Transcript Slide 1

Aqueous Cholesteric Liquid Crystals Using
Uncharged Rodlike Polypeptides
Enrico G. Bellomo, Patrick Davidson, Marianne Impéror-Clerc, and Timothy J. Deming
J. Am. Chem. Soc., 126 (29), 9101 -9105, 2004
Presented by:
Erick Soto
April 28, 2006
1
Overview:
Introduction
Background
Experimental and Results
Discussion
Conclusions
Epilogue
2
Introduction
States of Matter
Solid
Liquid
Gas
Liquid Crystal
Adapted from: http://www.chem.purdue.edu/gchelp/atoms/states.html
3
Applications of liquid crystals
From: http://www.beyondconnectedhome.com/aboutus/press/downloads.html
5
Liquid Crystal Thermometers
From: http://www.petsolutions.com/Images/200/15522455.jpg
http://www.goratec.com/applications.php?ApplicationID=12&language=en
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From the physicist’s perspective:
50-1000 Å
5Å
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Characteristics of molecules capable of forming liquid crystals:
 Rod-like molecular structure
 Rigidness of the long axis
 Strong dipoles and/or easily polarizable susbtituents
Examples:
H
C
C4H9
O
N
CH3
Methoxybenzylidene Butylanaline (“MBBA”)
O
H
C
N
C10H21
O
C
H
CH
CH3
H2
C
C
O
CH
*
C2H 5
p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate ("DOBAMBC")
adapted from: http://liq-xtal.case.edu/lcdemo.htm#Diagrams
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1888 Friedrich Reinitzer discovered LC’s
O
O
Cholesteryl benzoate (604-32-0)
The crystals transformed at 145.5 ºC into a cloudy fluid which suddenly clarified only on heating
at 178.5 ºC. He observed colors.
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The distinguishing characteristic of the liquid crystalline state is the tendency of the
molecules (mesogens) to point along a common axis, called the director.
To quantify how much order is present in a material, an order parameter (S) is defined
1
S   3 cos2   1 
2
S=1
S=0
From: http://plc.cwru.edu/tutorial/enhanced/files/lc/intro.htm
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Liquid Crystal Phases:
The nematic liquid crystal phase is characterized by molecules that have no
positional order but tend to point in the same direction (along the director).
From: http://www.nat.vu.nl/~fcm/ComplexFluids/ComplexFluids.html
http://www.nat.vu.nl/~fcm/ComplexFluids/4a.gif
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Smectic phases:
Smectic A phase
From: http://portellen.phycmt.dur.ac.uk/sjc/thesis_dlc/node15.html
http://www.warwick.ac.uk/fac/sci/Chemistry/jpr/lq/liqcry.html
Smectic C phase
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Probably the best studied example of polypeptide liquid crystal is PBLG
H
H
N
C
O
C
n
O
O
Repeating unit of poly(-benzyl-L-glutamate) (PBLG)
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-helical structure of PBLG
H bonding holds it
From: http://pszichologusboy.freeblog.hu/Files/Alfa%20hélix.jpg
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PBLG exhibits liquid crystal behavior in several organic solvents. Unfortunately,
there is no aqueous phase polypeptide analogue of PBLG.
PBLG liquid crystal in pyridine
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There are some water soluble polypeptides. For example poly(glutamic acid)
and polylysine.
Problem: Charge repulsion
H
H
N
C
O
C
O
H
N
CH
C
n
n
O
Na
O
NH3 Br
Poly (glutamic acid) sodium salt
Polylysine·HBr
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There was considerable interest in development of non-ionic, water-soluble,
conformationally regular polypeptides derived from chemically modified
aminoacids. The best materials resulting from this work are poly(N-hydroxyalkylL-glutamines).
(a) Lupu-Lotan, N.; Yaron, A.; Berger, A.; Sela, M. Biopolymers 1965, 3, 625-655. (b)
Lupu-Lotan, N.; Yaron, A.; Berger, A. Biopolymers 1966, 4, 365-368. (c) Okita, K.;
Teramoto, A.; Fujita, H. Biopolymers 1970, 9, 717-738
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poly(N-hydroxyalkyl-L-glutamines)
 The aminolysis reaction resulted in
significant backbone cleavage
 Studies on their helical structure were
limited because they contained substantial
random coil content when dissolved in water.
 Nearly all the side chains could be
functionalized.
 The resulting polymers were found to
be water soluble.
The best example, poly(N-hydroxybutyl-L-glutamine), PHBG, is ca. 65% helical
in neutral water at 20 ºC
O
H
N
C
CH
O
n
C
NH
OH
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Biocompatibility:
An important feature of -helical water soluble polypeptides would be
biocompatibility. This is for biomedical applications.
