Synthesis, structure, and thermal properties of 1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering lipid

Article abstract of DOI:10.1016/S0009-3084(99)00110-3

A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase bilayers at 20°C with extremel

Full text of DOI:10.1016/S0009-3084(99)00110-3

Chemistry and Physics of Lipids
104 (2000) 67–74
www.elsevier.com/locate/chemphyslip
Synthesis, structure, and thermal properties of
1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering
lipid
M.P. Pollastri a,1, N.A. Porter a,2, T.J. McIntosh b,*, S.A. Simon c
a Department of Chemistry, Duke Uni6ersity, Durham, NC 27706, USA
b Department of Cell Biology, Duke Uni6ersity Medical Center, Durham, NC 27710, USA
c Department of Neurobiology, Duke Uni6ersity Medical Center, Durham, NC 27710, USA
Received 26 April 1999; received in revised form 13 August 1999; accepted 13 August 1999
Abstract
A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray
diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase
bilayers at 20°C with extremely narrow interbilayer fluid separations, indicating that apposing DPGG bilayers
strongly adhere to each other. Differential scanning calorimetry shows that fully hydrated DPGG exhibits a
pretransition exotherm (3.7 kcal/mol) at 52°C and a high enthalpy (11.3 kcal/mol) main endothermic transition at
69°C. These thermal properties are similar to those of galactosylceramides with similar hydrocarbon chain composi-
tions. The adhesive and thermal properties of DPGG are likely due to both intermolecular hydrogen-bonding and
hydrophobic interactions between the aromatic rings on the gallic acids. © 2000 Published by Elsevier Science Ireland
Ltd. All rights reserved.
Keywords: Adhesion; X-ray diffraction; Calorimetry; Polyphenol
1. Introduction
tive and repulsive energies. For most phospho-
lipid bilayers, the major repulsive interactions
include electrostatic, hydration, and steric pres-
sures (Parsegian et al., 1979; Rand and Parsegian,
1989; McIntosh and Simon, 1994; Israelachvili
and Wennerstrom, 1996), and the major attractive
interaction is the van der Waals pressure (Parse-
gian et al., 1979). However, compared to most
membrane lipids, two classes of lipids have unusu-
ally large adhesion energies — electrically neutral
glycolipids, such as cerebrosides, and the zwitteri-
The adhesive properties of lipid bilayers depend
on the balance of a number of non-specific attrac-
* Corresponding author. Fax: +1-919-681-9929.
E-mail address: t.mcintosh@cellbio.duke.edu (T.J. McIn-
tosh)
1 Present address: Pfizer Central Research, Box 8002, East-
ern Point Road, Groton, CT 06340-8002, USA.
2 Present address: Department of Chemistry, Box 1822, Sta-
tion B, Vanderbilt University, Nashville, TN 37235, USA.
0009-3084/00/$ - see front matter © 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.
PII: S0009-3084(99)00110-3

