Researchers have discovered that longer molecules can pass faster through dynamic nanoscale pores than shorter ones. This finding challenges conventional understanding of molecular transport. The study involved self-assembled molecular "nanocubes" in water.
Professor Shuichi Hiraoka at the University of Tokyo and Professor Masanori Tachikawa at Yokohama City University led the research. Their team quantitatively analyzed how molecules move through these flexible pores. The study focused on linear alkane molecules and found an unexpected phenomenon.
The transport rates are influenced by pore size, gate flexibility, and weak interactions at the outer pore surface. Biological processes like ion channels and aquaporins in cell membranes rely on similar nanoscale transport. Unlike rigid artificial filters, biological pores constantly change shape due to thermal motion.
The researchers used cube-shaped molecular assemblies with hydrophobic inner cavities and flexible pores. They prepared three nanocubes with varying flexibilities. Time-resolved luminescence measurements analyzed the uptake rates of hydrocarbon molecules.
Linear alkanes entered the nanocubes much faster than branched alkanes of the same carbon number. This indicates that the dynamic pores discriminate based on molecular shape. Transport rates increased with hydrocarbon chain length, which contradicts typical macroscopic expectations.
The team proposed a two-step transport mechanism. Molecules first form an "encounter complex" at the nanocube's outer surface. They then pass through the fluctuating pore. Stronger interactions with the outer surface increase the probability of entry when the gate opens. Molecular dynamics simulations supported this mechanism. The findings could inform the design of selective artificial channels and molecular recognition systems.
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