Size-dependent ultrafast structural dynamics inside phospholipid vesicle bilayers measured with 2D IR vibrational echoes

Significance Plasma membranes hold a wide variety of proteins and other biomolecules that are essential to cell functions. In addition, membranes are the solvent bath, which is intimately coupled to transmembrane processes. Membranes are frequently modeled using planar phospholipid bilayers and vesicles. However, vesicles are small and have relatively small radii of curvature. Research has shown that the bilayer curvature influences structural and thermodynamic properties, but much less is known about the interior dynamics of model membranes. Here we address ultrafast structural dynamics inside vesicle and planar bilayers using 2D IR vibrational echo experiments. The results show that the interior dynamics depend on the phospholipid chain length and are curvature dependent, with the dynamics becoming faster as the vesicle size decreases. The ultrafast structural dynamics inside the bilayers of dilauroylphosphatidylcholine (DLPC) and dipalmitoylphosphatidylcholine vesicles with 70, 90, and 125 nm diameters were directly measured with 2D IR vibrational echo spectroscopy. The antisymmetric CO stretch of tungsten hexacarbonyl was used as a vibrational probe and provided information on spectral diffusion (structural dynamics) in the alkyl region of the bilayers. Although the CO stretch absorption spectra remain the same, the interior structural dynamics become faster as the size of the vesicles decrease, with the size dependence greater for dipalmitoylphosphatidylcholine than for DLPC. As DLPC vesicles become larger, the interior dynamics approach those of the planar bilayer.