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Mechanism of the Lamellar/Inverse Hexagonal Phase Transition Examined by High Resolution X-Ray Diffraction

Michael Rappolt ^*, Andrea Hickel , Frank Bringezu  and Karl Lohner 

^* Institute of Biophysics and X-Ray Structure Research, Austrian Academy of Sciences, c/o Sincrotrone Trieste, 34012 Basovizza, Italy; and Institute of Biophysics and X-Ray Structure Research, Austrian Academy of Sciences, A-8042 Graz, Austria

Correspondence: Address reprint requests to Michael Rappolt. Tel.: 39-040-375-8708; Fax: 39-040-938-0902; E-mail: Michael.Rappolt{at}elettra.trieste.it.

For the first time the electron density of the lamellar liquid crystalline as well as of the inverted hexagonal phase could be retrieved at the transition temperature. A reliable decomposition of the d-spacings into hydrophobic and hydrophilic structure elements could be performed owing to the presence of a sufficient number of reflections. While the hydrocarbon chain length, d_C, in the lamellar phase with a value of 14.5 Å lies within the extreme limits of the estimated chain length of the inverse hexagonal phase 10 Å < d_C < 16 Å, the changes in the hydrophilic region vary strongly. During the lamellar-to-inverse hexagonal phase transition the area per lipid molecule reduces by 25%, and the number of water molecules per lipid increases from 14 to 18. On the basis of the analysis of the structural components of each phase, the interface between the coexisting mesophases between 66 and 84°C has been examined in detail, and a model for the formation of the first rods in the matrix of the lamellar phospholipid stack is discussed. Judging from the structural relations between the inverse hexagonal and the lamellar phase, we suggest a cooperative chain reaction of rod formation at the transition midpoint, which is mainly driven by minimizing the interstitial region.

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