Modulated phase of phospholipids with a two-dimensional square lattice.

We report an observation of a modulated phase of phospholipids in which intrabilayer density modulations form an in-plane square lattice. Similar to the well-known ripple (P'(beta)) phase, this phase can be induced by either dehydration or cooling from the liquid crystalline (L(alpha)) phase. However, further lowering of hydration or temperature induces either an untilted straight-chain gel phase (L(beta)) , or the tilted L'(beta) phase after a brief appearance of the P'(beta) phase. The structural characteristics of this phase support the notion that the coupling between variations in local chain tilt and bilayer shape plays an important role in the formation of modulated phases.