Anomalous swelling in phospholipid bilayers is not coupled to the formation of a ripple phase

Aligned stacks of monomethyl and dimethyl dimyristoyl phosphatidylethanolamine (DMPE) lipid bilayers, like the much studied dimyristoyl PC (DMPC) bilayers, swell anomalously in a critical fashion as the temperature is decreased within the fluid phase towards the main transition temperature, T-M. Unlike DMPC bilayers, both monomethyl and dimethyl DMPE undergo transitions into a gel phase rather than a rippled phase below T-M. Although it is not fully understood why there is anomalous swelling, our present results should facilitate theory by showing that the formation of the phase below T-M is not related to critical phenomena above T-M.