Micropores in Crystalline Dipeptides as Seen from the Crystal Structure, He Pycnometry, and¹²⁹Xe NMR Spectroscopy

Abstract

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 P6(1)), 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 (129)Xe 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 A for AV and <4 A 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 approximately AI approximately VV > VI > LS, with a helix diameter of approximately 2 A for VA, IA, and IV and approximately 1 A 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.

Observation of Single‐File Diffusion in Dipeptide Nanotubes by Continuous‐Flow Hyperpolarized Xenon‐129 NMR Spectroscopy

“…[24][25][26][27][28][29][30] Its large chemical shift range allows the adsorbed phase to be distinguished from the free gas, and reveals interactions with the channel interiors and other Xe atoms. l-Alanyl-l-valine (AV) can form hydrophobic crystalline channel structures with interior walls lined with CH 3 groups [31] and an inner diameter of 5.13 , which is just slightly larger than the 4.5 van der Waals diameter of Xe atom.…”

Hydrophobic dipeptide crystals: a promising Ag-free class of ultramicroporous materials showing argon/oxygen adsorption selectivity

“…The structure of these molecular crystals is stabilised by H-bonds and hydrophobic interactions, being essentially the same for all VA-class dipeptides. This crystal structure displays an array of identical, parallel unidimensional micropores 2,7 . The pore walls are formed by the aliphatic side-chains of the dipeptides, making the pores highly hydrophobic 1 .…”

“…Different side-chain combinations create slightly different crystal structures, with different pore sizes 1 . The micropores are helical and have an approximately circular crosssection, somewhat uniform throughout the unit cell 4,7 . Nominal, average pore sizes of VA-class dipeptide crystals range from 0.37 nm to 0.50 nm 2,7 .…”

“…In this article, we report the adsorption isotherms and heats of adsorption of Ar, O2 and N2 in four crystalline dipeptides. The four dipeptides used have the smallest pores of the VA-class; VI, IA, IV and VV, having, respectively, 0.370, 0.374, 0.390 and 0.439 nm average crystallographic diameters 2 , corresponding, respectively, to porosities of 5.7, 6.0, 6.3 and 8.3 % (as determined from the crystal structure) 7 . The adsorbents show inverted Ar/O2 selectivity, a highly abnormal result, extremely interesting in the context of the very important industrial process of O2 production by pressure swing adsorption (PSA).…”

Thermal analysis of clathrates of tripeptide LLL with organic compounds and water

“…Short-chain oligopeptides are capable of forming selfassembled architectures and nanomaterials [1][2][3][4]. Such materials are of interest in the design of sensors [5] and new devices for energy storage [6], the preparation of new hybrid [5,7,8] and smart [9] materials, the separation of biologically active substances [10] or enantiomers [11], and in the creation of transmembrane pores and channels [12,13].…”