A tape-reading molecular ratchet

Cells process information in a manner reminiscent of a Turing machine(1), autonomously reading data from molecular tapes and translating it into outputs(2,3). Randomly processive macrocyclic catalysts that can derivatise threaded polymers have been described(4,5), as have rotaxanes that transfer building blocks in sequence from a molecular strand to a growing oligomer(6-10). However, synthetic small-molecule machines that can read and/or write information stored on artificial molecular tapes remain elusive(11-13). 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 pulse(14,15) of chemical fuel(16). Further fuel pulses transport the macrocycle through a series of compartments of the tape via an energy ratchet(14,17-22) 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 case(24) of a Turing machine that moves in one direction through a string-encoded state sequence, giving outputs dependent on the occupied machine state(25,26). It opens the way for the reading-and ultimately writing-of information using the powered directional movement of artificial nanomachines along molecular tapes.