Aqueous Cholesteric Liquid Crystals Using Uncharged Rodlike Polypeptides

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

2. Overview: Introduction Background Experimental and Results Discussion Conclusions Epilogue

3. Introduction States of Matter Solid Liquid Gas Liquid Crystal Adapted from: http://www.chem.purdue.edu/gchelp/atoms/states.html

4. 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

6. From the physicist’s perspective: 50-1000 Å 5 Å

7. Methoxybenzylidene Butylanaline (“MBBA”) p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate ("DOBAMBC") • 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

8. Cholesteryl benzoate (604-32-0) 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.

9. S=1  = 0º  = 54.7º S=0 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

10. 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

11. Smectic phases: 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

12. Probably the best studied example of polypeptide liquid crystal is PBLG Repeating unit of poly(-benzyl-L-glutamate) (PBLG)

13. -helical structure of PBLG H bonding holds it From: http://pszichologusboy.freeblog.hu/Files/Alfa%20hélix.jpg

14. PBLG exhibits liquid crystal behavior in several organic solvents. Unfortunately, there is no aqueous phase polypeptide analogue of PBLG. PBLG liquid crystal in pyridine

15. There are some water soluble polypeptides. For example poly(glutamic acid) and polylysine. Problem: Charge repulsion Poly (glutamic acid) sodium salt Polylysine·HBr

16. 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. Biopolymers1965, 3, 625-655. (b) Lupu-Lotan, N.; Yaron, A.; Berger, A. Biopolymers1966, 4, 365-368. (c) Okita, K.; Teramoto, A.; Fujita, H. Biopolymers1970, 9, 717-738

17. 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

18. 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.

19. PEG  Water soluble  Non ionic  Not recognized by immune systems They wanted to incorporate these attractive properties of PEG into polypeptides. 

20. Strategy: (1) (2) (3)

21. Polymer synthesis PEG(short) PEG(short)* pre-monomer pre-monomer– PEG(short) PEG(short)*

22. monomer– PEG(short) pre-monomer– PEG(short) monomer– PEG(short) polymer F1

23. Structure and schematic representation of polymer L-1

24. 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

25. 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).

26. LC I I + LC fpolymer Adapted from: Miller et al., 1974.

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

28. 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%

29. 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

30. Mw= 62 kDa Mw= 120 kDa 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.

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

32. 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.

34. The fingerprint patterns are caused by the cholesteric pitch. half pitch distance

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

36. 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.

37. Cholesteric pitch tuning:

38. 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%.

39. 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 %.

40. 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 a-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.

41. 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  

42. Be the judge PBLG liquid crystal Conmar Robinson 1957 Polylysine-PEG Tim Deming 2004

43. 55 mol % of L-polypeptide (60 wt %) Magnetic field aligned (8 T) F1 I versus  I versus  Order parameter, S = 0.85 ± 0.05. It’s large but it’s typical of lyotropic nematic phases. Maximum at 2.6 nm-1 which corresponds to the average distance between the rods of 2.4 nm

44. 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.

45. Radial X-ray diffraction 100 mol % L polymer, high shear 55 mol % L polymer, low shear I versus  I versus  Tangential X-ray diffraction showed no anisotropy.

46. 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.

47. 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

48. 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. Mw= 120 kDa

49. 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.

50. 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.