Amphiphilic Poly(phenyleneethynylene) - Ka Yee C. Lee - University ...

ARTICLES Ishitsuka et al.
Figure 2. (A) XR of 1 monolayers at 10 and 20 mN/m, and (B) their
corresponding model density profile. Solid line for panel A is the best fit
using one box model. Solid and dashed lines for panel B are smeared and
unsmeared Fel(z) profiles of data in panel A, respectively.
Results and Discussion
Antibacterial and Hemolysis Results. The antibacterial and
hemolytic activity of 1 and 2 were tested using standard
microbroth dilution protocols to determine their MIC and HC50.
In accordance with previous findings, 19,38 1 was relatively
nonpolar and no antibacterial activity was measured up to 100
µg/mL, beyond which solubility became an issue. By contrast,
the much smaller compound, 2, exhibited potent activity against
both gram-negative E. coli and gram-positive methicillin
resistant S. aureus with an MIC of 0.8 and 0.5 µg/mL,
respectively. In comparison to these MIC values, HC50 of 2 (75
µg/mL) showed 94 and 155-fold selectivity of prokaryotes over
eukaryotes, respectively. Comparison of the HC50 and MIC
values shows that 2 is significantly more active toward bacteria
than RBCs.
mPE Monolayers. Compound 1 formed a stable monolayer
at the air-buffer interface and XR measurements were carried
out at 10 and 20 mN/m (Figure 2). A single slab representing
the entire polymer monolayer produced the best fit for both data
sets. At 10 mN/m, the Fel of the polymer layer was 0.421 (
0.003 e - /Å 3 with a layer thickness of 13.1 ( 0.3 Å. Two values
of surface roughness for the subphase-polymer (σS-P; 4.8 (
0.2 Å) and the polymer-air (σP-A; 3.28 ( 0.03 Å) interfaces
were used to account for the two interfaces. At 20 mN/m, the
fit gave a similar Fel value of 0.414 ( 0.003 e - /Å 3 , but the
thickness increased to 15.34 ( 0.09 Å, with a single roughness
of 3.68 ( 0.01 Å. A layer thickness of 13-15 Å agrees well
with predictions from molecular models and powder X-ray
diffraction. 44 GIXD measurements were performed at both
pressures but no diffraction peaks were observed for either case,
indicating the lack of any in-plane ordering.
Compound 2 was designed and synthesized both to enhance
water solubility and to allow increased control over molecular
size. As a result, it does not form a stable monolayer at the
air-buffer interface as indicated by the absence of change in Π
upon compression.
Insertion of 1 into DPPC and DPPG Monolayers. Constant
pressure insertion studies of 1 with DPPC and DPPG monolayers
in conjunction with XR and GIXD measurements were
performed at 30 mN/m to investigate its membrane selectivity,
the location of the inserted 1 in the lipid matrix, as well as its
membrane disordering effect, respectively.
∆A/A values obtained after the injection of 1 were significantly
different between DPPC and DPPG, demonstrating the
membrane selectivity of 1. While the maximum ∆A/A was
(44) Kim, T.; Arnt, L.; Atkins, E.; Tew, G. N. Chem. Eur. J. 2006, 12, 2423-
2427.
13126 J. AM. CHEM. SOC. 9 VOL. 128, NO. 40, 2006
Figure 3. XR and the corresponding electron density, Fel(z), profile of
DPPC and DPPC/1 (A and B) and DPPG and DPPG /1 (C and D) at 30
mN/m. Solid lines for panels A and C are the best fits of the models to the
experimental data. Solid and dashed lines for panels B and D are smeared
and unsmeared Fel(z) profiles of corresponding XR curves, respectively.
merely 2% for the model mammalian membrane DPPC, a ∆A/A
equal to 19% was observed for the model bacterial membrane
DPPG.
The different degrees of insertion are also reflected by
changes in the XR curves of DPPC and DPPG before and after
the injection of 1 (Figure 3). XR curves normalized by Fresnel
reflectivity are shown in Figure 3A,C, with their corresponding
Fel profiles fitting in Figure 3B,D. Zero on the x-axis is chosen
at the boundary between the tail and air, with the solid and
dashed lines indicating the smeared and unsmeared Fel(z)
profiles, respectively. For clarity, all XR data, corresponding
fitted curves, Fel profile, and corresponding slab models have
been offset vertically. All XR fit parameters are in Table 1.
Both DPPC and DPPG monolayers were fitted using a twoslab
model by varying the density Fel(i), thickness t(i) and surface
roughness σ(i) for each slab (i). Both sets of data have also been
fitted using a three-slab model, but the fits reverted back to
those obtained using a two-slab model. The final fit was
achieved by minimizing the -square value while ensuring that
parameters obtained were physically meaningful.
For insertion of 1 into a DPPC monolayer, only a minute
change is observed in the XR curves before and after polymer
injection (Figure 3A,B), indicating a weak interaction. The XR
curve for the DPPC/1 system can be fitted using a two-slab
model (headgroup (H) and tail group (T)), showing only minor
thickness and roughness differences compared to pure DPPC.
For DPPC, no additional slab for any adsorbed polymer (P)
was necessary to fit the data after the injection of 1, suggesting
that the majority of the polymer is distributed evenly in the
subphase and has a weak interaction with the DPPC film.
By contrast, a large change in the XR profile of DPPG was
observed with 1 injected into the subphase (Figure 3,C,D), in
agreement with the large ∆A/A observed for this system (Table
1). Because Fel of 1 at the air-buffer interface is lower than that
of the lipid headgroup but higher than that of the tail, the
insertion of 1 should reduce the headgroup Fel but increase that
of the tail. Should the inserted 1 penetrate partially into the tail,
the region could be represented by two different slabs: one with
the polymer and one without. The former should have a higher
Fel than that of the pure lipid tail, and Fel of the latter should go