Indeed, the more planar and less sterically hindered E-isomers now presents a clear and rather deep free energy minimum at the polar head region, and more specifically at the polarhead/hydrophobic-core interface. The absolute minimum can be reached from the water buffer overcoming a quite small barrier of about 2-3 kcal/mol. While no full penetration inside the membrane and hence cellular internalization can be forecasted due to the high free energy barrier for membrane crossing exceeding 12 kcal/mol, the polar-head aggregate (Figure ) will be characterized by a long persistence-time and hence will be suitable to modify the physical parameters of the membrane. Note that these results obtained by enhanced sampling techniques, are also coherent with equilibrium MD simulations, in which the spontaneous formation of the stable aggregate for the E-isomers has been consistently observed (Figure ). Unsurprisingly, the formation of the persisting aggregate is due on the one side by the interaction of the butyl moiety of CyP-OBu with the hydrophobic core, and on the other hand by the formation of a favorable hydrogen-bond network with the polar groups. As can be seen in Figure , the hydroxyl group of the phenyl ring interacts with the closer oxygens of the lipidic esters yielding a stabilizing interaction which can be confirmed looking at the time evolution of the distances between the two moieties, which remains stable even in presence of further interactions with other vicinal lipids developed along the trajectory. Likewise, it can be noted that some water molecules are also forming auxiliary hydrogen bonds with the carbonylic oxygen of the pyrone derivative, contributing to the global stability.