PAPER www.rsc.org/greenchem | Green Chemistry Water soluble synthetic dieptide-based biodegradable nanoporous materials† Samit Guha, a Tushar Chakraborty b and Arindam Banerjee* a Received 17th December 2008, Accepted 20th April 2009 First published as an Advance Article on the web 15th May 2009 DOI: 10.1039/b822607e Two water-soluble synthetic dipeptides, b-alanyl-L-phenylglycine and its retro analogue L-phenylglycyl-b-alanine form a new class of dipeptide-based nanoporous materials, which are composed of a hybrid of a and b-amino acids. These materials adsorb N 2 gas with adsorption capacity of 173 cc g -1 and 71 cc g -1 and BET surface area of 56.76 m 2 g -1 and 41.73 m 2 g -1 for these dipeptides, respectively. Moreover, these nanoporous materials are found to be biodegradable towards soil bacterial consortium making them an interesting class of eco-friendly dipeptide-based nanoporous materials. Introduction Nanoporous materials based on purely inorganic materials such as zeolites 1 (aluminosilicates) and hybrid materials, consisting of both organic and inorganic counterparts such as metal–organic frameworks (MOFs), 2 have been extensively studied. These porous materials can be used in gas storage, chiral recognition, molecular separation, ion exchange, catalysis, and in sensors. 3 However, zeolites and MOFs are not based on purely organic molecules. There are a few examples of nanoporous materials based on purely organic molecules and these are termed as organo zeolites. 4–6 These include cyclic bis-urea based porous materials that bind reversibly with guest molecules, 4 Gorbitz’s Val-Ala and Phe-Phe class structures formed from hydropho- bic dipeptides 5 and Ripmeester’s dipeptide-based microporous (nanoporous) materials that can adsorb inert gas such as Xe. 6 Recently Sozzani and co-workers have reported dipeptide- based nanoporous materials and investigated their absorption, separation, and storage of gases such as methane, carbon dioxide, and hydrogen. 7 Some dipeptide-based compounds are able to host small organic molecules. 5f,8 A recent report describes that a polymeric sorbent has been used for very efficient removal of H 2 S from a mixture of gases for hydrogen purification. 9 Nanoporous materials based on water soluble peptide-based molecules have advantages over purely inorganic and hybrid materials, because they are of biological origin and generally non-toxic, eco-friendly and may be used as nanobiomaterials. Environmentally benign nanomaterials are an emerging field of current research. 10 However, none of the previously mentioned organic nanoporous materials are experimentally proven as biodegredable nanomaterials. a Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India. E-mail: arindam.bolpur@yahoo.co.in, bcab@iacs.res.in; Fax: (+)91-332473-2805 b Department of Cell Biology and Physiology, Indian Institute of Chemical Biology, Jadavpur, Kolkata, 700 032, India †Electronic supplementary information (ESI) available: Spectra and biodegradation data for dipeptides 1 and 2. See DOI: 10.1039/b822607e It is also very challenging to degrade non-protein amino acid based peptides. Seebach and coworkers have reported that soil bacterial consortium 11 can grow with b-amino acid contain- ing peptides as the sole carbon and energy source. 12 More- over, two aminopeptidases capable of degrading N-terminal b-alanine containing peptides, namely, L-aminopeptidase-D- alanine-esterase/amidase (DmpA) from Ochrobactrum anthropi and b-Ala-Xaa dipeptidase BapA from Pseudomonas sp. MCI3434 (Ps BapA), have been identified and characterized. 13 In this paper we report, a new class of water soluble dipeptide- based nanoporous materials, which are composed of a hybrid of b,a-amino acids having sequences b-alanyl-L-phenylglycine (b-Ala-L-Phg, 1) and its retro analogue L-phenylglycyl-b-alanine (L-Phg-b-Ala, 2). b-Alanine (b-Ala) is a non-coded naturally occuring b-amino acid and it has a role in a variety of biological processes. 14 It is believed to be a neurotransmitter in the central nervous system, binding to receptor sites common to glycine and g -amino butyric acid (GABA), and acting in the visual system. 14a b-Ala is one of the constituents of the naturally occuring dipeptides anserine and carnosine. 14c,d a-Phenylglycine (Phg) is a non protein a-amino acid. Its molecular structure is more rigid than naturally occuring a-phenylalanine (Phe) with a lower degree of rotational freedom, as it has one CH 2 unit less than phenylalanine. Nanoporous materials formed by dipeptides 1 and 2 are structurally different from the previously reported dipeptide-based nanoporous materials, 5,6 as they are composed of both a and b-amino acids instead of just a-amino acids. Moreover, these hybrid b,a-amino acid based nanoporous materials are found to be biodegradable towards a consortium of bacteria, 11 making them a novel, ecofriendly, new class of nanoporous materials. These compounds also adsorb nitrogen gas. 2a–e,j Results and discussion Two water soluble dipeptides, where b-alanine and L- phenylglycine are used as constituents, b-Ala-L-Phg (1), and its retro analogue L-Phg-b-Ala (2) (Fig. 1a and b) have been synthesized by conventional solution-phase methodology, 15,16 This journal is © The Royal Society of Chemistry 2009 Green Chem., 2009, 11, 1139–1145 | 1139