Smart materials, which respond to external stimuli, are well known for a long time. Typically these materials undergo an (ir)reversible property change upon exposure to an external stimulus. In the majority of cases the stimulus is a change in temperature, pH or is light induced. A relatively new concept that was recently developed involves smart materials that produce a change in response to the action of an enzyme, so called Enzyme Responsive Materials (ERMs). The response that the ERM produces can be any of a wide range of changes such as a change in fluorescence, the release of a drug or the generation of some form of physical aggregation such as the formation of micelles. Enzymes do not only exhibit regio-selectivity but they generally exhibit extremely high stereo-selectivity as well, i.e. the enzyme exclusively acts on one enantiomeric form of a molecule whilst leaving the other enantiomer untouched. Our approach to ERM synthesis is based on this principle, where the extent of the response that the ERM produces to the enzyme is encoded within the chirality of the material. By using different ratios of the enantiomers (building blocks) within the ERM, the extent of the response can be programmed-in without causing any changes in the general physical properties of the material. With this principle in mind, a number of acetophenone-derivatives with different functional groups in the para-position of the phenyl ring were synthesized (Chapter 2). In an attempt to reduce these prochiral ketones to their enantiomeric forms by employing two commercially available alcohol dehydrogenases (ADHs), namely (R)-producing Lactobacillus (ADH-LB) and (S)-producing Thermoanaerobacter sp. (ADH-T) as the catalysts for the reductions, a significant difference in the substrate acceptance by these enzymes was observed despite claims concerning their broad substrate spectrum. The examination of the various factors such as the solubility of the substrate in the reaction medium, the size of the substrate and the electronic character of the substituent at the para-position of the acetophenone derivatives did not offer any trend to explain the difference in the substrate acceptance by ADH-LB and ADH-T. Instead, we realized that non-polar para-substituents were favored over polar or ionizable para-substituents in the reductions. Mapping of the electronic charge distribution in the molecules revealed a correlation between the location of the highest electron density on the molecule and the success of the reaction. The substrates with the highest electron density on the carbonyl group could be reduced almost quantitatively to their enantiomeric forms. However, if the highest electron density is located on a different functional group of the substrate, then the substrate could not be reduced. ADH-catalyzed reduction of acetophenone-derivatives resulted in the synthesis of two types of enantiopure building blocks having an enzyme sensitive unit (phenyl ethanol): one with a polymerizable group, ((R)- and (S)-1-(4-vinylphenyl)ethanol), and the other one with a clickable moiety, ((R)- and (S)-1-(4-ethynylphenyl)ethanol). In chapter 3, homo and copolymers were synthesized from enantiopure (R)- and (S)-1-(4-vinylphenyl)ethanol and styrene by RAFT (co)polymerization. Well-defined polymers with low polydispersities were obtained. Kinetic investigations confirmed that the enantiopure monomers and styrene have similar reactivity ratios resulting in random copolymers. In chapter 4, block copolymers comprising two blocks with pendant hydroxy groups of opposite chirality were synthesized in which the length of starting block, (R)-block, was kept constant at 7,200 g/mol. The length of the second block, (S)-block, was varied from 2,100 to 6,400 g/mol. The optical rotation decreased from 29.8° to 1.5° with increasing (S)- to (R)- block length ratio. Noteworthy is that the blocky character of these chiral polymers would only manifest itself in the optical rotation since it is reasonable to claim that the chemical and physical properties of the individual chiral (S)- and (R)-blocks are identical. In chapter 5, linear and star-shaped poly(n-butyl acrylate)s (BAs) were prepared via ATRP of n-butyl acrylate by employing different ATRP initiators. Consecutive click reactions with (R)-1-(4-ethynylphenyl) ethanol after the chain end functionalization with an azide were not quantitative, probably due to loss of some of the end groups. In addition, chiral dendrimers were prepared from different mixtures of (R)- and (S)-1-(4-ethynylphenyl)ethanol, and the matching azide-functional bisMPA dendrimers using click chemistry. The specific optical rotation of the