Sequence-Controlled Multi-Block ... - American Chemical Society

Chapter 22

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date (Web): September 22, 2014 | doi: 10.1021/bk-2014-1170.ch022

Sequence-Controlled Multi-Block Glycopolymers via Cu(0) Mediated Living Radical Polymerization Qiang Zhang,1 Jennifer Collins,1 Athina Anastasaki,1 Russell Wallis,2 Daniel A. Mitchell,3 C. Remzi Becer,1 Paul Wilson,1 and David M. Haddleton1,* 1Department

of Chemistry, University of Warwick, CV4 7AL, Coventry, UK of Biochemistry, University of Leicester, LE1 9HN, Leicester, UK 3Clinical Sciences Research Institute, Warwick Medical School, University of Warwick, CV2 2DX, Coventry, UK *E-mail: [email protected]

2Department

Synthetic glycopolymers with pendant saccharides bind to mammalian carbohydrate-recognition proteins with high affinity due to their multivalency and primary structure. However, glycoproteins function in nature via an exquisitely tuned "glycocode" and mimicking this code remains an interesting challenge in polymer chemistry. In order to address this we have synthesized glycomonomers via a [3+2] cycloaddition reaction between sugar-alkyne and azido-acrylates and these monomers were used to synthesize a series of sequence controlled glycopolymers (SCGP) using single electron transfer living radical polymerization (SET-LRP).

Introduction Glycan-protein interactions are responsible for many physiological processes including cell-cell recognition, cell adhesion, cell signalling, pathogen identification and differentiation. These non-covalent interactions also play an © 2014 American Chemical Society In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


essential role in infectious disease processes such as pathogen-cell interactions and immune responses. This work was designed to utilize developments in SET-LRP (single electron transfer living radical polymerization), a Cu(0)-mediated controlled/living radical polymerization technique. This relatively new chemistry allows access to well-defined, high MW polymers at ambient and near ambient temperatures, SET-LRP gives polymers with outstanding PDI and end group fidelity. Typical PDI values are in the range of 1.05-1.10 up to very high monomer (>99%) conversion with little quantifiable bimolecular termination observed for a variety of acrylates. This advance allows sequence control of functionality within polymer chains. Our targets were driven by dendritic cells (DC) recognition events, which are the most antigen presenting cells and form a major component of the human immune system. Dendritic cells act as messengers between the innate and adaptive immunity and their main function is to process antigen material and present it on the surface to other cells of the immune system such as T-cells. Dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN; CD209) is a C-type lectin (carbohydrate-binding protein) present on both macrophages and dendritic cell subpopulations. DC-SIGN binds to microorganisms and host molecules by recognizing surface rich mannose containing glycans through multivalent glycan-protein interactions and notably serves a target molecule for several viruses such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV) (1–6). Therefore, synthetic lectins are of interest with Davis et. al. reporting the discovery of a simple monocyclic host, which was prepared in five steps and 23% overall yield instead of 21 steps and 0.1% yield (7). Alternatively, non-carbohydrate inhibitors of mammalian lectins can be used to prevent the interaction between DC-SIGN and gp120 (8–10). The architectures of the multivalent ligands can have a large effect on carbohydrate binding to lectins and the use of linear polymers on effective lectin binding has been demonstrated by several research groups (11–16). Carbohydrate sequence and conformation potentially supply a vast source of information and act to transfer biological information beyond the genetic code, namely “sugar code” or “glyco code”, which has been proved to play a critical role during evolution (17–19). Sequence control in polymer synthesis had been largely ignored mainly due to the difficulty in precise control and characterization during monomer sequencing (20–22). Templated polymerization and step-growth polymerization could also result in sequence-specific polymers (23–25). Chain-growth copolymerization tends to be more promising for complex monomer sequence construction, including random, block, alternate and gradient microstructures (25). Synthetic polymer chemistry has developed rapidly in the last few decades owing to much to the discovery of controlled/living radical polymerization and more recently the combination of this methodology with efficient click reactions (26–30). Currently, polymerization of functional monomers with desired chain length, architecture and composition is straightforward whereas sequence control structures (24, 31) and the and the control of folding of synthetic macromolecules remain important challenges in polymer chemistry (32). There are a few recent reports where sufficient control has been achieved in controlling the monomer 328 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


