Temperature and compositional dependence of the structure of hydrated dimyristoyl lecithin

• Published in: The Journal of biological chemistry Vol. 254; no. 13; pp. 6068 - 6078
• Main Authors: Janiak, M J, Small, D M, Shipley, G G
• Format: Journal Article
• Language: English
• Published: United States American Society for Biochemistry and Molecular Biology 10-07-1979
• Subjects: Calorimetry, Differential Scanning; Membranes, Artificial; Molecular Conformation; Myristates; Phosphatidylcholines; Temperature; Water; X-Ray Diffraction
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/s0021-9258(18)50520-2

Differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the structure and phase behavior
of hydrated dimyristoyl lecithin (DML) in the hydration range 7.5 to 60 weight % water and the temperature range -10 to +60
degrees C. Four different calorimetric transitions have been observed: T1, a low enthalpy transition (deltaH approximately
equal to 1 kcal/mol of DML) at 0 degrees C between lamellar phases (L leads to Lbeta); T2, the low enthalpy "pretransition"
at water contents greater than 20 weight % corresponding to the transition Lbeta leads to Pbeta; T3, the hydrocarbon chain
order-disorder transition (deltaH = 6 to 7 kcal/mol of DML) representing the transition of the more ordered low temperature
phases (Lbeta, Pbeta, or crystal C, depending on the water content) to the lamellar Lalpha phase; T4, a transition occurring
at 25--27 degrees C at low water contents representing the transition from the lamellar Lbeta phase to a hydrated crystalline
phase C. The structures of the Lbeta, Pbeta, C, and Lalpha phases have been examined as a function of temperature and water
content. The Lbeta structure has a lamellar bilayer organization with the hydrocarbon chains fully extended and tilted with
respect to the normal to the bilayer plane, but packed in a distorted quasihexagonal lattice. The Pbeta structure consists
of lipid bilayer lamellae distorted by a periodic "ripple" in the plane of the lamellae; the hydrocarbon chains are tilted
but appear to be packed in a regular hexagonal lattice. The diffraction pattern from the crystalline phase C indexes according
to an orthorhombic cell with a = 53.8 A, b = 9.33 A, c = 8.82 A. In the lamellae bilayer Lalpha strucure, the hydrocarbon
chains adopt a liquid-like conformation. Analysis of the hydration characteristics and bilayer parameters (lipid thickness,
surface area/molecule) of synthetic lecithins permits an evaluation of the generalized hydration and structural behavior of
this class of lipids.