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M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
onic phospholipid, phosphatidylethanolamine
(PE) (Evans and Needham, 1987; McIntosh and
Simon, 1996). Both cerebrosides and PEs form
hydrogen bonds in the plane of the membrane
(Boggs 1987; Pascher et al., 1987), and it has been
argued that such H-bond networks may be critical
to the adhesive properties of these bilayers (Sed-
don et al., 1984; Damodaran and Merz, 1994).
Related to the strong self-adhesion (and therefore
low hydration) of cerebrosides and PEs are their
higher melting temperatures (Tm) compared to
phosphatidylcholines and phosphatidylglycerols
with similar hydrocarbon compositions (Nagle,
1976, 1980; Ruocco and Shipley, 1983). Again,
the H-bond characteristics of cerebrosides and
PEs could be one factor in their high melting
temperatures. In the case of the cerebroside galac-
tosylceramide (GalCer) it has been argued that
the higher Tm involves stabilization of the gel
phase via intra- and intermolecular H-bonds in-
volving the galactose (Ruocco and Shipley, 1983).
In the case of PE, weak and transient H-bonding
between the phosphate and ethanolamine groups
could raise Tm by inhibiting lateral expansion of
the bilayer (Nagle, 1976, 1980).
The adhesion and partial dehydration of neu-
tral phospholipid bilayers can also be promoted
by the exogenous addition of tannic acid or other
polyphenolic compounds (Simon et al., 1994; Huh
et al., 1996). These polyphenols appear to associ-
ate with phosphatidylcholine bilayers by hydro-
phobic interactions arising from the aromatic
rings on their gallic acids (Oh et al., 1980), by
donating hydrogen bonds to H-bond accepting
groups (Haslam, 1974), and by p-dipole interac-
tions between the choline headgroup and the aro-
matic gallates (Haslam, 1974). By similar types of
interactions tannic acid also aggregates or precipi-
to tether proteins, membranes, or cells to glass or
other substrates for experimental use or as a
biocompatible coating for medical implants. The
second goal is to compare the structure and ther-
mal properties of DPGG to naturally occurring
lipids, such as galactosylceramide and PE, which
have large adhesion energies but different hydro-
gen-bonding patterns. Of particular interest is a
comparison of DPGG with GalCer, since the
headgroups of both lipids have multiple hydroxy
groups, but DPGG lacks the sphingosine back-
bone that may be important in intramolecular
H-bonding in GalCer (Pascher and Sundell,
1977).
2. Materials and methods
2.1. Chemical synthesis
The scheme for the synthesis of 1,2-dipalmitoyl-
galloylglycerol (1) is outlined in Fig. 1. 2,3-
Dipalmitoylglycerol (2) was obtained from Avanti
Polar Lipids (Alabaster, AL). Other chemicals
were obtained from Aldrich (St. Louis, MO) and
were used as received. Tri-O-benzylgalloyl chlo-
ride (3) was synthesized by benzylation of methyl
gallate, followed by hydrolysis, acidification, and
treatment of the resulting carboxylic acid with
phosphorous pentachloride. Chemical synthesis
reactions were run in flame-dried glassware under
1
Argon, and all solvents were distilled. H and 13C
NMR spectra were recorded on a Varian IN-
OVA-400 spectrometer. Chemical shifts are re-
ported in ppm using TMS as a reference. FTIR
spectra were recorded on a Bomem Michelson
Series BM-10 FTIR spectrophotometer and mass
spectra were obtained on a JEOL 5X-10Z using
fast atom bombardment (FAB). Melting points
were obtained on a Hoover apparatus and are
uncorrected.
tates
a
variety of proteins and polymers
(Goldstein and Swain, 1965; Nash et al., 1966;
Haslam, 1974; Oh et al., 1980).
In this paper we synthesize and characterize the
new lipid, 1,2-dipalmitoylgalloylglycerol (DPGG),
which is a diacyl glycerol-based lipid containing a
polyphenolic head group. This study has two
primary goals. The first is to develop a lipid
molecule that forms bilayers with strong adhesive
properties. Such bilayers could potentially be used
2.1.1. Dipalmitoyl(3,4,5-tri-O-benzylgalloyl)-
glycerol (4)
To a solution of 80 mg dipalmitoylglycerol (2)
(0.1 mmol, 1 eq.) in 25 ml CH2Cl2 was added 54
mg tri-O-benzylgalloyl chloride (3) (0.11 mmol,
1.1 eq.) and 14 mg DMAP (0.11 mmol, 1.1 eq.)

