Supramolecular Chemistry in Water

High affinity crown ether complexes in water: thermodynamic analysis, evidence of crystallography and binding of NAD+

Improving traditional crown ether to the water-soluble and high binding ability host molecule is critical to our efforts to model or mimic biological supramolecular systems. In this paper, we converted two traditional crown ethers, 1,5-dinaphtho-32-crown-8 and 1,5-dinaphtho-38-crown-10, into the water-soluble tetrasulfonated 1,5-dinaphtho-32-crown-8 and tetrasulfonated 1,5-dinaphtho-38-crown-10, evaluated their complexation with three dicationic bipyridiniums in aqueous solution by microcalorimetric titration, UV-vis, and NMR experiments, and then determined the crystal structures of three tetrasulfonatocrown ether-bipyridinium complexes. The equilibrium association constants of tetrasulfonated 1,5-dinaphtho-32-crown-8 with these bipyridiniums reach up to 10(7) M(-1), while those of tetrasulfonated 1,5-dinaphtho-38-crown-10 are just in the range of 10(5) M(-1) order of magnitude. The thermodynamic data obtained show that the complexation of two tetrasulfonatocrown ethers with dicationic bipyridiniums is absolutely enthalpy-driven in water with an accompanying little entropic gain, and each monocationic pyridinium moiety in guest molecules can provide about -10 to -15 kJ·mol(-1) enthalpy contribution irrespective of the size of ether crowns. Moreover, we also investigated the recognition capability of the two water-soluble crown ethers with NAD(+) and NADH by microcalorimetric titration and NMR experiments, indicating that tetrasulfonated 1,5-dinaphtho-32-crown-8 shows exclusive selectivity to NAD(+). The water-solubility and high affinity of this system as well as the flexible and non-preorganized characteristic of these crown ethers make it suitable to serve as a model for mimicking biological systems.

Molecular hydrogels from bolaform amino acid derivatives: a structure-properties study based on the thermodynamics of gel solubilization

Insight is provided into the aggregation thermodynamics associated to hydrogel formation by molecular gelators derived from L-valine and L-isoleucine. Solubility data from NMR measurements are used to extract thermodynamic parameters for the aggregation in water. It is concluded that at room temperature and up to 55 °C, these systems form self-assembled fibrillar networks in water with quite low or zero enthalpic component, whereas the entropy of the aggregation is favorable. These results are explained by considering that the hydrophobic effect is dominant in the self-assembly. However, studies by NMR and IR spectroscopy reveal that intermolecular hydrogen bonding is also a key issue in the aggregation process of these molecules in water. The low enthalpy values measured for the self-assembly process are ascribed to the result of a compensation of the favorable intermolecular hydrogen-bond formation and the unfavorable enthalpy component of the hydrophobic effect. Additionally, it is shown that by using the hydrophobic character as a design parameter, enthalpy-controlled hydrogel formation, as opposed to entropy-controlled hydrogel formation, can be achieved in water if the gelator is polar enough. It is noteworthy that these two types of hydrogels, enthalpy-versus entropy-driven hydrogels, present quite different response to temperature changes in properties such as the minimum gelator concentration (mgc) or the rheological moduli. Finally, the presence of a polymorphic transition in a hydrogel upon heating above 70 °C is reported and ascribed to the weakening of the hydrophobic effect upon heating. The new soft polymorphic materials present dramatically different solubility and rheological properties. Altogether these results are aimed to contribute to the rational design of molecular hydrogelators, which could be used for the tailored preparation of this type of soft materials. The reported results could also provide ground for the rationale of different self-assembly processes in aqueous media.

Molecular recognition of hydrocarbon guests by a supramolecular capsule formed by the 4:4 self-assembly of tris(Zn(2+)-cyclen) and trithiocyanurate in aqueous solution

We have previously reported that the trimeric Zn(2+)-cyclen complex (tris(Zn(2+)-cyclen), [Zn(3)L(1)](6+)) and the trianion of trithiocyanuric acid (TCA(3-)) assembled in a 4:4 ratio to form a cuboctahedral supramolecular cage, [(Zn(3)L(1))(4)(TCA(3-))(4)](12+) (hereafter referred to as a Zn-cage), in neutral aqueous solution (cyclen=1,4,7,10-tetraazacyclododecane). Herein, we examined the molecular recognition of C(1)-C(12) hydrocarbons (C(n)H((2n+2)) (n≈1-12)), cyclopentane, cyclododecane, cis-decalin, and trans-decalin by the Zn-cage under normal atmospheric pressure. This cage complex was also able to encapsulate guest molecules that had larger volumes than that of the inner cavity of the Zn-cage, thereby suggesting that the inner shape of the Zn-cage was flexible. Computational simulations of Zn-cage-guest complexes provided support for this conclusion. Moreover, the solvent-accessible surface areas (SASA) of the Zn-cage host, guest molecules, and the Zn-cage-guest complexes were calculated and the data were used to explain the order of stability determined by the guest-replacement experiments. The storage of volatile molecules in aqueous solution by the Zn-cage is also discussed.

