In a discovery that defies everyday logic, scientists in Japan have found that longer molecules actually move faster through tiny pores than shorter ones—a finding that could reshape how we design filters and separation systems inspired by nature.
The research, led by Professor Shuichi Hiraoka at the University of Tokyo and Professor Masanori Tachikawa at Yokohama City University, reveals something counterintuitive about how molecules navigate through dynamic nanoscale gates. When a long pencil is longer than a short one, you'd expect it to take longer to pass through a narrow doorway. But inside self-assembled molecular "nanocubes"—cube-shaped structures that form spontaneously in water—the opposite happens. This discovery matters because understanding how molecules move through flexible pores could unlock better ways to filter, separate, and recognize materials, and it brings us closer to mimicking the elegance of biological transport systems like the ion channels and aquaporins that regulate what passes in and out of living cells.
The team created three different nanocubes with varying degrees of flexibility and used time-resolved luminescence measurements to track how various hydrocarbon molecules moved through them. What they observed was remarkable: linear alkane molecules entered the nanocubes at faster rates as their carbon chains grew longer. But there was more to the story. The researchers discovered that molecular shape mattered far more than size alone. Linear alkanes zipped through at much higher speeds than branched alkanes with the same number of carbon atoms, proving that the dynamic pores were remarkably selective. When the team added double or triple bonds to the ends of molecules, transport accelerated even further. However, introducing oxygen atoms slowed the journey down significantly.
To explain this paradox, Hiraoka and Tachikawa proposed a two-step transport mechanism that reveals the hidden intelligence of these nanoscale gates. Molecules don't simply squeeze through the pore—they first form what the researchers call a transient "encounter complex" at the outer surface of the nanocube. Molecules that interact more strongly with this outer surface linger there longer, which paradoxically increases their chances of slipping through when the gate temporarily opens due to thermal motion. It's like a revolving door that opens briefly but more frequently for those who stand closer and wait patiently. The flexibility of the pore itself—its constant fluctuation and shape-shifting—becomes a feature rather than a hindrance. Molecular dynamics simulations visualized these gate openings directly, confirming the proposed mechanism.
Unlike rigid artificial filters that maintain fixed dimensions, biological pores are in constant motion, squirming and shifting with thermal energy. The challenge has always been understanding this dance at the molecular level. Now, thanks to this work published in the journal Chem, we have a new kinetic principle for molecular transport through dynamic gates. The implications extend far beyond basic science. These insights could inspire the design of selective artificial channels for medical diagnostics, molecular recognition systems for detecting specific substances, and separation materials that harness nature's elegant approach to controlling what passes through and what stays behind.