Transcript Introduction Into Cubic Phase Lipids

Introduction Into Cubic Phase
Lipids
Matt Chandler
Polymorphism
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In general, it describes
multiple possible states
for a single property.
Also known as
mesomorphism.
Ex. Carbon can exist as
diamond or graphite.
Polymorphism in Lipids
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The ability of a given mixture
of lipids to form
crystallographically diverse
structures.
A - lamellar liquid crystal
phase, L.
B - inverted hexagonal
phase, HII.
C - hexagonal phase, HI.
Why Study Lipid Polymorphs?
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They exhibit the broadest range of polymorphic
structures of any known class of molecules.
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By studying the structural polymorphism observed
with isolated lipids, we can gain an understanding
of the forces that are locked up in biomembranes
and that affect the organization and function of
proteins.
Terminology

Non-Bilayer Phase - nonlamellar phase, or liquid
crystalline phases that are not L phases.
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Inverted or Water-In-Oil Phase - refers to one in
which the lipid/water interface has the same sign
curvature as an HII phase, i.e. a net concave
curvature when viewed from the water domain.
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Liquid-Crystalline - refers to phases that are
intermediate to the rigorously crystalline solids and
true isotopic liquids, including systems that do not
have long flexible chains.
Lipid Phases
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We try to understand the physical basis of lipid
phases, because an understanding of this
basis gives insight into the forces at play in
lipid bilayers.
A polar biomembranes lipid interaction with
water allows for a variety of structures, or
polymorphs, not normally found in cells.
These include, lipid bilayers, as well as tubes,
rods (hexagonal phases), and three
dimensional assemblies, aka cubic phase
lipids.
Lamellar Crystalline Phase, L
Bilayer (cylindrical).
 Composed of lipid
molecules, usually
phospholipids.
 These
phospholipids have
glycerol backbones
with polar head
groups and long
hydrocarbon,
hydrophobic tails.
Inverted Hexagonal Phase, HII
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Reverse micelle
aggregates that form
tubes and rods.
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Concave curvature.
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Precipitates out of an
aqueous solution.
Hexagonal Phase, HI
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Tubular micelle aggregates.
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Convex curvature.
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Will suspend in aqueous solution.
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Mostly comprised of
lysophospholipids (monoacyl).
Snapshot of a configuration of lipid
aggregates in the form of a filament of
rod-like micelles formed by H3(T5)2
lipids and simulated by dissipative
particle dynamics. The model
parameters are adapted from Groot and
Rabone (2001). The red beads
represent hydrophilic head groups (H)
and the green beads represent the tail
beads (T). For the sake of clarity, the
water beads are not shown.
Cubic Phase Lipids
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Cubic phases have an interesting
thermodynamically stable structure
consisting of curved bicontinuous lipid
bilayer in three dimensions, separating
two congruent networks of water
channels.
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It is suggested that the cubic phase is
an intermediate of a phase transition
between hex II and the lamellar phase,
and is stabilized at a particular
temperature.
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Spontaneously formed when
amphiphilic lipids are placed in
aqueous environments.
Most Common Lipids in CLP
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Common cubic phase lipids:
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1-monooleoyl-racglycerol (MO)
1-monopalmitoleoyl-racglycerol (MP)
Palmitoyl
lysophosphatidylcholine
(PLPC)
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Cubic Phase Lipids (Cont.)
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CLPs have been studied extensively, however, this has
proven to be difficult both due to the structural complexity
and because crystallographically well-formed samples are
difficult to obtain.
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Crystallographic structures are difficult to obtain due to the
low enthalpies associated with bicontinuous cubic
transitions.
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This suggests that the formation of cubic phases is not
strongly favored and involves large energy of activation
barriers.
Cubic Phase Lipids (Cont.)
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Take on a PMS topology, or
periodic minimal surface,
which is a three-dimensional
surface that periodically has
zero mean curvature (H=0)
everywhere.
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Since the surface topology
has a PMS-like structure, the
cubic phase is in between
the L phase (H=0) and the
HII phase (H is large), using
mean curvatures.
Most cubic PMS structures
are just lipid monolayers that
drape both sides of PMS-like
structures, which is why they
are classified as nonlamellar.
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3 Common Motifs (Space Groups)
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Primitive P, Im3m
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Double diamond D,
Pn3m
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Gyroid G, Ia3d
Purpose and Uses
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These cubic phases are being used to grow wellordered, three dimensional crystals of smaller
membrane proteins.
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Understanding the mechanisms of membrane
proteins requires the elucidation of their structures to
high resolution.
Purpose and Uses (Cont.)
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The complexity of the cubic
phase allows for nucleation
sites, or seeding sites, for
membrane proteins to
integrate and support growth
by lateral diffusion of protein
molecules in the membrane.
This is known as feeding.
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Proteins, once integrated,
tend to aggregate toward the
valley of the cubic phase
matrix. This population
increase allows for the
crystallization of smaller
membrane proteins.
Bacteriorhodopsin crystal
obtained in the cubic-lipid phase
(Pebay-Peyroula et al., Science 97,
Belrhali et al., Structure 99).
Purpose and Uses (Cont.)
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Cubic phase lipids have also been used on a nanometer level
as a drug delivery mechanism.
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Cubic phases have been shown to deliver small molecule drugs
and large proteins by oral and parenteral routes in addition to
local delivery in vaginal and periodontal cavity.
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Release of drugs from cubic phase typically show diffusion
controlled release from a matrix as indicated by Higuchi's
square root of time release kinetics (international journal of
pharmaceutics. Volume 160, issue 2, 26 January 1998, pages 207-212).
Purposes and Uses (Cont.)
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Incorporation of drug in cubic phase can cause phase
transformation to lamellar or inverted hexagonal
phase depending on the polarity and concentration of
the drug, which could also affect the delivery.
Why CLP Is Good for Drug Delivery
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Biodegradability.
Phase behavior.
Ability to deliver drugs of different size
and polarity.
Ability to enhance chemical/physical
stability of drugs or proteins.
Drawback
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Shorter release duration and the extremely high
viscosity may limit CLPs use to specific applications
such as periodontal, mucosal, vaginal and short
acting oral and parenteral drug delivery.
Future
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A new potential application of the cubic phase
(monoolein/water; 70:30, w/w) which is being studied,
involves delivering pro-drugs and a photosensitizer
for topical application in photodynamic therapy
(PDT).
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Crystallization efforts will continue as well.
References:
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Yeagle, P. (2005 ).The Structure of Biological Membranes (2nd. Ed.).
Philadelphia: CRC Press.
Landau, E. M. and Rosenbusch, J.P. (1996). Lipidic Cubic Phases: A novel
concept for the crystallization of membrane proteins. PNAS, 93(25), 1453214535.
Seddon, J. M. and Conn, C. (2003). Pressure-jump Studies of Liquid-crystalline
Cubic Phase Transition in Lipids.University of Dortmund, Germany.
Grabe, M. and Neu, J. (2003). Protein Interactions and Membrane Geometry.
Biophysical Journal. 84, 854-868.
Saludjian, P. and Reiss-Husson, F. (1980). Structure of the body-centered cubic
phase of lipid systems. Proc. Natl. Acad. Sci. USA. 77(12), 6991-6995.
Shah JC, Sadhale Y, and Chilukuri DM. Adv Drug Deliv Rev. 2001 Apr 25;47(23):229-50.