Acid-base controllable recognition properties of a highly versatile calixcrypturea.
ABSTRACT Versatile concave receptors with binding properties that can be controlled by external stimuli are rare. Herein, we report on a calixcrypturea (1) that features two different binding sites in close proximity, that is, a tris(2-aminoethyl)amine (tren)-based tris-ureido cap that provides convergent hydrogen-bond-donor sites and a hydrophobic cavity suitable for the inclusion of organic guests. The binding properties of this heteroditopic receptor have been evaluated by NMR spectroscopic studies. Compound 1 behaves as a remarkably versatile host that strongly binds neutral molecules, anions, or contact ion pairs. Within each family of guests, compound 1 is able to discriminate between different guests with a high degree of selectivity. Indeed, neutral molecules that possess hydrogen-bond donor and acceptor groups, chloride anions, and linear ammonium ions associated to F(-) or Cl(-) are particularly well recognized. In comparison with all the related receptors, compound 1 displays several unique features: 1) charged or neutral species are also recognized in polar or protic solvents, 2) thanks to the flexibility of the calixarene structure, induced-fit processes allow the binding of large, biologically relevant ammonium salts such as neurotransmitters, and 3) the protonation of the basic cap leads to a positively charged receptor, 1.H(+), which is reluctant to host anions and in which host properties are now governed by strong charge-dipole interactions with the guests. In other words, compound 1 presents an acid-base controllable tris-ureido recognition site protected by a hydrophobic corridor that can select guests through induced-fit processes. Thus, its versatile host properties can be allosterically controlled by protonation and selective guest-switching processes are possible. To illustrate all these remarkable features, a sophisticated three-pole supramolecular switch, based on the interconversion of host-guest systems displaying either charged or neutral guests, is described.
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ABSTRACT: The construction of self-assembled receptors based on flexible concave subunits is a challenging task and constitutes an interesting approach to mimic binding processes occurring in biological systems. The receptors studied herein are based on flexible calixarene skeletons bearing three (or more) acid-base functionalities at their narrow rim. When complementary, they self-assemble in a tail-to-tail manner to give a diabolo-like complex, provided that each calixarene subunit hosts a guest. The allosterically-driven multi-recognition pattern is highly selective and leads to stable quaternary adducts. In order to evaluate the scope of this system, various polyamino and polyacidic calixarenes have been studied. It is shown that modifications of the nature of the wide rim substituents do not alter the efficiency of the quaternary self-assembling process, even with the more flexible macrocycles that lack tBu substituents. On the contrary, the replacement of the latter by smaller groups led to receptors with broader scope, as larger guests such as tryptamine and dopamine derivatives were stabilized in the cavities. Implementation of extra-functionalities at the narrow rim were revealed also to be of high interest. Indeed, it is shown that secondary interactions take place between the two calix-subunits when they present additional and complementary functions such as carboxylate and ureido moieties. The ureido arms are also capable of binding the counter anion Cl(-) of the ammonium guest, thus leading to a quinternary neutral complex. Such remarkable behavior is due to the versatility of the calixarene platform, which allows the implementation of a high number of functions, leading to multiple non-covalent attractive interactions, whereas the macrocycle remains flexible, thus allowing induced-fit processes to occur.Organic & Biomolecular Chemistry 02/2011; 9(7):2387-96. · 3.57 Impact Factor
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ABSTRACT: Two novel calixcryptamides bearing a tren-based cap have been synthesized and their host-guest properties have been investigated by (1)H and (19)F NMR spectroscopy. One of them behaves as a remarkable heteroditopic receptor toward either polar neutral guests, anions or contact ion pairs. It has been shown that only F(-) can be encapsulated into the tris-amido cap of this host. Moreover, the fluoride anion acts as an allosteric activator by favoring the inclusion of ammonium ions into the calixarene cavity. The ammonium fluoride salts are bound as contact ion pairs and, remarkably, the calixcryptamide host is reluctant to other ammonium salts. To our knowledge, such an highly cooperative and selective process toward contact ammonium fluoride salts is unique in the literature. Allosteric regulation of all the host-guest systems can also be achieved through protonation of the aza-cap. Indeed, guest release can be triggered by addition of various acids. In comparison to related calixarene-based receptors, all these unique properties are due to the smallness and the higher preorganization of the binding site provided by the convergent hydrogen bond donor groups of the tris-amido cap.Organic & Biomolecular Chemistry 10/2010; 8(20):4607-16. · 3.57 Impact Factor