M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
69
and stirred at room temperature over night. The
reaction was diluted with CH2Cl2 and washed 2×
1 M HCl, 2× 1 M NaOH, 1× brine, dried over
MgSO4, filtered, and evaporated. The residue was
chromatographed in 1:1 Et2O:hexane (Rf=0.61).
Evaporation provided (4), mp 56°C, in 72% yield.
1H NMR: (CDCl3, ppm): 7.40 (m, 17H), 5.41 (m,
1H), 5.14 (s, 4H), 5.12 (s, 2H), 4.45 (dd, J1=4.8
Hz, J2=11.6 Hz, 1H), 4.38 (m, 2H), 4.20 (dd,
J1=6 Hz, J2=12 Hz, 1H), 2.30 (t, 7.6 Hz, 4H),
1.61 (m, 4H), 1.22 (brm, 48H), 0.88 (t, J=7.2
Hz). 13C NMR (CDCl3, ppm): 201.9, 196.2, 152.6,
128.6, 128.2, 128.0, 127.5, 109.2, 75.1, 71.2, 63.0,
62.1, 34.3, 34.0, 31.9, 29.7, 29.5, 29.4, 29.3, 29.1,
Hz). 13C NMR (CDCl3, ppm): 174.0, 173.7, 166.2,
142.6, 120.7, 109.9, 69.1, 63.0, 62.3, 34.3, 34.1,
31.9, 30.3, 29.7, 29.5, 29.4, 29.3, 29.1, 24.9, 22.7,
14.1. IR(CHCl3 solution): 3329 cm−1 2926 cm−1
,
2853 cm−1, 1725 cm−1. MS (HiRes, Fab+): cal-
culated for C42H73O9 (MH+) 721.5255, found
721.5255. [a]D(CHCl3): +0.040.
2.2. Differential scanning calorimetry
Lipid samples were prepared by weighing
DPGG into an aluminum sample pan and adding
appropriate amounts of water. The pans were
hermetically sealed and cycled repeatedly between
20 and 80°C (which is above the main endother-
mic lipid transition temperature, Fig. 2). The lipid
was then heated and thermograms recorded at a
scan rate of 2.5°C/min in a Perkin-Elmer DSC7
calorimeter. The reference cell contained a volume
of water equal to the amount in each of the
hydrated samples.
24.9, 22.7, 14.1. IR(CHCl3 solution): 2926 cm−1
,
2853 cm−1, 1725 cm−1. MS (HiRes, Fab+):
calculated for C63H90O9 (M+): 990.6585, found:
990.6542. [a]D(CHCl3): +0.105.
2.1.2. Dipalmitoylgalloylglycerol (DPGG) (1)
A solution of (4) in THF was hydrogenated
over 10% Pd/C at 80 psi H2 overnight, filtered
through Celite, and the solvent evaporated. The
residue was then dissolved in acetone and treated
with decolorizing carbon to provide (1), a white
2.3. X-ray diffraction
X-ray diffraction patterns of both dry and hy-
drated DPGG were recorded on Kodak DEF
X-ray film. Hydrated DPGG was prepared by
addition of excess water and repeated heating and
cooling cycles between 20 and 80°C, a tempera-
1
solid, mp 88°C, in 75% yield. H NMR: (CDCl3,
ppm): 7.21 (s, 2H), 6.20 (brs, 3H), 5.44 (m, 1H),
4.39 (m, 3H), 4.26 (m, 1H), 2.34 (t, J=7.6 Hz),
1.61 (m, 4H), 1.25 (brm, 48H), 0.88 (t, J=6.8
Fig. 1. Scheme for the synthesis of 1,2-dipalmitoylgalloylglycerol (DPGG).

70
M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
(McIntosh and Simon, 1986a). Electron density
profiles, z(x), on a relative electron density scale
were calculated from
z(x)=(2/d) % exp{iƒ(h)} · F(h) · cos(2yxh/d)
(1)
where x is the distance from the center of the
bilayer, d is the lamellar repeat period, ƒ(h) is the
phase angle for order h, and the sum is over h.
,
The resolution of the profiles was d/2hmax:8 A.
3. Results
3.1. Differential scanning calorimetry
Fig. 2 shows a thermogram of DPGG in excess
water. DPGG exhibited exothermic pretransition
peaks at 52.5 and 55.4°C, a main endothermic
transition at 69.3°C with an endothermic shoulder
centered at 64°C, and an additional minor en-
dothermic transition at 71.1°C. The total enthalpy
of the pretransition peaks (DHpre) was −3.7 kcal/
mole, whereas the total enthalpy of the main
endothermic transition and shoulder (DHm) was
11.3 kcal/mol (Table 1). This thermogram dis-
played similar features to the one obtained for the
fully hydrated galactosylceramide N-palmitoyl-
galactosylsphingosine (NPGS) (Ruocco et al.,
1981; Ruocco and Shipley, 1983), which has an
exothermic pre-transition at 52°C followed by a
main endothermic transition at 82°C (Table 1).
However, the thermal transitions of DPGG dif-
fered markedly from those of a phosphatidyl-
choline (DPPC) and a phosphatidylethanolamine
(DPPE) with the same hydrocarbon composition
as DPGG (Table 1).
Fig. 2. Differential scanning calorimetry thermogram of fully
hydrated DPGG. The arrow points in the direction of an
endothermic transition.
ture above the lipid’s main transition temperature.
The mixture was vortexed extensively. Both the
dry powder and hydrated DPGG samples were
loaded in quartz glass X-ray capillary tubes and
mounted in a point collimation X-ray camera and
exposed at 20°C. X-ray films were processed by
standard techniques and densitometered with a
Joyce-Loebl microdensitometer as described pre-
viously (McIntosh and Simon, 1986a,b). After
background subtraction, integrated intensities,
I(h), were obtained for each order h by measuring
the area under each diffraction peak. The struc-
ture amplitude F(h) was set equal to {h2I(h)}1/2
Table 1
Thermal properties of DPGG and related lipidsa
Lipid
Tpre (°C)
DHpre (kcal/mol)
Tm (°C)
DHm (kcal/mol)
DPGG
NPGS
DPPC
DPPE
52.5, 55.4
52
34.9
−3.7
69.3
82
41
11.3
17.5
8.3
−0 to −8b
1.5
64
8.5
a NPGS data are from Ruocco et al. (1981), and DPPC and DPPE data are from Marsh (1990).
b Depends on the scan rate and thermal history.

