Micropores in crystalline dipeptides as seen from the crystal structure, He pycnometry, and 129Xe NMR spectroscopy

Eight crystalline dipeptides were studied: AV (Ala-Val), VA ( Val-Ala), AI (Ala-Ile), VV (Val-Val), IA (Ile-Ala), IV (Ile-Val), VI (Val-Ile), and LS (Leu-Ser) ( all LL isomers). The first seven form an isostructural series (space group P 61), whereas LS has a different structure (P6(5)). All structures display H-bonded tubular assemblies of the dipeptide molecules resulting in open ultramicropores in the form of isolated one-dimensional (1D) channels. The total porosity of the materials ranges from 4 to 12% ( micropore volume from 0.04 to 0.12 cm(3)/g). Calculations based on the crystal structures, He pycnometry, and solid-state Xe-129 NMR methods were used to obtain a comprehensive description of the geometry and properties of the micropores. The following order was established for the channel diameter: AV > VA > AI > VV > IA > IV > VI, with > 5 angstrom for AV and < 4 angstrom for VI; LS is close to AI. The observed sorption behavior cannot be described adequately based on the crystal structure and can only be understood if one takes into account the dynamics of the host matrix. The pores are chiral, with the center of the channel describing a right-handed helix ( left- handed for LS). The following order was established for the channel helicity: VA > IA > IV > AV approximate to AI approximate to VV > VI > LS, with a helix diameter of similar to 2 angstrom for VA, IA, and IV and similar to 1 angstrom or less for the remaining dipeptides. A comparison of the dipeptides studied with other supramolecular materials is given and the potential for applications is discussed.