A phosphate group steps along a glycerol track for hours on carbodiimide fuel alone

The walker here is a phosphate group, and the track is a polyol whose hydroxy groups are the footholds ^[1]. Activation was never the hard part. EDC, a water-soluble carbodiimide, has been used for years to activate organophosphates ^[11]. Getting the bond open again was the obstacle, because the P-O bonds in an ordinary phosphodiester are too inert to hydrolyse under either acidic or basic conditions ^[12]. Without hydrolysis the fuel leaves the phosphate wherever it landed, and there is no cycle ^[12].

The fix is a five-membered cyclic phosphate as the activated species ^[13]. RNA does this to itself. The 2' hydroxy group on ribose closes onto the phosphate, and the resulting transient five-membered ring hydrolyses many orders of magnitude faster than a linear phosphodiester such as dimethyl phosphate ^[14]. That same ring keeps the phosphate on the track. Earlier small-molecule walkers depended on carefully chosen reversible covalent reactions to ensure one foot could come off the track and not both ^[22][16]. Here the cyclic diester spans two footholds while selective hydrolytic ring opening leaves the phosphate on one of them ^[2], so processivity comes from the intermediate itself ^[15].

Autonomy in this system means the fuel drives the stepping with nobody adding reagents between steps, and that is the attribute small-molecule walkers had been missing ^[8]. A motor is a stronger claim. Motor proteins catalyse the exergonic conversion of ATP to ADP and use kinetic asymmetry in the cycle to produce net directional motion along helically chiral tracks ^[10]. The evidence for asymmetry in this work is a kinetic model ^[5], and what the abstract reports is a phosphate distribution held away from equilibrium for hours, stopping short of cargo moved or work done against a load ^[3].

Non-enzymatic cycles that hold a system away from equilibrium have been studied for about a decade, producing oscillations and liposome division among other behaviour ^[19]. None had been used to run a small-molecule walker ^[18]. DNA walkers had already reached autonomy ^[9].

Non-enzymatic phosphodiester formation has been studied in the context of phospholipids and RNA oligomerization ^[21], and the group's earlier cycles used phosphoramidates and acylphosphates as the activated species; the phosphodiester version had stayed out of reach ^[17]. The authors describe what they have added as "migratory endergonic synthesis" on biologically relevant phosphate scaffolds ^[20].