M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
71
teristic of an Lb phase (Tardieu et al., 1973;
McIntosh, 1980). This pattern was very similar in
both repeat period and intensity distribution to
the one obtained at 20°C for dipalmitoylphos-
phatidylethanolamine (DPPE), which gave
a
,
lamellar repeat period of 63 A and a single sharp
,
wide-angle reflection centered at 4.15 A (McIn-
tosh, 1980). For comparison, the X-ray patterns
of NPGS at full hydration (after being cooled
from above its transition temperature) had two
,
lamellar repeat periods of 58 and 55 A with a
,
sharp wide angle reflection at 4.55 A and a more
,
diffuse reflection at 4.16 A (Ruocco et al., 1981).
To obtain more structural information regard-
ing the Lb gel phase of fully hydrated DPGG, we
calculated an electron density profile (top profile
in Fig. 3). Due to the similarities in the patterns
from DPGG and DPPE, this profile was calcu-
lated assuming the same phase angles as previ-
ously used for DPPE (McIntosh, 1980). The
profile of DPGG (Fig. 3) shows two apposing
bilayers with the origin located at the center of the
bilayer on the left. For the bilayer on the left the
Fig. 3. Electron density profiles for fully hydrated DPGG,
DPPE, and DPPC multilayers. Each profile shows two unit
cells containing two bilayers and the fluid spacing between
apposing bilayers. DPPE and DPPC data are from McIntosh
(1980).
3.2. X-ray diffraction
,
high electron density peaks at 925 A correspond
to the relatively high density galloyl head groups,
Low angle diffraction patterns of unheated
DPGG powder yielded four orders of diffraction
,
the trough at 0 A corresponds to the terminal
methyl groups of the fatty acid chains, and the
medium density regions between the terminal
methyl trough and the head group peaks corre-
spond to the fatty acid methylene chains. The
,
corresponding to a lamellar repeat period of 51 A.
There were also seven sharp wide-angle reflec-
tions, the two most intense corresponded to 3.93
,
and 3.67 A, indicating a crystalline packing of the
,
medium density region centered at 31 A corre-
saturated acyl chains lipids in the plane of the
membrane. For comparison, anhydrous N-palmi-
toylgalactosylsphingosine (NPGS) gave a diffrac-
sponds to the center of the fluid space between
apposing bilayers.
For comparison, electron density profiles of
DPPE and DPPC are also shown in Fig. 3, with
each profile containing two unit cells, including
two apposing bilayers and the fluid space between
each bilayer. The shapes of the profiles were
similar for DPGG, DPPE, and DPPC. However,
for the three systems the distances between the
high density head group peaks varied, both across
each bilayer and between adjacent bilayers. The
distance between head group peaks across the
,
tion pattern with a lamellar repeat period of 56 A,
,
and wide-angle reflections at 4.4 A (sharp) and 3.8
,
A (broad) (Ruocco et al., 1981).
Complete hydration of DPGG, as evidenced by
a visible excess water phase, was achieved by
addition of excess water and cycling between
room temperature and 80°C (which is above Tm).
The X-ray pattern for fully hydrated DPGG con-
sisted of four low angle reflections that indexed as
,
orders of a lamellar repeat period 62 A, and a
,
bilayer hydrocarbon region was 49, 49 and 42 A
,
single sharp wide-angle reflection at 4.09 A. The
for DPGG, DPPE, and DPPC, respectively,
whereas the distance between head group peaks
sharpness of the single reflection indicates that
chains were packed in a hexagonal lattice and not
significantly tilted from the bilayer plane, charac-
,
from apposing bilayers was 13, 14, and 22 A for
DPGG, DPPE and DPPC, respectively. Another