dendrimers increased linearly with increasing percentage of (R) end-groups in the dendrimer, which indicates that (R)- and (S)-building blocks had been incorporated into the dendrimer in agreement with the enantiomeric feed ratio in the click reaction. The molar rotation values of the dendrimers were found to be directly proportional to the number of (R)-building blocks clicked to the periphery, implying that each stereogenic group at the periphery behaves like an isolated molecule and does not induce any additional chiral substructure. Prior to the exposure of the synthesized chiral polymers to Candida Antarctica Lipase B to study the response of these macromolecules to an enantioselective enzyme, model reactions were performed to investigate the optimal reaction conditions. Since the chiral homopolymers synthesized in this study were not soluble in common hydrophobic solvents like hexane or toluene, in which CALB generally shows optimum activity, different organic solvent systems, both polar as well as mixtures of polar and apolar solvents, were employed for the CALB-mediated esterification of phenylethanol units with vinylacetate. Toluene/THF (2/1 v/v) was found to be the most appropriate solvent mixture for the post-functionalization of (R)-polymer. It was shown that the lipase-catalyzed polymer analogous esterification of the chiral hydroxy groups was strongly (R)-selective, in agreement with the preferred lipase enantioselectivity. However, even after extended reaction times, esterification on the (R)-polymers was limited to around 50 %, which was much lower than the results obtained with small model compounds. Increasing the molecular weight of (R)-polymers from 5,400 g/mol to 16,200 g/mol resulted in a decrease of the esterification yield (55 % to 42 %). This suggests that steric factors play a role in the esterification, although it cannot be ruled out that a decrease of polymer solubility in toluene/THF (2/1) with increasing molecular weight also contributes to this result. When (R)-1-(4-vinylphenyl)ethanol-styrene copolymers with different chirality were exposed to CALB, an increase in the extent of esterification (from 21 to 53 %) was observed whilst the composition ratio of (R)- 1-(4-vinylphenyl)ethanol/styrene was increased from 0.25 to 4. However, further increase in the enzyme-sensitive monomer concentration in the backbone did not further improve the extent of esterification (the maximum reached in these conditions with homopolymer was 55 %). This might suggest that not only steric effects play a role but that possibly also the local environment of the hydroxy groups is important for the extent of the reaction. CALB-catalyzed esterification of the chiral block copolymers was stereoselective and only one of the present blocks was esterified, thereby converting these chiral block copolymers into block copolymers with chemically and physically distinguishable blocks. CALB-catalyzed esterification of these (R)-phenyl ethanol groups at the chain end of polyBAs was successful and principally validated the proposed strategy of enantioselective enzymatic esterification of globular multifunctional chiral materials. The extremely high selectivity of CALB towards the R enantiomer of the 1-phenyl-ethanol moiety was retained also on the periphery of dendrimers. Furthermore, the chirality of the dendrimer directly correlates to a chemical reaction yield using an enantioselective catalyst. In the last chapter, vinyl methacrylate was used as the acyl donor in the CALB-catalyzed post-polymerization modification of selected (R)-chiral polymers as a comparison to vinyl acetate. These modifications provided pendant methacrylic double bonds which were utilized in thiol-ene reactions, viz. the Michael addition of poly(ethylene glycol) methyl ether thiol (PEG-SH), for further modification. The preliminary results showed that vinyl methacrylate could be used successfully as the acyl donor in the CALB-catalyzed esterifications. However, the extent of the esterification of pendant (R)-OHs with vinyl methacrylate was lower (35 %) compared to that with vinyl acetate (55%) performed under the same reaction conditions. In conclusion, the proof of principle described in this dissertation can be employed to program reactivity into otherwise indistinguishable molecules. It can be stated that chirality can be used as a means of encoding macromolecules that and a key characteristic of enzymes, i.e. enantioselectivity, can be utilized to read-out this code by correlating it to a chemical reaction.
|Qualification||Doctor of Philosophy|
|Award date||9 Jan 2013|
|Place of Publication||Eindhoven|
|Publication status||Published - 2013|