Molecular machines won a Nobel prize nine years ago, but we still can’t decide what one is

A new project aims to define the key terms used in the field of molecular machines. It is hoped that the new definitions will allow researchers to avoid semantic debates, and could help provide clarity in any future legal battles involving the technology.

Even the phrase ‘molecular machine’ is used inconsistently by the field’s researchers

Molecular machines are generally understood to include an array of nanoscale pumps, switches, motors and ratchets. The field emerged in the early 1980s, and three of its pioneers – Ben Feringa, Jean-Pierre Sauvage and Fraser Stoddart – shared the 2016 Nobel prize in chemistry for their groundbreaking miniature creations.

But despite decades of research and numerous accolades bestowed upon the field’s top scientists, debate still surrounds many of the most fundamental terms used to describe the work. In fact, even the phrase ‘molecular machine’ is used inconsistently by the field’s researchers.

‘There’s actually a whole bunch of different definitions in literature. If you read Fraser’s papers, and Jean Pierre-Sauvage’s papers and David Leigh’s papers and Ben Feringa’s papers – each of them gives you a different definition of a molecular machine,’ says Carson Bruns, a nanomaterials researcher at the University of Colorado Boulder, US. ‘And actually, even just one author will change their mind about what a molecular machine is over time – so in the ‘90s, they were kind of saying different stuff than what they were saying in the 2000s and so on.’

Bruns is leading the new project, which is backed by the International Union of Pure and Applied Chemistry (Iupac) – the body that codifies nomenclature used by chemists. He explains that the idea took shape after he received a lot of feedback to an article he’d written about molecular machine terminology in 2022. One response came from a division leader at Iupac, who encouraged Bruns to launch an official project and assemble a committee to investigate the issue.

‘I wanted to make sure I had people on the committee who disagreed with me,’ he adds. ‘I figured, if we had a committee of people who are really invested in the field and who disagree with each other – if that committee could come to a consensus, then maybe the broader community could come to a similar consensus.’

The project is currently at the information-gathering stage, with the committee keen to hear the views of the community before it makes any recommendations. This year, they are planning to attend conferences in Japan and the US to talk face-to-face with researchers in the field. The committee has also produced two interactive polls so that Chemistry World readers can participate in the project.

An old problem

The University of Manchester’s David Leigh has been working on molecular machines for decades, and wrote about the need for the field to adopt ‘suitably defined’ scientific language almost 20 years ago. Leigh, who is not a member of the Iupac project team, says that the new initiative could be helpful in preventing misuse of some terms. However, he also cautions that any new definitions will need to be carefully considered so that they remain relevant in future.

‘It’s that delicate balance of not being too prescriptive in your use of terms and realising that things will evolve and that there will be some correction in time,’ he says. ‘And if you’re too liberal with it and it means anything to everyone, then that’s bad, because then the language literally has no meaning.’

Leigh also points out that terms like molecular pump and molecular motor are widely used by scientists working in many different fields, and so any new definitions would need to be consistent with these wider uses.

‘There’s probably a hundred times more papers on molecular motors by biologists and physicists than there are by chemists,’ he notes. ‘There are thousands of papers on kinesin on myosin published every year – so, again, I would just be a little bit worried if chemists were too prescriptive.’

While the project may eventually tease out definitions for terms like molecular switch, motor, rotor, gear, ratchet and pump, Bruns says that the most important phrase to clarify is molecular machine. ‘Because that’s the name of the field, that also feels like the most important to us, the one that we want to resolve first,’ he says. ‘If it’s not clear what a molecular machine is, then it’s also not clear if all these other terms are machines or not.’

‘If you don’t know what a molecular machine is, then you don’t know if a switch is a machine or not, or if a motor is a machine or not,’ he adds. ‘And so we’ve really kind of narrowed our focus on the term molecular machine, because it’s clearly the highest priority and the most contentious.’

Aside from settling niche debates between researchers, Bruns notes that having accepted definitions of scientific terminology can have more serious ramifications. For example, he points to court cases that have relied on definitions of per- or polyfluoroalkyl substance (PFAS) to determine a company’s liability for its pollution.

‘Sometimes there are these important and high-impact scenarios that hinge on a formally recognised definition. We don’t quite see that happening yet in molecular machines – probably because nobody’s invented a molecular machine that’s going to totally transform the world,’ he says. ‘But someday that probably will happen. When there’s an artificial molecular machine that has [a major] impact on society, there’s going to be lawsuits – and then we’re going to need to be really clear on what a molecular machine is.’