation of their biological activities, while mechanistic details of how capric acid and monocaprin interact with phospholipid bilayers remain elusive. Herein, we investigated the effect of these two compounds on the morphological and fluidic properties of prefabricated, supported lipid bilayers (SLBs). The critical micelle concentration (CMC) of each compound was determined by fluorescence spectroscopy measurements. At or above its CMC, capric acid induced the formation of elongated tubules protruding from the SLB, as determined by quartz crystal microbalance-dissipation and fluorescence microscopy experiments. By contrast, monocaprin induced the formation of elongated tubules or membrane buds below and above its CMC, respectively. Fluorescence recovery after photobleaching (FRAP) experiments indicated that capric acid increased bilayer fluidity only above its CMC, whereas monocaprin increased bilayer fluidity both above and below its CMC. We discuss these findings in the context of the two compounds' structural properties, including net charge, molecular length and hydrogen-bonding capacity. Collectively, the findings demonstrate that capric acid and monocaprin differentially affect the morphological and fluidic properties of SLBs, and that the aggregation state of the compounds plays a critical role in modulating their interactions with phospholipid membranes.