Page 94 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN009G-417 July 10, 2001 15:10

Membrane Structure

Anna Seelig
Joachim Seelig
University of Basel

I. Introduction
II. Membrane Lipids
III. The Membrane–Water Interface
IV. Hydrophobic Core Region
V. Phase Behavior of Lipids and Membrane
Domain Formation
VI. Interaction of Membrane Lipids with Amphiphilic
Molecules and Transmembrane Proteins
VII. Concluding Remarks

GLOSSARY fast molecular motions such as rotational and flexing
movements are thus characterized, on the average, by
Biological membrane A very thin sheath of biological a cylindrical symmetry with the bilayer normal as the
material (thickness ∼10 nm to 15 nm) which consti- axis of motional averaging.
2
tutes the envelope of living cells and also of intracellu- Deuterium nuclear magnetic resonance ( H-NMR) By
lar organelles, separating them from the environment. means of chemical synthesis or biochemical incorpo-
Membranes are made up from a lipid bilayer into which ration, protons in lipid molecules can be selectively
proteins are embedded. They are highly organized but replaced by deuterium atoms. Since the van der Waals
are nevertheless fluid enough to allow considerable radii of the two isotopes are identical, this substitution
translational, rotational, and flexing movements of the leaves the membrane virtually unchanged, which is in
constituent lipid and protein molecules. contrasttootherbulkierreportergroups.Deuteriumnu-
Lipid bilayer A double layer of lipid molecules orga- clear magnetic resonance provides information on the
nized in a tail-to-tail arrangement. It is an anisotropi- order and mobility of the molecule. Structural informa-
cally ordered fluid that has a number of properties in tion comes from the deuterium quadrupole splitting,
common with smectic liquid crystals. The normal to the dynamic information is derived from NMR relaxation
surface of the lipid bilayer constitutes an axis of mo- times.
tional averaging. From an optical point of view, bilayer Quadrupolar splitting (∆ν Q ) The deuterium nucleus
membranes thus behave like uniaxial crystals with the has a spin = 1 and, due to its electric quadrupole mo-
bilayer normal as the optical axis (director axis n). All ment, the anisotropic motion within the membrane will

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