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The hydrophobic peptide gramicidin is shown by 31P-NMR, freeze-fracture electron microscopy and small-angle X-ray diffraction, to induce a hexogonal H t~-phase lipid organization when incorporated in liquid crystalline saturated and unsaturated synthetic and natural phosphatidyicholines if the length of the fatty acids exceeds a 16 carbon atoms chain. The amount of hexagonaily organized lipid increases with increasing fatty acid chain length. With phosphatidylcholines possessing shorter fatty acid chains, as well as with the longer phosphatidylcholines in the gel state, a lamellar organization results. Of the various possible models to explain the induction of the hexagonal H u phase by gramicidin, one in which gramicidin dimers span adjacent cylinders of the hexagonal H~t phase seems most plausible. In phosphatidylcholines with intermediary chain lengths gramicidin induces intermediary structures, such as lipidic particles and possibly cubic phases. These experiments suggest that the balance between the length of the hydrophobic domain o| a peptide and the membrane thickness is of critical importance for the structure of the membrane. In relation to this observation, the possible involvement of non-bilayer lipid structures in insertion and anchoring of membrane proteins is dicussed.

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