An even more nuanced picture emerges when we consider the TR-SAXS data of POPC/PG vesicles presented in figure . In this case the lipids, which have one palmitoyl (saturated, 16-C) tail and a oleoyl (monounsaturated, 18-C) tail, have a very low phase transition temperature (∼ -2 °C) despite the long tails. Being in fluid phase and hence softer, a large degree of deformation is observed when the LUVs are subjected to the same osmotic shock. Interestingly, the scattering curves of the of POPC/PG vesicles appear very similar to the ones for DMPC/PG membranes without AMPs measured at 37 °C (top plot of fig. ), both in shape, but also in the fact that complete deformation occurs before the minimum measurement time of 7.8 ms. Despite being in fluid phase and different from the DM membranes, AMP incubation does not appear to permeabilize the PO membrane, least not completely. The membrane is still severely deformed, although to a somewhat lesser degree, even when incubated with LL-37 (Fig. ). These results suggest that, while membrane phase might also play a role, the permeabilization is largely dependent on the lipid tail length. We speculate that this is likely due to the significantly higher energy cost of lipid flip-flop in thicker membranes, even in the presence of AMPs, making the transient pores induced by the flipping too rare to facilitate sufficient ion-transport. While these thicker membranes are impervious to ions, and remain so even after AMP incubation, this is not the case for water. By preparing the LUVs in H 2 O and subsequently mixing them with D 2 O (which has a very different SLD), we can track the equilibration of water across the membrane using TR-SANS. The water diffusion across the lipid membrane results in a time-dependent reduction of the scattering intensity, which can be expressed using the relaxation function in eq. ( ). The relaxation function for DS and PO membranes, as well as a schematic illustration of the method, is shown in figure . In the case of DS membranes at 9 °C, the water diffusion is quite slow and equilibration takes approximately 15 s, as shown by the solid dark blue scattering points. While not surprising, this confirms that water can readily diffuse through the membrane . It is, however, more surprising that neither indolicidin nor LL-37 appear to have any effect on the rate of water equilibration across the membrane. This shows that no pores, neither structural or transient, are formed, as this would dramatically increase the rate of water equilibration. Additionally, as these AMPs have no apparent effect on neither the salt nor water permeability of the DSPC/PG membranes, these results appear to be consistent with lipid flip-flop induced pores which are shown to occur at a rate several orders of magnitude lower in DSPC membranes compared to DMPC membranes at 20 °C .