Poly(N-hydroxyalkyl-L-glutamines) are recognized are foreign and rapidly
degraded in vivo.
Most biocompatability strategies employ polyethylene glycol (PEG), which is
typically grafted onto other polymers.
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PEG
 Water soluble
 Non ionic
 Not recognized by immune systems
They wanted to incorporate these attractive properties of PEG into
polypeptides.
backbone
X
n
+
PEG
Y

backbone
n
PEG
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Strategy:
pre-monomer
+
pre-monomer
PEG(short)
(1)
PEG(short)
ring closing
pre-monomer
PEG(short)
monomer
PEG(short)
+ Initiator
monomer
(2)
PEG(short)
ring opening
polymerization
monomer
n
(3)
PEG(short)
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Polymer synthesis
O
O
O
OH
O
+
N
HO N
O
C
O
N
O
THF
O
O N
O
O
O
PEG(short)
HHO
N
O
O
PEG(short)*
O
OH
O
+
O
O
O N
O
O
H2N
NaHCO3
THF/H2O
HHO
N
O
OH
O
O
O
O
NH
O
O
pre-monomer
PEG(short)*
pre-monomer– PEG(short
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HHO
N
O
OH
O
O
O
Cl
+
O
CH CH3
Cl
O
C
HN O
CH C
O
CH2Cl2
reflux
NH
O
O
O
O
NH
O
O
monomer– PEG(short)
pre-monomer– PEG(short)
O
C
HN O
CH C
O
O
O
H
N
(PMe3)4Co
O
C
CH n
NH
O
O
monomer– PEG(short)
O
O
NH
O
O
polymer
F1
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Structure and schematic representation of polymer L-1
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Circular Dichroism
The magenta vector can be decomposed in two green vectors of
equal intensity. The change in intensity of the magenta electric field
vector causes these two vectors to rotate oppositely.
From: http://www.imb-jena.de/ImgLibDoc/cd/tut1a.html
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Circular Dichroism
[] at 222 nm as a function of temperature for 1 (Mn= 93100) and PHBG ( Mn= 105000) in H2O
([polymers] = 0.5 mg/mL, pH 7).
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130
110
I
LC
90
T 70
50
30
I + LC
10
-10 0
0.2
-30
0.4
0.6
0.8
1
f polymer
Adapted from: Miller et al., 1974.
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Birefringence:
From: http://www.microscopyu.com/articles/polarized/polarizedintro.html
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Polymer solutions observed between cross polarizers
Test tubes filled with increasing weight fractions of L-1 in deionized water. Samples
were imaged between crossed polarizers. (A) (Mw = 62 kDa): i = 32.7%; ii = 39.3%; iii
= 41.9%; iv = 43.5%; v = 45.4%; vi = 47.9%
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Polymer solutions observed between cross polarizers
B) (Mw = 120 kDa): i = 15.4%; ii = 18.7%; iii = 23.8%; iv = 25.2%; v = 38.5%; vi =
39.8%. All sample compositions are in weight percent
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Mw= 62 kDa
32.7%
39.3%
41.9%
43.5%
45.4%
47.9%
Mw= 120 kDa
15.4%
18.7%
23.8%
25.2%
38.5%
39.8%
Between the concentrated and the dilute regime, a biphasic domain is found where
samples display macroscopic phase separation. These observations show the
existence of a first-order phase transition between the isotropic and liquid crystalline
phases.
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Optical Microscopy
From: http://www.microscopyu.com/articles/polarized/polarizedintro.html
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 The textures of the liquid-crystalline samples held in flat glass
capillaries were clear enough to allow mesophase identification.
 Biphasic samples displayed banded birefringent liquid spherulites
floating in a dark isotropic liquid.
 Fully liquid-crystalline samples displayed large dark homeotropic regions
separated by bright regions containing fingerprint patterns.
 All of these features are typical of the cholesteric (chiral nematic) phase.
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36
The fingerprint patterns are caused by the cholesteric pitch.
half pitch
distance
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Cholesteric pitch
From: “Ordered phases of filamentous viruses”, (to be published in Current opinion in Colloid and Interface Science)
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 From a practical standpoint the two major challenges in cholesterics are
adjusting and controlling the pitch.
 The pitch depends on solvent and temperature but it also varies with
the optical purity of the sample.
 If a cholesteric liquid-crystalline compound is mixed with its opposite
enantiomer (i.e. L and D) the pitch should increase and approach
infinity as the mixture become racemic, and then the liquid crystalline
phase will become nematic.
