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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

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Overview:

Introduction

Background

Experimental and Results

Discussion

Conclusions

Epilogue

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Solid Liquid Gas

Liquid Crystal

Introduction

States of Matter

Adapted from: http://www.chem.purdue.edu/gchelp/atoms/states.html

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Applications of liquid crystals

From: http://www.beyondconnectedhome.com/aboutus/press/downloads.html

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From: http://www.petsolutions.com/Images/200/15522455.jpg http://www.goratec.com/applications.php?ApplicationID=12&language=en

Liquid Crystal Thermometers

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From the physicist’s perspective:

50-1000 Å

5 Å

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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:

adapted from: http://liq-xtal.case.edu/lcdemo.htm#Diagrams

p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate ("DOBAMBC")

CH

N CH

HC C

O

H2C

CH

CH3

C2H5

O

OC10H21

*

Methoxybenzylidene Butylanaline (“MBBA”)

N

HC O

CH3C4H9

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1888 Friedrich Reinitzer discovered LC’s

The crystals transformed at 145.5 ºC into a cloudy fluid which suddenly clarified only on heating at 178.5 ºC. He observed colors.

Cholesteryl benzoate (604-32-0)

O

O

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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

From: http://plc.cwru.edu/tutorial/enhanced/files/lc/intro.htm

1cos32

1 2 S

= 0º

S=1

= 54.7ºS=0

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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 A phase Smectic C 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 phases:

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Probably the best studied example of polypeptide liquid crystal is PBLG

HN C C

OH

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

HN C C

OH

n

O

ONa

HN CH C

O

NH3

n

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-hydroxyalkyl-L-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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The best example, poly(N-hydroxybutyl-L-glutamine), PHBG, is ca. 65% helical in neutral water at 20 ºC

HN

CH

C

O

C

NH

O

OH

n

Nearly all the side chains could be functionalized. The resulting polymers were found to be water soluble.

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.

poly(N-hydroxyalkyl-L-glutamines)

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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.

backbonen

X

PEG+ Y backbonen

PEG

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pre-monomer + PEG(short) pre-monomer

PEG(short)

Strategy:

pre-monomer

PEG(short)

ring closing monomer

PEG(short)

(1)

(2)

(3)monomer

PEG(short)

+ Initiator ring openingpolymerization monomer

PEG(short)

n

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N

O

O

HOOOH

O

CN N

N

O

O

OOO

OO

OO

+

THF

OHN

OH

O

H2N

O

HN

O

O

OOO

OO

NaHCO3THF/H2O+

OHN

OH

O

NH

O

H

OO

OO

Polymer synthesis

pre-monomer

PEG(short) PEG(short)*

PEG(short)* pre-monomer– PEG(short)

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OHN

OH

O

NH

O

H

OO

OO

+ CHO

CH3

Cl

Cl

CH2Cl2reflux

HNCH C

OC

O

NHO

OO

O

O

HNCH C

OC

O

NHO

OO

O

O

(PMe3)4Co

HN

CHC

O

NHO

OO

O

n

F1

polymer

pre-monomer– PEG(short) monomer– PEG(short)

monomer– PEG(short)

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Structure and schematic representation of polymer L-1

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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

Circular Dichroism

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[] 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).

Circular Dichroism

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-30

-10

10

30

50

70

90

110

130

0 0.2 0.4 0.6 0.8 1

T

LC

I + LC

I

polymer

Adapted from: Miller et al., 1974.

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From: http://www.microscopyu.com/articles/polarized/polarizedintro.html

Birefringence:

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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%

Polymer solutions observed between cross polarizers

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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

Polymer solutions observed between cross polarizers

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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.

32.7% 39.3% 41.9% 43.5% 45.4% 47.9%

15.4% 18.7% 23.8% 25.2% 38.5% 39.8%

Mw= 62 kDa

Mw= 120 kDa

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From: http://www.microscopyu.com/articles/polarized/polarizedintro.html

Optical Microscopy

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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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The fingerprint patterns are caused by the cholesteric pitch.

half pitch distance

38From: “Ordered phases of filamentous viruses”, (to be published in Current opinion in Colloid and Interface Science)

Cholesteric pitch

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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 polymer10-15% increments

Circular Dichroismto verify the degreeof enantiomeric mixing

Cross polarized microscopyto measure the cholesteric pitch

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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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100 mol % L polymer 55 mol % L polymer, aligned by magnetic field

CCD camera and rotating anode X-ray source were used to record the scattering patterns.

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Be the judge

PBLG liquid crystal Conmar Robinson 1957

Polylysine-PEG Tim Deming 2004

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Magnetic field aligned (8 T)F1

55 mol % of L-polypeptide (60 wt %)

I versus I versus

Maximum at 2.6 nm-1 which corresponds to the average distance between the rods of 2.4 nm

Order parameter, S = 0.85 ± 0.05. It’s large but it’s typical of lyotropic nematic phases.

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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.

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100 mol % L polymer, high shear 55 mol % L polymer, low shear

I versus I versus

Radial X-ray diffraction

Tangential X-ray diffraction showed no anisotropy.

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55 mol % of L-polypeptide (60 wt %)

Maximum at 2.6 nm-1 which is the same as obtained by magnetic field alignment.

Order parameter, S = 0.88 ± 0.05. Which is comparable to the obtained by magnetic 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)

32.7% 39.3% 41.9% 43.5% 45.4% 47.9%

Mw= 62 kDa

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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.

The electrostatic interactions are negligible in this system because similar properties were observed when samples were prepared in 100 mM NaCl.

15.4% 18.7% 23.8% 25.2% 38.5% 39.8%

Mw= 120 kDa

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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)

+

NH2

CH3

CH3CH3

NH2

NH2

CH3CH3CH3

H2NH3C

H2N

H3C

H3C

H2N

H3C HN

CH C

O

C

O

NH

O

O

O

O

O

Potential candidates for drug delivery

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Thanks for your attention