Interactions and crowding in the hydrophilic region have also been suggested to affect the propensity for interdigitation. In the case of phospholipids, substituting PC ester linkages with ether linkages allows the lipids to form interdigitated structures at atmospheric pressure. The tendency of ether-linked PCs for interdigitation has been attributed to decreased attraction in the hydrophilic region, i.e., through a reduction in the extent of hydrogen bonding through water molecules. The fact that our results for the "interdigitating" MMG-1 show decreased levels of intermolecular MMG-MMG and MMG-water hydrogen bonding supports this idea. In contrast, distinct conformations in the hydrophilic region of MMG-6 may allow the molecules to form more intermolecular hydrogen bonds, especially through the hydroxyl group and carbonyl oxygen found in the lipid acid moiety. The headgroup size has also been shown to play a role for interdigitation; more bulky headgroups have been shown to correlate with interdigitation in the case of PC versus PE due to increased headgroup repulsion and crowding, as well as formation of voids in the hydrophobic region.