A small molecule that walks non-directionally along a track without external intervention.

Kinesin, dynein, and some myosin motor proteins transport cargoes within the cell by “walking” along polymeric filaments, that is carrying out successive, repetitive, mostly directional changes of their point of contact with the molecular track without completely detaching from it. These extraordinary biomolecules are inspiring scientists to mimic aspects of their dynamics to create artificial molecular transport systems. Recently, the first small molecules that are able to walk down short molecular tracks were described. However, external intervention (the addition of chemical reagents and/or irradiation with light) are required to mediate each step taken by the walker units in the nonDNA systems reported to date. Although migrations of submolecular fragments occur in many different types of chemical reaction, few systems appear to offer the potential for multiple successive and cumulative transport necessary for developing small-molecule walkers. An interesting exception are the so-called equilibrium transfer alkylating crosslinking (ETAC) reagents introduced in the 1970s by Lawton and co-workers for the dynamic cross-linking of biomolecules. These reagents reversibly form covalent bonds between pairs of accessible nucleophilic sites on proteins through a series of interand intramolecular Michael and retro-Michael reactions until the most thermodynamically stable crosslink is located (Scheme 1a). We wondered whether it would be possible to apply a similar concept, focusing instead on chemistry where the cross-linked products are less stable than those attached by a single covalent bond, to make synthetic small molecules that migrate with a high degree of processivity along a linear molecular track. Herein we describe the attachment of a-methylene-4nitrostyrene to oligoethylenimine tracks and the dynamics of its subsequent migration from amine group to amine group without fully detaching by a sequence of intramolecular Michael and retro-Michael reactions. In this way the amethylene-4-nitrostyrene units move towards the most thermodynamically favored distribution of walkers on oligoamine tracks (Scheme 1b). A model walker-track conjugate, 1, was synthesized in which a-methylene-4-nitrostyrene was attached to an outer amine group of a triamine track and then allowed to exchange between the secondary amine footholds (Scheme 2a; see the Supporting Information for experimental procedures and characterization data). The reaction was followed by H NMR spectroscopy through the different chemical shift of vinyl protons (Hc/c’ and Hd/d’) of isomers 1 and 2 (Figure 1). The kinetics of the amine-to-amine migration of the amethylene-4-nitrostyrene unit (“walking”) is highly solventdependent. Starting from pristine 1 (5 mm), no formation of 2 was observed in CDCl3 or CD2Cl2 over 15 h at room temperature and the reaction only proceeded slowly in CD3OD (< 10% conversion over 15 h) or CD3CN (< 25% conversion over 15 h). However, the interconversion of 1 with 2 reached a close-to-1:1 steady-state ratio of 1:2 over 15 h in [D7]DMF or 4.5 h in [D6]DMSO (298 K, 5 mm). Increasing Scheme 1. a) The migration of an ETAC reagent between nucleophilic sites of a protein by Michael/retro-Michael reactions towards the most thermodynamically stable cross-linked product. b) Processive (intramolecular) migration of a-methylene-4-nitrostyrene along a polyamine track. Michael addition of a track amine group to the olefin of the “two-legged walker” results in a bridged intermediate (both “feet” attached to the track, shown in square brackets) that can subsequently undergo a retro-Michael reaction to either side, unmasking the double bond and leaving the walker attached to the track through a single covalent bond.