Synthesis, Helical Chirality, and Self-Assembling Hierarchical

Downloaded by NORTH CAROLINA STATE UNIV on September 1, 2012 | http://pubs.acs.org Publication Date: March 4, 2002 | doi: 10.1021/bk-2002-0812.ch010

Chapter 10

Synthesis, Helical Chirality, and Self-Assembling Hierarchical Structures of Amino Acid-Containing Polyacetylenes 1,

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Ben Zhong Tang *, Kevin K. L. Cheuk , Fouad Salhi , Bingshi Li , Jacky W. Y. Lam , John A. K. Cha , and Xudong Xiao 1

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Departments ofChemistryand Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China *Corresponding author: e-mail: [email protected]

Creation of unnatural helical macromolecules and construction of biomimetic hierarchical structures are of both scientific value and technological implication. In this work, we synthesized a group of amphiphilic polyacetylenes of high molecular weights (up to million daltons) in high yields. The helical chirality of the macromolecular chains can be continuously and reversibly tuned by simple external stimuli such as solvent and pH. The helical polymers, in response to the changes in their environments, self-associate into macromolecular assemblies reminiscent of natural organizational structures such as double helix, twisted cable, spherical vesicle, hairpin loop, extended fibril, coiling ribbon, honeycomb pattern, and mollusk shape.

Helicity is a structural feature of biomacromolecules and is expressed at all organizational levels of the molecular machinery of living systems, e.g., α helix of proteins, double helix of DNA, triple helix of collagen, and spiral bacterium of Spirillum (1, 2). Hierarchy is a characteristic of biological self-assembling processes, as exemplified by protein folding, which involves a sequential set of structural intermediates, each being more organized at a higher structural level than the one before it. Many biological functions performed by biomacromolecules are directly associated with their well-defined hierarchical structures, whose formation is largely regulated by the helical conformation of the polymer chains and by the packing information encoded in their building blocks. Much, however, remains to be learned about the helicity and hierarchy in nature because of the involved complexity.

© 2002 American Chemical Society In Synthetic Macromolecules with Higher Structural Order; Khan, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Development of biomimetie helical polymers is a topic of current interest (3-17). Studies of the simple unnatural systems may lead to a better understanding of the complex natural systems, to the possible control of the elementary steps in the natural construction processes, and to the exploration of efficient strategies for assembling molecular components into hierarchical architectures. We here report a group of amphiphilic polyacetylenes that possess helical chirality and are capable of selfassembling. The helicity of the macromolecular chains can be readily tuned and the assembled structures are reminiscent of the shapes and patterns found in living world.

Synthesis of Amino Acid-containing Polyacetylenes Polyacetylene is an archetypal conjugate polymer of high electrical conductivity (18) and its derivatives with appropriate substituents show unique electronic and optical properties such as photoconductivity, liquid crystallinity, and photo- and electroluminescence (19-21). Wrapping the molecular wires of the conductive polyacetylene backbones with biocompatible pendants of naturally occurring building blocks such as amino acids may lead to the development of biomaterials for bioinspired technologies such as artificial nerve systems and photosynthesis devices. We thus designed a synthetic route to amino acid-containing polyacetylenes (Scheme 1).

Ο R = Me CHMe

2

,CNHC*H(R)COMe 1

CH C0 Me 5

2e

CH Ph

6

Ph

7

CH CHMe2 3e 2

2

^CNHC*H(R)COH CT R = CHMe

2

2

2

2a

CH CHMe;> 3a 2

(CH ) SMe 4 2 2

Scheme 1. Synthesis of amino acid-containing polyacetylenes 1-7: (i) H2SO4, MeOH, reflux, 2 h, 90%; (ii) Me SiC^ZH, (Ph P) PdCl , Cul, Et N, 25 °C, 8 h, 96%; (Hi) KOH, MeOH, reflux, 4 h, 84%; (iv) SOCl , DMF, DCM, 25 °C, and then L-amino acid methyl ester hydrochloride, DCM, pyridine, 25 °C, 8 h, 55-63%; (v) [Rh(nbd)Cl] , Et N, THF, 25 °C, 24 h, 76-97%, M 0.17-1.47 MDa; and (vi) KOH, MeOH, 25 °C, 0.5-1 h, 100%, M 0.41-1.05 MDa [All the esters (e) can he converted to the corresponding acids (a) by the base-catalyzed hydrolysis, two of which are performed in the present study]. 3

3

2

2

3

2

2

3

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In Synthetic Macromolecules with Higher Structural Order; Khan, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

135

A (chloroform)

X_ 8 ¥ " M

0

β f 9 H/H{CH ) 3

2

-ι—•—'—·—ι—·—»—«—ι—'— —*—Γ" 4 2 Η,Ο

0


^-f>< x dΗ

1

-OCH3

8

O

6

Me,CO Β (acetone)

-τ— 14

12

10 Chemical shift (ppm)

Figure 1. NMR spectra of2e in (A) chloroform-d and (B) acetone-d and (C) 2a in DMSO-d 6

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Esterification of 4-bromobenzoic acid followed by Pd-catalyzed coupling with (trimethylsilyl)acetylene yields methyl [4-(trimethylsilyl)ethynyl]benzoate, hydrolysis of which gives 4-ethynylbenzoic acid (22). Condensation of the acid with L-amino acid methyl esters results in the formation of the p-substituted phenylacetylene monomers, whose polymerizations are readily initiated by a rhodium catalyst (23). Selective hydrolysis of the esters 2e and 3e produces respectively the acids 2a and 3a in quantitative yields. All the polymers except 4 are soluble in common solvents, possess high molecular weights (M up to -1.5 x 10 Da), and can be processed into robust engineering forms such as mechanically strong fibers and films. The polymers are characterized spectroscopically and a few examples of their nuclear magnetic resonance (NMR) spectra are given in Figure 1. The spectrum of 2e in chloroform corresponds well to its expected molecular structure. Isolated amide 6

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In Synthetic Macromolecules with Higher Structural Order; Khan, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


136 protons are known to resonate at δ

Synthesis, Helical Chirality, and Self-Assembling Hierarchical

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