72
M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
difference between the profiles from the three
lipids was that the electron density of the fluid
spacing between apposing bilayers differed. In
particular, using the electron density of the hydro-
carbon region of the bilayers as a reference, one
can see that the electron density of the interbilayer
space was greater for DPGG than for DPPC.
bonding between the sphingosine backbone and
the galactose moiety in NPGS. Such interactions
would not be present in DPGG.
Table 1 also shows that both Tm and DHm were
greater for DPGG than for DPPE and DPPC,
two glycerol-based phospholipids with the same
hydrocarbon chains. Since these three lipids have
the same glycerol backbone and acyl chains, it
follows that the polar head group is critical. The
larger DHm seen for DPGG compared to DPPC
or DPPE could derive from several phenomena,
including different hydrocarbon chain packing in
the gel phase. However, this difference may arise,
at least in part, from the additional energy in the
gel phase required to break some of the attractive
interactions between neighboring gallates.
4. Discussion
The calorimetry and X-ray diffraction results
presented in this paper show that DPGG has
similar properties to other lipids, such as cere-
brosides and PE, that have large adhesion ener-
gies and narrow interbilayer fluid spacings.
4.1. Differential scanning calorimetry
4.2. X-ray diffraction
The calorimetry data for DPGG (Fig. 2 and
Table 1) were quite similar to those obtained from
NPGS, a cerebroside with an amide linked fatty
acid with the same acyl chain length as the fatty
acids in DPGG. This suggests that the specific
H-bonding network found in cerebrosides (in part
between the galactose and sphingosine backbone,
Pascher and Sundell, 1977) is not necessary to
account for all the unusual thermal properties of
NPGS and other cerebrosides. For NPGS, the
exothermic phase transition (at 52°C) is associ-
ated with the hydration of the polar head group
occurring at a phase transition between two gel
phases, the lower temperature phase having no
hydrocarbon chain tilt and the higher temperature
phase having tilted chains (Ruocco et al., 1981).
Given that the exothermic transition of NPGS
and DPGG both occur at the same Tm and have
approximately the same enthalpy (Table 1), it is
likely that similar phase transitions are present in
DPGG. For NPGS the main endothermic transi-
tion has Tm=82°C and DHm=17.5 kcal/mol
(Table 1) resulting from a transition between a
titled gel phase (Lb%) and liquid–crystalline (La)
phase (Ruocco et al., 1981). The main endother-
mic transition of DPGG has a lower Tm and DHm
than NPGS, which likely reflects different packing
of the acyl chains for the two lipids and the
additional work of breaking up the strong H-
The X-ray data clearly show that there are large
structural differences between dry and fully hy-
drated DPGG. Hydration of the lipid converted a
crystalline hydrocarbon chain packing into a
hexagonal packing, typical for hydrated gel phase
phospholipid bilayers with untilted hydrocarbon
chains (Tardieu et al., 1973; McIntosh, 1980). In
addition, the lamellar repeat period increased
when the lipid was hydrated, most likely due to a
decrease in chain tilt and possibly a change in
hydration and conformation of the polar head
group.
Although the electron density profile in the
fully hydrated phase is not at high enough resolu-
tion to give information on the conformation of
the polyphenolic head group, the profiles do
provide information on the widths of the bilayer
and fluid space between bilayers. The approxi-
,
mately 49 A separation between electron high
density peaks in the profile of DPGG (Fig. 3) is
similar to that obtained from phospholipid bilay-
ers having the same hydrocarbon chain composi-
tion and no hydrocarbon chain tilt. That is, both
fully hydrated DPPE (Fig. 3) and DPPC in the
presence of tetradecane (which removes chain tilt)
have similar headgroup separations in comparable
resolution electron density profiles (McIntosh,
1980). The profile for DPGG (Fig. 3) also dis-
plays an extremely narrow and relatively high

M.P. Pollastri et al. / Chemistry and Physics of Lipids 104 (2000) 67–74
73
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