A tape-reading molecular ratchet.

Cells process information in a manner reminiscent of a Turing machine1, autonomously reading data from molecular tapes and translating it into outputs2,3. Randomly processive macrocyclic catalysts that can derivatise threaded polymers have been described4,5, as have rotaxanes that transfer building blocks in sequence from a molecular strand to a growing oligomer6-10. However, synthetic small-molecule machines that can read and/or write information stored on artificial molecular tapes remain elusive11-13. Here we report on a molecular ratchet in which a crown ether (the 'reading head') is pumped from solution onto an encoded molecular strand (the 'tape') by a pulse14,15 of chemical fuel16. Further fuel pulses transport the macrocycle through a series of compartments of the tape via an energy ratchet14,17-22 mechanism, before releasing it back to bulk off the other end of the strand. During its directional transport, the crown ether changes conformation according to the stereochemistry of binding sites along the way. This allows the sequence of stereochemical information programmed into the tape to be read out as a string of digits in a non-destructive manner through a changing circular dichroism response. The concept is exemplified by the reading of molecular tapes with strings of balanced ternary digits ('trits'23), -1,0,+1 and -1,0,-1. The small-molecule ratchet is a finite-state automaton: a special case24 of a Turing machine that moves in one direction through a string-encoded state sequence, giving outputs dependent on the occupied machine state25,26. It opens the way for the reading-and ultimately writing-of information using the powered directional movement of artificial nanomachines along molecular tapes.

α-Amino-iso-Butyric Acid Foldamers Terminated with Rhodium(I) N-Heterocyclic Carbene Catalysts.

To investigate how remotely induced changes in ligand folding might affect catalysis by organometallic complexes, dynamic α-amino-iso-butyric acid (Aib) peptide foldamers bearing rhodium(I) N-heterocyclic carbene (NHC) complexes have been synthesized and studied. X-ray crystallography of a foldamer with an N-terminal azide and a C-terminal Rh(NHC)(Cl)(diene) complex showed a racemate with a chiral axis in the Rh(NHC) complex and a distorted 310 helical body. Replacing the azide with either one or two chiral L-α-methylvaline (L-αMeVal) residues gave diastereoisomeric foldamers that each possessed point, helical and axial chirality. NMR spectroscopy revealed an unequal ratio of diastereoisomers for some foldamers, indicating that the chiral conformational preference of the N-terminal residue(s) was relayed down the 1 nm helical body to the axially chiral Rh(NHC) complex. Although the remote chiral residue(s) did not affect the stereoselectivity of hydrosilylation reactions catalysed by these foldamers, these studies suggest a potential pathway towards remote conformational control of organometallic catalysts.