Permselective Crystals DOI: 10.1002/ange.201000007 Dipeptide Crystals as Excellent Permselective Materials: Sequential Exclusion of Argon, Nitrogen, and Oxygen** RuiV. Afonso, Joana Dur¼o, AdØlio Mendes, Ana M. Damas, and Luís Gales* Gas storage and gas separation using porous solids are important technologies that have attracted great attention because of their environmental and energetic applications. Highly porous materials, such as zeolites, silicate, and carbon- based materials, [1] have long-established specific applications. The key for new applications is the development of new frameworks. Advances in gas sorption capacities were ach- ieved through the synthesis of materials such as metal– organic frameworks (MOFs), organic polymers, and micro- porous organic crystals. [2] Recently, crystals formed by dipeptides were tested as adsorbents [3] with significant results in hydrogen absorption and methane purification from carbon dioxide. [3b] Dipeptides can form microporous materials with channels of tunable size. Although the dipeptides self-assemble through a net of hydrogen bonds, the crystal matrix is conserved upon exchange of guest molecules. Moreover, crystalline dipeptides show a very high density of single-size micropores with very low tortuosity, which makes them excellent materials for storage or selective separation pur- poses. Finally, there is the remarkable feature that pores of crystalline dipeptides are perfectly aligned (along the crys- tallographic c axis), which indicates that they are excellent candidates for use as permeation-selective barriers. Herein, we report for the first time the use of dipeptide crystals as permselective materials. Although this looks like an obvious engineering application for the kind of porous topology present in the crystals, there are issues that call for experimental support: 1) potential crystal defects, such as twinning or fractures, may greatly diminish their actual selectivity; and 2) the potential lack of rigidity of the crystal structure allows the pores to adapt to some extent to the size of the guest molecules. The dynamic behavior of the matrix of peptide crystals has already been observed by He picnometry and 129 Xe NMR methods. [4] We envisage the selective permeation of argon, nitrogen, and oxygen (the main components of air) through dipeptide crystals. This is a highly relevant industrial separation process and is also a very ambitious one given the similarity of the molecular sizes of the individual components. [5] The dipeptide crystals that were tested as single-crystal membranes were l- leucyl-l-serine (LS) (Scheme 1), l-valyl-l-isoleucine (VI), and l-alanyl-l-alanine (AA) crystals. The peptides were crystallized and their structures determined by X-ray diffraction (Figure 1). The structures of all three peptides had been resolved previously. [6] The VI crystal packing has hexagonal symmetry with molecules forming helices with six dipeptides per turn. LS crystals have a unique crystal packing with the inner walls formed by leucine side chains and with right-handed helicity. AA packs in the tetragonal space group I4 and the crystal arrangement is characterized by the segregation of the hydrophobic methyl groups into columns. The calculated void volumes in the three crystal structures that are accessible to He, the molecule with the smallest kinetic diameter (2.6 ), are shown in Figure 2. [7] LS and VI contain nanochannels while AA should be considered non- porous. The average channel diameters of LS and VI are displayed in Table 1. Scheme 1. Dipeptides used in this study. Figure 1. Crystal structures of the dipeptides viewed along the crystal- lographic c axis. [*] R. V. Afonso, J. Dur¼o, Prof. Dr. A. M. Damas, Prof. Dr. L. Gales Instituto de Biologia Molecular e Celular Rua do Campo Alegre 823, 4150-180 Porto (Portugal) Fax: (+ 351) 226-099-157 E-mail: lgales@ibmc.up.pt R. V. Afonso, Prof. Dr. A. M. Damas, Prof. Dr. L. Gales Instituto de CiÞncias BiomØdicas Abel Salazar Largo Prof. Abel Salazar 2, 4099-003 Porto (Portugal) R. V. Afonso, Prof. Dr. A. Mendes Laboratory for Process, Environmental, and Energy Engineering Faculdade de Engenharia da Universidade do Porto Rua Dr. Roberto Frias, s/n 4200-465 Porto (Portugal) [**] This work is supported by Fundaç¼o para a CiÞncia e Tecnologia (project PTDC/CTM/64191/2006) and by a PhD scholarship to R.A. (SFRH/BD/43821/2008). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201000007. Zuschriften 3098  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2010, 122, 3098 –3100