Photoswitchable gel assembly based on molecular recognition

The formation of effective and precise linkages in bottom-up or top-down processes is important for the development of self-assembled materials. Self-assembly through molecular recognition events is a powerful tool for producing functionalized materials. Photoresponsive molecular recognition systems can permit the creation of photoregulated self-assembled macroscopic objects. Here we demonstrate that macroscopic gel assembly can be highly regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules. A photoregulated gel assembly system is developed using polyacrylamide-based hydrogels functionalized with azobenzene (guest) or cyclodextrin (host) moieties. Reversible adhesion and dissociation of the host gel from the guest gel may be controlled by photoirradiation. The differential affinities of α-cyclodextrin or β-cyclodextrin for the trans-azobenzene and cis-azobenzene are employed in the construction of a photoswitchable gel assembly system.

Self-assembly through molecular recognition events is used in the production of functionalized materials. This study shows that macroscopic gel assembly can be regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules.

Capsular complexes of nonpolar guests with octa amine host detected in the gas phase

Nanocapsules, made up of the deep cavitand octa amine and several guests, were prepared in aqueous acidic solution and were found to be stable in the gas phase as detected by electrospray ionization mass spectrometry (ESI-MS). The observed gas phase host-guest complexes contained five positive charges and were associated with several acid molecules (HCl or HBr).

Adaptive supramolecular nanomaterials based on strong noncovalent interactions

Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.

Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.

A simple ionic triphenylene receptor for catecholamines, serotonin and D-glucosamine in buffered water

The combination of hydrophobic effects and ionic pairing within a triphenylene-based receptor were exploited for the binding of biological phenylethylamines, serotonin and D-glucosamine in phosphate buffered water.

Determinants of cyanuric acid and melamine assembly in water

While the recognition of cyanuric acid (CA) by melamine (M) and their derivatives has been known to occur in both water and organic solvents for some time, analysis of CA/M assembly in water has not been reported (Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. J. Am. Chem. Soc.1999, 121, 1752-1753; Mathias, J. P.; Simanek, E. E.; Seto, C. T.; Whitesides, G. M. Macromol. Symp.1994, 77, 157-166; Zerkowski, J. A.; MacDonald, J. C.; Seto, C. T.; Wierda, D. A.; Whitesides, G. M. J. Am. Chem. Soc.1994, 116, 2382-2391; Mathias, J. P.; Seto, C. T.; Whitesides, G. M. Polym. Prepr.1993, 34, 92-93; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1993, 115, 905-916; Zerkowski, J. A.; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1992, 114, 5473-5475; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1990, 112, 6409-6411; Wang, Y.; Wei, B.; Wang, Q. J. Chem. Cryst.1990, 20, 79-84; ten Cate, M. G. J.; Huskens, J.; Crego-Calama, M.; Reinhoudt, D. N. Chem.-Eur. J.2004, 10, 3632-3639). We have examined assembly of CA/M, as well as assembly of soluble trivalent CA and M derivatives (TCA/TM), in aqueous solvent, using a combination of solution phase NMR, isothermal titration and differential scanning calorimetry (ITC/DSC), cryo-transmission electron microscopy (cryo-TEM), and synthetic chemistry. While the parent heterocycles coprecipitate in water, the trivalent system displays more controlled and cooperative assembly that occurs at lower concentrations than the parent and yields a stable nanoparticle suspension. The assembly of both parent and trivalent systems is rigorously 1:1 and proceeds as an exothermic, proton-transfer coupled process in neutral pH water. Though CA and M are considered canonical hydrogen-bonding motifs in organic solvents, we find that their assembly in water is driven in large part by enthalpically favorable surface-area burial, similar to what is observed with nucleic acid recognition. There are currently few synthetic systems capable of robust molecular recognition in water that do not rely on native recognition motifs, possibly due to an incomplete understanding of recognition processes in water. This study establishes a detailed conceptual framework for considering CA/M heterocycle recognition in water which enables the future design of molecular recognition systems that function in water.

Unexpected stable dimerisation of an anionic imidopyrrolecarboxylate in polar solution

The imidopyrrolecarboxylate 3(-) unexpectedly forms stable dimers (K(ass) = 130 M(-1) in CHCl(3)/DMSO, 1 : 1, v/v) despite the fact that two anions have to interact. The dimer is more stable than an analogous neutral amidopyrrolecarboxylic acid dimer (K(ass) < 10 M(-1)) underlining the importance of charged H-bonds compared to neutral ones.