Abstract
Oligomers equipped with complementary recognition units have the potential to encode and express chemical information in the same way as nucleic acids. The supramolecular assembly properties of m-phenylene ethynylene polymers equipped with H-bond donor (D = phenol) and H-bond acceptor (A = phosphine oxide) side chains have been investigated in chloroform solution. Polymerisation of a bifunctional monomer in the presence of a monofunctional chain stopper was used for the one pot synthesis of families of m-phenylene ethynylene polymers with sequences ADnA or DAnD (n = 1–5), which were separated by chromatography. All of the oligomers self-associate due to intermolecular H-bonding interactions, but intramolecular folding of the monomeric single strands can be studied in dilute solution. NMR and fluorescence spectroscopy show that the 3-mers ADA and DAD do not fold, but there are intramolecular H-bonding interactions for all of the longer sequences. Nevertheless, 1 : 1 mixtures of sequence complementary oligomers all form stable duplexes. Duplex stability was quantified using DMSO denaturation experiments, which show that the association constant for duplex formation increases by an order of magnitude for every base-pairing interaction added to the chain, from 103 M−1 for ADA·DAD to 105 M−1 for ADDDA·DAAAD. Intramolecular folding is the major pathway that competes with duplex formation between recognition-encoded oligomers and limits the fidelity of sequence-selective assembly. The experimental approach described here provides a practical strategy for rapid evaluation of suitability for the development of programmable synthetic polymers.
Highlights
Linear oligomers of different monomeric building blocks are the key functional molecules of biological systems.[1,2,3,4,5,6] Properties are encoded by the sequence of monomers in the polymer chain, and in principle, it should be possible to encode function in synthetic copolymers in the same way
The supramolecular assembly properties of mphenylene ethynylene polymers equipped with H-bond donor (D 1⁄4 phenol) and H-bond acceptor (A 1⁄4 phosphine oxide) side chains have been investigated in chloroform solution
We describe a synthetic strategy for rapidly accessing mixed sequence oligomers in order to characterise the competing equilibria of intramolecular folding and duplex formation
Summary
Linear oligomers of different monomeric building blocks are the key functional molecules of biological systems.[1,2,3,4,5,6] Properties are encoded by the sequence of monomers in the polymer chain, and in principle, it should be possible to encode function in synthetic copolymers in the same way. The DAnD and ADnA oligomers could fold via intramolecular interactions or self-associate via the doubly H-bonded AD$AD duplex motif shown in Fig. 5 to give supramolecular polymers. DAD forms a duplex in the solid state, and in agreement with the solution-phase NMR experiments, there is no intramolecular H-bonding in the crystal structure (Fig. 7).
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