A new approach to characterizing sorption in materials with flexible micropores

Microporous dipeptides, also known as organic zeolites or biozeolites, as examples of small-pore peptide nanotubes provide a convenient set of materials for developing a systematic approach based on Xe-129 NMR spectroscopy for the derivation of thermodynamic and molecular scale information on temperature dependent pore filling. The sorption of xenon in the isolated 1D chiral nanochannels of eight microporous dipeptides Ala-Val (AV), Val-Ala (VA), Leu-Ser (LS), Ala-Ile (AI), Val-Val (VV), Ile-Ala (IA), Ile-Val (IV), Val-Ile (VI) (all LL isomers) was monitored in situ with continuous-flow Xe-129 NMR spectroscopy over a temperature range of 173-343 K. The materials all showed strongly anisotropic signals, with isotropic chemical shift changing from 95 to 281 ppm depending on the dipeptide used and/or temperature. The isosteric heats of sorption (q(st)) and entropy factors were determined from two independent models. The sorption process was complicated by reversible phase transformations of some dipeptides and irreversible changes due to aging of samples, both of which may be of considerable importance in applications of soft materials. The interpretation of the line shapes and chemical shift anisotropy as a function of temperature provided information on the structure of the xenon-cavity complex and made it possible taking into account the helicity and flexibility of the nanochannels and the dynamics of xenon. The approach illustrates a powerful way of analyzing pore space in soft microporous materials, yielding a quantitative thermodynamic description of sorption and the characteristics of the pore space and sorption events that occur on molecular-scale level during pore filling.