Effect of Functional Group and Solvent On the Conformational Ordering of m-Phenylene Ethynylene Foldamers: a Simulation Study On Folding Driving Force

The ester functionalized meta-polyphenylene ethynylene (mPPE) is known order itself into a helical conformation in suitable solvents. This folding phenomenon results from the contribution of three distinctive properties of the polymer: the meta-connected backbone, the solvophilic tails and the ester functional groups. While the roles of the first two are clear and well supported, the actual role of the ester functional group is still elusive. To explore this problem, ab initio quantum simulations and Replica Exchange Molecular Dynamics modeling, which employed the OPLS force field, were used to investigate the effect of specific functional groups on the folding behavior of a range of mPPEs. Specifically, this procedure was used to simulate the ester mPPE in 7 different explicit solvents. Further, 20 mPPEs with different functional groups were simulated in explicit acetonitrile. The 6 other mPPEs with side chains that have an activating effect on the benzyl ring did not show any folding event. For the 14 mPPEs with side chains that have a deactivating effect on the benzyl ring, 11 exhibited the tendency to form more compact conformations than the initial structures, while the other three remained unfolded. However, these results also indicate that in this study only the ester functionalized mPPE was able to form a stable and well defined helical secondary structure from a random coil structure. These results suggested that a combination of pi-stacking and CH/pi interactions could be employed to explain mPPE folding behaviors. Further, the difference between pi-stacking and CH/pi interactions was proposed as a driving force in mPPE conformation formation.