Deuteration of Water Enables Self-Organization of Phospholipid-Based Reverse Micelles

Phospholipid-based reverse micelles are composed of branched cylinders. Their branching points are known to attract themselves and to slide along branches. The rate of this sliding is governed by the lifetime of H(D)-bonded water bridges between phospholipid molecules. This lifetime is increased when the water is deuterated. On condition that the water contains at least 40 D atoms%, water/dipalmitoylphosphatidylcholine (DPPC)/deuterated pyridine reverse micelles with the composition 1.11:250 (v/v) have been shown to self-organize into a liquid crystal in the 310-316 K temperature range. The mechanism of this self-organization is unraveled by following the FTIR and (1)H NMR spectra of more concentrated micelles upon heating. During the preparation of micelles, pyridine-(D(+))H(+) ions are formed. They give rise to hydron transfers, under the influence of the DPPC electric charges, evidenced by two broad FTIR absorptions above (BB1) and below (13132) the nu(C-0) stretch. These hydron transfers occur along strong (D(+))H(+) bonds of pyridinium ions with pyridine (BB1) and DPPC C=O groups (BB2). The proton transfers at the interface of micelles, relayed in the continuous pyridine medium, create a tenuous link between separated micelles, thus facilitating their organization. Upon heating, DPPC heads shrink and DPPC chains expand to make wedge-shaped DPPC molecules. The micelles then change in shape: cylinders constrict and enclosed water drifts towards branching points, which swell. Branching points of neighboring micelles come into contact. Due to the deuteration of water these contacts are prolonged and H bonds are formed between DPPC molecules located in each branching point. Upon storage at 39 degrees C, these branching points fuse. The lateral diffusion of DPPC molecules becomes free, as evidenced by a narrowing of all (1)H NMR resonances. Upon further heating, reorganization into a liquid crystal occurs.