The effect of tail-length mismatch in binary DMPC/DSPC lipid bilayers

Bilayer heterogeneity has been long hypothesized to drive raft formation and promote complex functionality in lipid membranes. The highly dynamic nature of the membrane however is thought to play a critical role in this delicate balance between structure and performance. To probe the effect of lateral heterogeneity on membrane dynamics, we investigate the thermal response of unilamellar-vesicle systems of mixed dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) with DMPC/DSPC ratios of 50/50 and 70/30. Both lipids experience a transition from an ordered gel phase, with stiff stretched tails, to a melted fluid phase, with more coiled flexible tails, as they are heated through their melting temperature, T$_{\mathrm{m}}$(DMPC) $\approx $ 21 $^{\circ}$C and T$_{\mathrm{m}}$(DSPC) $\approx $ 51 $^{\circ}$C. The distinct T$_{\mathrm{m}}$'s of the two lipids provide a broad gel-fluid phase with a significant mismatch ($\approx $ 20 {\AA}) between the tail-lengths of the DMPC and DSPC molecules. The structural properties of the vesicles were determined by small-angle neutron and x-ray scattering and the collective lipid dynamics in the bilayer were investigated by neutron spin-echo (NSE) spectroscopy on selectively deuterated samples. The NSE results indicate a slowdown of thickness fluctuations in the gel-fluid coexistence phase and an intriguingly strong enhancement in the thickness fluctuation amplitude for T \textgreater T$_{\mathrm{m}}$(DSPC) compared to our previous work on single component vesicles.