Xenon in l-Alanyl-l-Valine Nanochannels: A Highly Ideal Molecular Single-File System

Abstract

Xenon-129 pulsed field gradient NMR and hyperpolarized xenon-129 spin tracer exchange NMR experiments were performed on polycrystalline l-Ala-l-Val (AV) and l-Val-l-Ala (VA) nanochannels under identical conditions of temperature and Xe pressure. Displacements of up to 3 μm were measured in specimens with mean crystallite lengths of around 50–100 μm. The combination of the two NMR techniques yielded the most definitive evidence for molecular single file diffusion to date, in which the time-scaling of the mean squared displacement is proportional to the square-root of time. The PFG-NMR echo attenuation data yield single-file mobility factors of 6 ± 0.7 × 10–13 m2 s–1/2 and 4.4 ± 0.2 × 10–13 m2 s–1/2. The results establish Xe in AV nanochannels, in particular, as an ideal experimental model system for fundamental studies of single-file diffusion dynamics.