Characterization of the sub-transition of hydrated dipalmitoylphosphatidylcholine bilayers: X-ray diffraction study

Abstract

The structural changes accompanying the recently described sub-transition of hydrated dipalmitoylphosphatidylcholine (Chen, S.C., Sturtevant, J.M. and Gaffney, B.J. (1980) Proc. Natl. Acad. Sci. USA 77, 5060–5063) have been defined using X-ray diffraction methods. Following prolonged storage at −4°C the usual L β′ gel form of hydrated dipalmitoylphosphatidylcholine (DPPC) is converted into a more ordered stable ‘crystal’ form. The bilayer periodicity is 59.1 Å and the most striking feature is the presence of a number of X-ray reflections in the wide angle region. The most prominent of these are a sharp reflection at 1 4.4 A ̊ −1 and a broader reflection at 1 3.9 A ̊ −1 . This diffraction pattern is indicative of more ordered molecular and hydrocarbon chain packing modes in this low temperature ‘crystal’ bilayer form. At the sub-transition ( T rmsub = 15–20° C) an increase in the bilayer periodicity occurs ( d=63.6 A ̊ ) and a strong reflection at approx. 1 4.2 A ̊ −1 with a shoulder at approx. 1 4.1 A ̊ −1 is observed. This diffraction pattern is identical to that of the bilayer gel (L β′) form of hydrated DPPC. Thus, the sub-transition corresponds to a bilayer ‘crystal’ → bilayer L β′ gel structural rearrangement accompanied by a decrease in the lateral hydrocarbon chain interactions. Differential scanning calorimetry and X-ray diffraction show that on further heating the usual structural changes L β′ → P β′ and P β′ → L α occur at the pre- and main transitions, at approx. 35°C and 41°C, respectively.