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Cholesteric pitch tuning:
L-polymer 10wt%
Mw=59.5 kDa
D-polymer 10wt%
Mw=64.9 kDa
L+D polymer
10-15% increments
Circular Dichroism
to verify the degree
of enantiomeric mixing
Cross polarized microscopy
to measure the
cholesteric pitch
40
Polarized optical micrographs
showing the dependence of
the cholesteric pitch on the
enantiomeric composition of
1. All samples were prepared
at 60 wt % in deionized water
and are defined as the mol %
of L-1 in a L-1 + D-1 mixture.
(A) = 0%; (B) = 15%; (C) =
30%; (D) = 40%; (E) = 60%;
(F) = 70%; (G) = 85%; (H) =
100%.
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Cholesteric pitch (m), measured from optical micrographs, as a function of the enantiomeric
composition of 1 (mol % of L-1 in a L-1 + D-1 mixture). The solid line is a fit of the data using a
hyperbolic divergence at 50 mol %.
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Sample Alignment:
In order to gain more information on the structure and stability of these
cholesteric phases, they sought to untwist the pitch by aligning the rodlike
polypeptide chains.
It is known that magnetic fields can align -helical polypeptides parallel to
the direction of the applied magnetic field.
In preliminary studies they found that a moderate magnetic field (1.7 T)
was not sufficient to align optically pure polypeptide samples, or even the
weakest cholesteric samples (i.e. near the equimolar L + D).
A stronger magnetic field (magnet of an NMR spectrometer ~8T) was able
to align the polypeptide molecules into a nematic phase.
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CCD camera and rotating anode X-ray source
were used to record the scattering patterns.
100 mol % L polymer
55 mol % L polymer, aligned by magnetic field
f

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Be the judge
PBLG liquid crystal Conmar Robinson 1957
Polylysine-PEG Tim Deming 2004
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55 mol % of L-polypeptide (60 wt %)
Magnetic field aligned (8 T)
F1
I versus f
Order parameter, S = 0.85 ± 0.05. It’s
large but it’s typical of lyotropic
nematic phases.
I versus 
Maximum at 2.6 nm-1 which
corresponds to the average distance
between the rods of 2.4 nm
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Shear alignment:
The inability to record structural information in situ during magnetic field
alignment, a Couette cell setup was used.
Application of a shear flow is a very powerful way of aligning viscous
mesophases and has been successfully applied to liquid crystal phases of
PBLG in m-cresol.
47
Radial X-ray diffraction
100 mol % L polymer, high shear
I versus f
55 mol % L polymer, low shear
I versus 
Tangential X-ray diffraction showed no anisotropy.
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55 mol % of L-polypeptide (60 wt %)
Order parameter, S = 0.88 ± 0.05.
Which is comparable to the obtained
by magnetic alignment.
Maximum at 2.6 nm-1 which is the
same as obtained by magnetic field
alignment.
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Discussion:
System behaved as predicted by the Onsager model. (Strong first order
isotropic/nematic phase transition, with phase coexistence)
The Onsager model predicts the volume fractions n= 4.2 D/L and i= 3.3
D/L for the isotropic and nematic phase transition.
For a short L- polymer (Mw= 62 kDa; L= 32.3 nm and D= 2.2 nm) the values
should be n= 29% and i= 23%. These results are in fair agreement with
the experimental values of 47% and 38% respectively. (n/ i~same)
Mw= 62 kDa
32.7%
39.3%
41.9%
43.5%
45.4%
47.9%
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For a longer L- polymer (Mw= 120 kDa; L= 62.6 nm and D= 2.2 nm) the
values should be n= 15% and i= 12%. These results are in fair agreement
with the experimental values of 39% and 17% respectively. This deviation
could be due to chain flexibility and polydispersity.
Mw= 120 kDa
15.4%
18.7%
23.8%
25.2%
38.5%
39.8%
The electrostatic interactions are negligible in this system because similar
properties were observed when samples were prepared in 100 mM NaCl.
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Conclusions:
 A new aqueous cholesteric liquid crystal has been reported which is
formed by uncharged rodlike polypeptide molecules.
 The mentioned system undergoes isotropic/nematic phase transition.
 The cholesteric pitch was found to be easily tuned.
 Important structural information was obtained by aligning the rodlike
polypeptide molecules by shear or magnetic field. (S and d)
The system was found to behave as predicted by the Onsager model.
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Epilogue:
This publication has been cited 3 times to date.
Research ideas:
Fundamental
To investigate the persistence length vs. molecular weight behavior.
To investigate D vs. concentration.
To observe their interaction with spheres. New phases may arise.
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Applied
To use a more rigid polypeptide backbone. (glutamate)
O
C
H3C
H2N
H3C
H3C
HN
NH2 CH
3
CH3
O
CH
NH2
CH3
C
O
+
NH2
H2N
CH3
H3C
H2N CH CH3
3
NH
O
O
O
O
Potential candidates for drug delivery
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Thanks for your attention
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