sequence along the polymer chain (33–37). Of note is a successful sequence controlled polymerization technique, single electron transfer living radical polymerization (SET-LRP) (35, 38–40), which allows for the facile synthesis of high-order multiblock copolymers via iterative monomer addition in an one-pot reaction featuring high yield, high chain end fidelity and requiring purification only at the last step (35). The synthesis of glycopolymers featuring well-defined macromolecular architectures has been developed by using different polymerization techniques and click reactions (41, 42). However, direct transition metal-catalyzed polymerization of unprotected glyco monomers is still limited mainly due to the difficulty in synthesis of unprotected glyco monomers and optimization of polymerization conditions (12, 37, 43–46). This inspired us to introduce SET-LRP for the synthesis of sequence-controlled glycopolymers for a glycopolymer code. and the demonstration of their binding to the human lectin DC-SIGN (47).

Results and Discussion Synthesis of Glycomonomers via CuAAC Click Reaction In order to obtain some degree of control over the sugar sequence, different carbohydrate units can be inserted along the polymer backbone either through polymerization of different sugar monomers or via post-modification after polymerization. Based on the demand of carbohydrate diversity, direct polymerization of different functional glyco monomers is the first choice compared with multistep chemical modification following polymerization. We decided to utilize a copper-catalyzed azide-alkyne cycloaddition (CuAAC), which provides a facile route for the synthesis of glycomonomers. The use of azide functionalized sugars and methacrylate type sugar monomers allows for novel 4-vinyl-1, 2, 3-triazole type sugar monomers to be synthesized via reaction with alkyne compounds in MeOH/H2O or THF/H2O with CuSO4/sodium ascorbate catalysis (12, 48). In order to demonstrate an alternative approach, a one-pot Fischer type glycosylation reaction was first conducted to prepare alkyne-functionalised sugars, which were then reacted with an azide acrylate intermediate via a CuAAC reaction in MeOH/H2O under the catalysis of CuSO4/sodium ascorbate. Three different stable solid acrylate glyco monomers were obtained through this protocol, Scheme 1. 1H NMR clearly revealed the appearance of a triazole ring proton at ~7.9 ppm and vinyl peaks at 5.5-6.5 ppm after the click reaction, Figure 1. The 13C NMR spectrum showed the existence of D-glucose C-1 peaks at 99.6 & 103.6 ppm, suggesting that the monomer is present as an anomeric mixture. The combination of this data with further ESI-MS and FT-IR analysis proved that targeted glyco monomer had been successfully synthesized. D-mannose and Lfucose acrylate monomers were also synthesized in same way. 329 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Scheme 1. Synthesis of glycomonomers via Fischer glycosylation & CuAAC.

Figure 1. 1H (top) and 13C (bottom) NMR spectra of D-glucose acrylate monomer in MeOD.

Homopolymerization of Glycomonomers by SET-LRP in DMSO at Ambient Temperature The glycomonomers had good solubility in DMSO and were polymerized at ambient temperature using the Cu(0)/Cu(II)/Me6TREN system with EBiB as initiator and DMSO as solvent (Scheme 2). Monomer conversion reached 91% in 4 h and after 24 h full conversion was observed, Figure 2. The number average molecular weight (Mn) as measured by DMF SEC generally increased linearly with monomer conversion. However, the Mn by SEC is higher than the theoretical molecular weight mainly due to the different structure of glycopolymers with the PMMA calibration standards, which cause a significant difference of hydrodynamic volume of polymers in DMF. The molecular weight 330 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


distribution remained narrow (Mw/Mn

Sequence-Controlled Multi-Block ... - American Chemical Society

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