Hierarchical self‐assembly of novel para‐aryltriazole helical foldamers

Pfukwa, Rueben (2012-03)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Hierarchical information transfer is investigated as a tool to prepare well‐defined nanostructures with high aspect ratios, via the self‐assembly of helically folding poly(paraaryltriazole) (P(p‐AT)) foldamers. A novel ‘helicity codon’ based on the 1,4‐linkage geometry in 1,4‐aryl‐disubstituted‐1,2,3‐ triazoles is developed. Helical folding is induced exclusively by directing all triazole moieties into a cisoid configuration. By linking the triazole rings in a para fashion about the aryl moiety, this helicity codon codes for a helix with a large internal cavity of ~ 3 nm. One turn of the putative helical conformation requires 14 repeat units and the helical pitch is ~ 0.38 nm. The aryltriazole backbone is appended with amphiphilic oligo(ethylene glycol) (oEG) units which have the dual roles of imparting solubility as well as instigating a solvophobic helical folding in solvents which poorly solvate the hydrophobic arytriazole backbone but, solvate the side chains fully. The helix interior is hydrophobic and the exterior is amphiphilic. A true polymer synthesis approach to the foldamer synthesis, based on the copper catalysed azide‐alkyne cycloaddition (CuAAC) AB step growth polymerization system, is developed. This is preceded by a facile synthetic protocol for the AB monomers. The subsequent P(p‐ AT)s have high molecular weights ensuring several turns in the helical foldamer. A DMF/H2O good solvent/bad solvent system is established. Twist sense bias in the helical foldamers is successfully imparted by installing enantiopure chiral oEG side chains. Spectroscopic signatures for the solvent dependent coil to helix transition are established enabling the tracking of the conformational transitions from primary to secondary and finally tertiary structure. Conclusive evidence for the formation of stable, long stacked helical columns, in the solution state, is provided via cryo‐TEM. The helical stacks are several microns long, but of random lengths and do not intertwine but rather run parallel to each other. The helical stacks, however, have indeterminate lengths. Control over the length and chirality of the self‐assembled helical stacks is successfully imparted by using a template which mimics the role of ribonucleic acid (RNA) in tobacco mosaic virus (TMV). The template used is the hydrophobic α‐helical polypeptide poly(γ‐ benzyl‐L‐glutamate) (PBLG). Self‐assembly is driven by solvophobicity in a DMF/H2O system, the PBLG template being encapsulated inside the hydrophobic cavities of the stacked/selfassembled helical foldamers. Information from the template, i.e. length and chirality, is used to control the length and the chirality of the stacked/self‐assembled construct. The templated self‐assembly process is solvent dependent. When carried out in the solvent regime at the coil to helix transition mid‐point of the foldamer host, system operates under a dynamic equilibrium. Under these conditions, the self‐assembly process is shown to take place between two distinct states, the foldamer helices and the helical template, the template threading through the foldamer helices. The resulting self‐assembled construct has a pseudo‐rotaxane architecture. Under dynamic equilibrium conditions, temperature induced dis‐assembly of the templated assembled construct, is shown to be a cooperative process, whilst re‐assembly is characterized by a large hysteresis. By increasing the volume fraction of water, the solvophobic character of the system is increased and template assembled construct is better stabilised. The assembly system, however, loses its dynamic equilibrium character and falls into kinetic traps. Temperature induced de‐threading, of the foldamer helices, becomes less favourable and loses its cooperative character although the hysteresis loop is reduced.

