Continuous wave electron paramagnetic resonance (CW-EPR) and pulsed electron spin echo (ESE) methods are used to investigate lamellar phases formed by dialkyl-lipids with ether-linked chains in a wide temperature range, from 77 K to 353 K.
In the cryogenically frozen state, dihexadecyl phosphatidylcholine (DHPC) lipids adopt a lamellar bilayer phase with interdigitated chains. The lipids undergo stochastic librations of small amplitude in the nanosecond time scale. The librational dynamics is driven by variation in the angular amplitude of the motion and a dynamical transition sets in at about 220 K. The interdigitated frozen DHPC membranes exhibit almost uniform profiles of water penetration and polarity across the hydrocarbon zone to an extent comparable to that found in the interfacial regions of cholesterol-containing lipid membranes.The trend of the profiles contrasts with the typical sigmoidal shape shown by lipid bilayers with noninterdigitated chains.
For temperatures throughout the interdigitated gel phase, DHPC lipids suffer of motional restriction at the terminal chain segments and show moderate packing density. Moreover, lipids with interdigitated chains coexist with non-interdigitated bilayers in the intermediate phase of DHPC, up until the transition to the liquid-crystalline phase.
In the fluid state at high temperature, DHPC dispersions form bilayer lamellae characterized by a flexibility profile of increasing mobility on proceeding down the chain and a typical sigmoidal transmembrane polarity profile. The features of the profiles correlate with structural parameters that are derived from X-ray diffraction and indicate increased conformational freedom and permeability for water and various solutes in DHPC relative to membranes formed by diacyl-lipids with ester linked-chains.