A cationic single-chain bolalipid forms stable vesicles with distinct interfacial behavior

The ability of single-chain bolalipids (or bolaamphiphiles) to self-assemble into vesicular structures remains poorly characterized. Here, we report the synthesis and self-assembly behavior of a new class of proline-based single-chain lipids, a symmetric bipolar molecule (2Pro-C18:1) and, for comparison, a unipolar analogue (1Pro-C18:1), both bearing an unsaturated C18 hydrocarbon chain. Confocal fluorescence microscopy, encapsulation assays, and fluorescence spectroscopy demonstrate that both amphiphiles form stable vesicles capable of entrapping small molecules. Vesicle formation occurs over a broad range of pH, with the bolalipid favoring higher pH, consistent with differences in apparent pK_a determined from zeta potential measurements. At the air-water interface, 2Pro-C18:1 exhibits lower surface activity than 1Pro-C18:1, and vibrational sum frequency generation (vSFG) spectroscopy is consistent with a bent or U-shaped interfacial conformation for the bolalipid, in contrast to a more extended geometry for the unipolar analogue. Both lipids display dynamic exchange between interfacial and bulk-associated aggregates at higher concentrations, with 2Pro-C18:1 exhibiting more complex concentration-dependent behavior. Taken together, these results demonstrate that single-chain bolalipids can form stable, dynamic aggregates with bulk and interfacial properties that are governed primarily by hydrophobic chain effects modulated by headgroup architecture. These findings provide insight into molecular design principles for functional amphiphilic systems.