AFRIKAANSE OPSOMMING: Hiërargiese inligtingsoordrag is bestudeer as ‘n hulpmiddel om goed gedefinieerde nanostrukture met ‘n goeie beeldverhouding voor te berei. Die nanostrukture word voorberei deur middel van self‐samestelling van heliese vouing van poli(para‐arieltriasool) (P(p‐AT)) ‘foldamers’. ‘n Nuwe heliese‐kodon gebaseer op die 1,4 koppelingsgeometrie in 1,4 arieldigesubstitueerde‐ 1,2,3‐triasool is ontwikkel. Heliese vouing word uitsluitlik geïnduseer as al die triasole in die sis konfigurasie is. Deur die triasole in ‘n para konfigurasie te bind, kodeer die heliese kodon vir ‘n heliks met ‘n groot interne kanaal van ~ 3 nm. Een draai van die heliks benodig 14 herhalende eenhede en die heliese gradiënt ~ 0.38 nm. Amfifiliese oligo(etileen glikol) (oEG) eenhede is aan die arieltriasoolruggraat aangeheg. Hierdie aanhegting van oEG eenhede bevorder oplosbaarheid en dit induseer ‘n solvofobiese heliese vouing in oplosmiddels wat nie die hidrofobiese arieltriasoolruggraat oplos nie, maar wel die sy‐kettings volledig oplos. Die binnekant van die heliks is hidrofobies en die buitekant is amfifilies. ‘n Polimeersintese benadering tot die ‘foldamer’ sintese (gebaseer op die koper gekataliseerde siklo‐addisie reaksie tussen ‘n asied en ‘n alkyn) AB stapsgewyse groei polimerisasiestelsel, is ontwikkel. Dit is voorafgegaan deur ‘n geskikte sintetiese protokol vir die AB monomere. Die daaropvolgende P(p‐AT) het ‘n hoë molekulêre massa wat verseker dat daar ‘n hele paar draaie in die heliese ‘foldamer’ is. ‘n DMF/H2O goeie oplosmiddel/ swak oplosmiddel sisteem is vasgestel. Draaiing van die heliks na ‘n spesifieke kant alleenlik is suksesvol geïnduseer deur die toevoeging van suiwer enantiomere van die chirale oEG sykettings. Spektroskopiese handtekeninge van die oplosmiddel‐afhanklike ketting tot heliks transformasie word vasgestel sodat die oorgangstoestande gevolg kan word vanaf primêre tot sekondêre en uiteindelik tesiêre struktuur. Beslissende bewyse vir die formasie van stabiele, lang gestapelde heliese kolomme in die opgeloste toestand is bewys met cryo‐TEM. Die heliese stapels is verskeie mikron lank, maar het verskillende lengtes. Die heliese stapels is parallel aan mekaar en oorvleuel nie. Die lengte van die heliese stapels is egter onbepaalbaar. Beheer oor die lengte en chiraliteit van die self‐samestellende heliese stapels is verkry deur gebruik te maak van ‘n templaat wat die rol van ribonukleïensuur (RNS) in die tabakmosaïekvirus (TMV) naboots. Hidrofobiese α‐heliese polipeptied poli(γ‐bensiel‐Lglutamaat) (PBLG) is gebruik as die templaat. Self‐samestelling word gedryf deur solvofobisiteit in ‘n DMF/H2O stelsel, met die PBLG templaat wat dan geënkapsuleer word binne die hidrofobiese holtes van die gestapelde/ self‐saamgestelde heliese ‘foldamers’. Die lengte en die chiraliteit van die templaat word gebruik om die lengte en chiraliteit van die gestapelde helikse te beheer. Die templaatbemiddelde self‐samestellende proses is afhanklik van die oplosmiddel. Die stelsel is by ‘n dinamiese ewewig wanneer, uitgevoer in ‘n oplosmiddel, die ketting na heliks oorgang die middelpunt van die ‘foldamer’ gasheer bereik het. By hierdie omstandighede vind die self‐samestellende proses plaas tussen twee afsonderlike toestande nl. die ‘foldamer’ helikse en die heliese templaat, en die templaat wat vleg deur die ‘foldamer’ helikse vleg. Die gevolglike struktuur het ‘n pseudo‐rotaxane argitektuur. By dinamiese ewewigstoestande veroorsaak temperatuur dat die self‐samestellende templaatstrukture weer disintegreer. Hierdie is ‘n koöperatiewe proses terwyl die hersamestelling gekarakteriseer word deur ‘n sloerende proses. Deur die waterfraksie te vermeerder, word die solvofobiese karakter van die sisteem verhoog en die templaat selfsamestellende struktuur beter gestabiliseer. Die samestellingsproses verloor egter sy dinamiese ewewigkarakter en val in kinetiese slaggate. Temperatuur geïnduseerde disintegrasie van die foldamer helikse word minder gunstig en dit verloor die koöperatiewe karakter alhoewel die sloering verminder is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/20370
This item appears in the following collections: