Ultrafast structural dynamics inside planar phospholipid multibilayer model cell membranes measured with 2D IR spectroscopy.

The ultrafast dynamics in the interior of planar aligned multibilayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (dilauroylphosphatidylcholine, DLPC) are investigated using 2D IR vibrational echo spectroscopy. The nonpolar and water insoluble vibrational dynamics probe, tungsten hexacarbonyl (W(CO)6), is located in the alkane interior of the membranes. The 2D IR experiments conducted on the antisymmetric CO stretching mode measure spectral diffusion caused by the structural dynamics of the membrane from ~200 fs to ~200 ps as a function of the number of water molecules hydrating the head groups and as a function of cholesterol content for a fixed hydration level. FT-IR studies of the lipid bilayers and the model liquids, hexadecane and bis(2-ethylhexyl) succinate, indicate that as the number of hydrating water molecules increases from 2 to 16, there are structural changes in the membrane that partition some of the W(CO)6 into the ester region of DLPC. However, the 2D IR measurements, which are made solely on the W(CO)6 in the alkane regions, show that the level of hydration has no observable impact on the interior membrane dynamics. FT-IR spectra and 2D IR experiments on samples with cholesterol concentrations from 0 to 60% demonstrate that there is a change in the membrane structure and an abrupt change in dynamics at 35% cholesterol. The dynamics are independent of cholesterol content from 10 to 35%. At 35%, the dynamics become slower and remain unchanged from 35 to 60% cholesterol.