DOI: 10.1002/chem.201202182 Tuning Chloride Binding, Encapsulation, and Transport by Peripheral Substitution of Pseudopeptidic Tripodal Small Cages InØs Martí, [a] Jenifer Rubio, [a] Michael Bolte, [b] M. Isabel Burguete, [a] Cristian Vicent, [c] Roberto Quesada, [d] Ignacio Alfonso,* [e] and Santiago V. Luis* [a] Introduction The study of macrocycles and cage-like structures derived from amino acids (peptides and pseudopeptides) is an emerging research topic, closely connected with synthetic chemistry, [1] natural products, [2] material science, [3] and bio- logical chemistry. [4] The geometrical constraints produced by the cyclic frame are extremely useful to implement a rigid conformation within a small peptidic sequence. [5] This has al- lowed the chemists to design structural scaffolds with inter- esting biological activities. [6] Moreover, this type of com- pounds has been used as receptors for some species of inter- est, either ions (cations [7] and anions [8] ) or small neutral mol- ecules. [9] Among these structures, those defining a three-di- mensional inner space are especially attractive for recognition purposes, because they usually lead to stronger interaction and higher selectivity towards a given comple- mentary guest. Thus, the study of three-dimensional cavities as molecular containers is a hot topic nowadays. [10] In this context, macrobicyclic pseudopeptides defining a cage-like architecture [11] have become very appealing as synthetic tar- gets for molecular recognition purposes [12] and biomimetic studies. [13] However, their preparation is very often challeng- ing, requiring multi-step synthetic pathways with low yields and tedious purification steps. In this regard, the key step is usually the reaction leading to the macrobicyclic structure, Abstract: A highly efficient synthesis of small pseudopeptidic cages from simple precursors has been achieved by the triple S N 2 reaction between tripo- dal tris(amido amines) and several 1,3,5-tris(bromomethyl)benzene elec- trophiles. The success of the macrobi- cyclization strongly depends on the central triamine scaffold, which dic- tates the correct preorganization of the intermediates. The chloride binding properties of the protonated pseudo- peptidic cages have been studied in the solid state (by X-ray diffraction) as well as in solution (by NMR spectro- scopy and ESI-MS) and in the gas phase (by collision-induced dissociation (CID)-MS). The crystal structure of the HCl salts of several cages show a chloride partially or completely caged within the cavity of the macrobicycle. Both the amino acid side chain and the substitution at the aromatic tripodal ring have an effect on the chloride binding ability. The cages derived from the 1,3,5-benzene moiety show low af- finity, whereas the triple substitution in the ring (either with Me or Et) increas- es the chloride binding by one order of magnitude. Besides, the cages derived from aliphatic amino acids display a stronger interaction than those derived from phenylalanine. The basis for the different mode of binding depending on the receptor structure is proposed according to the structural data (X-ray and NMR spectroscopy). Finally, the transport of the chloride anion through lipid bilayers has been studied for se- lected cages. Despite the important dif- ferences in the chloride binding, the transport properties are better correlat- ed with the lipophilicity of the mole- cules. Therefore, the pseudopeptidic cages sharing the same binding motif for chloride rendered very different in- teraction and transport properties de- pending on the peripheral substitution. Keywords: cage compounds · chlor- ide binding · pseudopeptides · su- pramolecular chemistry · transport [a] I. Martí, Dr. J. Rubio, Prof. Dr. M. I. Burguete, Prof. Dr. S. V. Luis Departamento de Química Inorgµnica y Orgµnica Universitat Jaume I Avda. Sos Baynat, s/n, 12071 Castellón (Spain) Fax: (+ 34) 964728214 E-mail : luiss@uji.es [b] Dr. M. Bolte Institut für Anorganische Chemie J.-W.-Goethe-Universität Max-von-Laue-Str.7 60438 Frankfurt/Main (Germany) [c] Dr. C. Vicent Serveis Centrals d’Instrumentació Científica Universitat Jaume I Avda. Sos Baynat, s/n 12071 Castellón (Spain) [d] Dr. R. Quesada Departamento de Química, Facultad de Ciencias Universidad de Burgos 09001 Burgos (Spain) [e] Dr. I. Alfonso Departamento de Química Biológica y Modelización Molecular Instituto de Química Avanzada de CataluÇa (IQAC-CSIC) Jordi Girona, 18-26 08034 Barcelona (Spain) Fax: (+ 34) 932045904 E-mail: ignacio.alfonso@iqac.csic.es Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201202182. # 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 16728 – 16741 16728