An artificial molecular machine that builds an asymmetric catalyst

Biomolecular machines perform types of complex molecular-level tasks that artificial molecular machines can aspire to. The ribosome, for example, translates information from the polymer track it traverses (messenger RNA) to the new polymer it constructs (a polypeptide)(1). The sequence and number of codons read determines the sequence and number of building blocks incorporated into the biomachine-synthesized polymer. However, neither control of sequence(2,3) nor the transfer of length information from one polymer to another (which to date has only been accomplished in man-made systems through template synthesis)(4) is easily achieved in the synthesis of artificial macromolecules. Rotaxane-based molecular machines(5-7) have been developed that successively add amino acids(8-10) (including beta-amino acids(10)) to a growing peptide chain by the action of a macrocycle moving along a mono-dispersed oligomeric track derivatized with amino-acid phenol esters. The threaded macrocycle picks up groups that block its path and links them through successive native chemical ligation reactions(11) to form a peptide sequence corresponding to the order of the building blocks on the track. Here, we show that as an alternative to translating sequence information, a rotaxane molecular machine can transfer the narrow polydispersity of a leucine-ester-derivatized polystyrene chain synthesized by atom transfer radical polymerization(12) to a molecular-machine-made homo-leucine oligomer. The resulting narrow-molecular-weight oligomer folds to an alpha-helical secondary structure(13) that acts as an asymmetric catalyst for the Julia-Colonna epoxidation(14,15) of chalcones.