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356 Membrane Structure

give rise to a quadrupole splitting, ν Q (kHz). In an un- Phosphorous nuclear magnetic resonance ( P-NMR)
31
oriented sample, as most membrane preparations are, No isotope labeling is required for 31 P-NMR spec-
the deuterium quadrupole interactions give rise to a troscopy. The chemical shielding anisotropy, σ,in
characteristic powder pattern. The spectrum has two 31 P-NMR is comparable to the deuterium quadrupole
distinct peaks, the separation of which is the so-called splitting in H-NMR and can be determined from the
2
deuterium quadrupole splitting, ν Q powder . The deu- edges of the spectrum.
terium quadrupole splitting may by used to calculate
the deuterium order parameter S CD according to
2 THE MAIN STRUCTURAL element of the biological
ν Qpowder = (3/4)(e qQ/h)S CD
membrane is the lipid bilayer. Lipid molecules, when
The static deuterium quadrupole coupling constant brought into contact with water, spontaneously organize
2
is (e qQ/h) = 170 kHz for aliphatic carbon-deuterium themselves into a bilayer leaflet: The polar lipid head-
(C D) bonds. A change in the residual quadrupole groups remain in the aqueous environment while the fatty
splitting can be caused by two different mechanisms. acid tails form the inner hydrophobic core. The lipid bi-
First, the angle of the molecular fluctuations may in- layer is thus a “sandwich”-like structure with the polar
crease or decrease, secondly, the molecule may un- groupasthe“bread”andthefattyacylchainasthe“butter.”
dergo a conformational change which alters the ori- The structure of the lipid bilayer and the interaction of the
entation of the C D bond vector with respect to the lipid molecules with their environment, such as metal ions,
bilayer normal. peptides, and proteins, are the themes presented here. Us-
Order parameter (S CD ) The deuterium order parameter ing solid-state nuclear magnetic resonance (NMR) tech-
is a measure of the motional anisotropy of the particu- niques, a quantitative analysis of the molecular ordering
lar C D bond investigated and yields its time-averaged and dynamics of a lipid bilayer has become possible with a
orientation. If denotes the instantaneous angle be- segment-to-segment resolution. Lipid bilayers—and also
tween the C D bond and the direction of the bilayer intact biological membranes—are not rigid but can be
normal then S CD is defined as classified in physical terms as smectic liquid crystals. The
lipids within each bilayer undergo rapid translational and
2
S CD = (1/2)(3 cos − 1)
rotational motion. The packing of the hydrocarbon chains
where the bar denotes a time average. is best described in terms of statistical order profiles. In
Order parameter (S mol ) Assuming an axial symmetry contrast, well-defined conformations are observed for the
of the segment motion S CD can further be related to the glycerol backbone and to some extent also for the polar
molecular order parameter S mol according to head groups. Both the order profile and the orientation of
the polar groups can vary considerably depending on the
S mol =−2S CD . external conditions and constitute regulatory elements for
the function of the biological membrane.
If the chains are fixed in an all-trans conformation and
are just rotating around the long molecular axis, the
molecular order parameter would be unity. The other I. INTRODUCTION
extreme is that of a completely statistical movement
through all angles of space, leading to S mol = 0. This
Biological membranes segregate cells and organelles, act
simple statistical interpretation of S CD is not possible
as barriers for the passive transport of matter, and support
if specific geometric effects come into play as, for ex-
a wide range of important metabolic processes, includ-
ample, in the case of the cis-double bond.
ing active transport, energy flow, signal transduction, and
Order profile of the lipid bilayer It shows the variation
motility. The two main components of membranes are
of the order parameter, S mol or S CD , with the position
lipids and proteins. Depending on the type of membrane,
of the segment in the chain and is an expression of the
lipids contribute between 20 and 80% by weight to the
average angular fluctuations around the bilayer normal.
total membrane mass, the rest being protein. The lipid
Spin-lattice relaxation time (T 1 ) The spin lattice relax-
molecules are predominantly arranged in a bilayer struc-
ation time depends on both the ordering (S CD ) and the
ture with the hydrophilic head groups facing the aqueous
rate of motion (correlation time, τ C ). Assuming a mo-
environments and the fatty acyl chains forming the in-
tion sufficiently characterized by a single correlation
ner hydrophobic core. Minor but functionally important
time, τ C , the following expression holds for the short
components of membranes are carbohydrates. They are
correlation time limit:
covalently attached to either lipids (glycolipids) or pro-
2 2
3 e qQ 2 teins (glycoproteins) and are restricted to the outer leaflet
1/T 1 = 1 − S CD τ C
8 h of the bilayer membrane.