Chemistry and Physics of Lipids 164 (2011) 62–69
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Chemistry and Physics of Lipids
journal homepage: www.elsevier.com/locate/chemphyslip
Sterol chemical configuration influences the thermotropic phase behaviour of
dipalmitoylphosphatidylcholine bilayers containing 5-cholestan-3- and 3-ol
Matthew G.K. Benesch, David A. Mannock
∗
, Ronald N. McElhaney
∗
Department of Biochemistry, School of Molecular and Systems Medicine, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, T6G 2H7 Canada
article info
Article history:
Received 28 June 2010
Received in revised form 20 October 2010
Accepted 27 October 2010
Available online 3 November 2010
Keywords:
Differential scanning calorimetry
Cholesterol
Phosphatidylcholine
Sterol–lipid interactions
Phase behaviour
Cholestanol
Epicholestanol
Dihydrocholesterol
abstract
It is commonly believed that all membrane sterols are rigid all-trans ring systems with a fully extended
alkyl side-chain and that they similarly influence phospholipid bilayer physical properties. Here, we
report the sterol concentration-dependent, thermotropic phase behaviour of binary dipalmitoylphos-
phatidylcholine (DPPC)/sterol mixtures containing two similar 5-H sterols with different functional
group orientations (3-OH or 3-OH), which adopt an ideal all-trans planar ring conformation but lack
the deformed ring B conformation of cholesterol (Chol) and epicholesterol (Echol), using differential
scanning calorimetry (DSC). Our deconvolution of the DSC main phase transition endotherms show dif-
ferences in the proportions of sterol-poor (sharp) and sterol-rich (broad) domains in the DPPC bilayer
with increasing sterol concentration, which delineate gel/liquid-crystalline (P
′ /L
) and disordered gel
(L
)/liquid-ordered (l
o
) phase regions. There are similarities in the DPPC main phase transition tempera-
ture, cooperativity and enthalpy for each 3-ol and 3-ol pair with increasing sterol concentration and
differences in the parameters obtained for both the sterol-poor and sterol-rich regions. The sterol-poor
domain persists over a greater concentration range in both 3-ol/DPPC mixtures, suggesting that either
those domains are more stable in the 3-ols or that those sterols are less miscible in the sterol-rich
domain. Corresponding parameters for the sterol-rich domain show that at sterol concentrations up to
20 mol%, the 5-H,3-ol is more effective at reducing the phase transition enthalpy of the broad compo-
nent (H
brd
m
) than Chol, but is less effective at higher concentrations. Although mixtures containing Echol
and 5-cholestan-3-ol have similar positive slopes below 7 mol% sterol, suggesting that they abolish the
L
/l
o
phase transition equally effectively at low concentrations, Echol is more effective than the saturated
3-ol at higher sterol concentrations. A comparison of H
brd
m
obtained for the saturated and unsaturated
pairs suggests that the latter sterols stabilize the l
o
phase and broaden and abolish the DPPC main phase
transition more effectively than the saturated sterols at physiologically relevant concentrations, support-
ing the idea that the double bond of Chol and Echol promotes greater sterol miscibility and the formation
of l
o
phase lipid bilayers relative to corresponding saturated sterols in biological membranes.
© 2010 Elsevier Ireland Ltd. All rights reserved.
Abbreviations: DSC, differential scanning calorimetry; FTIR, Fourier-transform
infrared; Chol, cholesterol; Echol, epicholesterol; DPPC, dipalmitoylphosphatidyl-
choline; 5-H,3-ol, 5-cholestan-3-ol; 5-H,3-ol, 5-cholestan-3-ol; T
shp
m
,
chain-melting phase transition temperature of the sterol-poor domain; T
shp
1/2
,
chain-melting phase transition peak-width at half height of the sterol-poor domain;
H
shp
m
, chain-melting phase transition enthalpy of the sterol-poor domain; T
brd
m
,
chain-melting phase transition temperature of the sterol-rich domain; T
brd
1/2
, chain-
melting phase transition peak-width at half height of the sterol-rich domain; H
brd
m
,
chain-melting phase transition enthalpy of the sterol-rich domain; P
′ , tilted rip-
ple gel phase; L, lamellar liquid-crystalline phase; L
, lamellar gel phase; lo,
liquid-ordered phase; HOMO, highest occupied molecular orbital; LUMO, lowest
unoccupied molecular orbital.
∗
Corresponding author. Tel.: +1 780 492 2412/2413; fax: +1 780 492 0886.
E-mail addresses: dmannock@ualberta.ca (David A. Mannock),
rmcelhan@ualberta.ca (Ronald N. McElhaney).
1. Introduction
Biological membranes contain a wide range of structurally
important polar phospho- and glycolipids, as well as many
amphiphilic molecules with different chemical configurations and
functions. The lipid bilayer is a fundamental structure in biologi-
cal membranes, acting as a barrier between the cytoplasm and the
external environment, dividing the cell into smaller compartments
for specific biochemical roles. Sterols, such as cholesterol (Chol),
are among the most abundant plasma membrane constituents in
eukaryotes. The majority of natural membrane sterols contain an
equatorially oriented C3–OH group () in ring A of the steroid
nucleus (Fig. 1), which is a tetracyclic system containing three
cyclohexane and a single cyclopentane ring arranged in an all-
trans configuration. This ring system may have two or more methyl
branches on one or both surfaces and is assumed to be planar and
0009-3084/$ – see front matter © 2010 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.chemphyslip.2010.10.003