In deuterium NMR spectra of phosphatidylethanolamine bilayers with an extremely high content of saturated fatty acids, each C1 deuteron of the glycerol backbone gave rise to a doublet [Yoshikawa et al.,(1988) Biochim. Biophys. Acta 944, 321–328]. This suggests the presence of two backbone conformations, the exchange between which is slow on an NMR time-scale. The origin of the two conformations has been investigated in this work using ...