Dipeptides as Microporous Materials

Abstract

The synthesis and characterization of hydrophobic porous frameworks designed for a variety of applications has attracted significant research efforts over the last few years. The key idea in such studies is to design building elements which allow for controllable assembly and disassembly, and which may be extensively replaced and modified to give porous frameworks with various topologies and functions. The natural world often provides excellent examples of how to proceed with the design of materials; for example, the initial experiences with natural zeolites has led to very successful efforts to produce a wide variety of industrially relevant synthetic materials. Recently, supramolecular architectures based upon weaker interactions have motivated a number of studies; polypeptides that can build tubular and helical structures, frequently referred as nanotubes, have attracted considerable attention because of their biological relevancy as ion channels, membrane pores, etc. These naturally occurring and artificially created materials reveal a sophisticated complexity and are often difficult to characterize in detail. Thus, the lower oligomeric peptides, being both simpler model systems and cheaper, would appear to be useful as practical porous materials, especially since some dipeptides demonstrate the ability to host small organic molecules. Moreover, the ability of single crystals of the dipeptides to sustain guest solvent exchange and removal was reported. Herein we examine the structure and sorption properties of the two closely related dipeptides l-alanyl-lvaline (AV) and l-valyl-l-alanine (VA; Scheme 1), and show that these lower oligomers have considerable promise as novel porous materials. Moreover, the wide variety of amino acids available should allow the assembly of materials with quite diverse structural motifs containing significant void space. The two dipeptides AV and VA are microporous crystalline materials. Hexagonal prisms of the dipeptides were grown from water and characterized by single-crystal X-ray diffraction (XRD) analysis at room temperature (Table 1).