Hydrogen-Bonded Hexameric Capsules of Resorcin[4]arene, Pyrogallol[4]arene and Octahydroxypyridine[4]arene are Abundant Structures in Organic Solvents: A View from Diffusion NMR

Solution-Phase Magnetic Mechanistic Study of Ni-Seamed Pyrogallol[4]arene Nanocapsules Reveal Presence of Novel Cylindrical and Spherical Species

The magnetic properties of nickel-seamed C-pyrogallol[4]arene (PgC3 Ni) hexamers and dimers are studied for the first time in solution. The combination of small-angle neutron scattering and superconducting quantum interference device magnetometer measurements of the solution species reveal their paramagnetic and weakly antiferromagnetic behaviour. Surprisingly, the magnetic results indicated the presence of an unprecedented 13 Å-radius species, larger than both the dimeric and hexameric nanocapsules with both octahedral and square-planar metal centers. To confirm the presence of this novel species, we performed a mechanistic study of PgC3 Ni as a function of temperature and solvent and deduced the presence of two additional new species: a) an 11 Å cylinder with Ni atoms seaming the tubular framework and b) an 8 Å-radius sphere with non-interacting Ni centers located within the internal cavity. Select parameters that shift the equilibrium towards desired species are also identified.

Anion-Based Self-assembly of Resorcin[4]arenes and Pyrogallol[4]arenes

Spatial sequestration of molecules is a prerequisite for the complexity of biological systems, enabling the occurrence of numerous, often non-compatible chemical reactions and processes in one cell at the same time. Inspired by this compartmentalization concept, chemists design and synthesize artificial nanocontainers (capsules and cages) and use them to mimic the biological complexity and for new applications in recognition, separation, and catalysis. Here, we report the formation of large closed-shell species by interactions of well-known polyphenolic macrocycles with anions. It has been known since many years that C-alkyl resorcin[4]arenes (R4C) and C-alkyl pyrogallol[4]arenes (P4C) narcissistically self-assemble in nonpolar solvents to form hydrogen-bonded capsules. Here, we show a new interaction model that additionally involves anions as interacting partners and leads to even larger capsular species. Diffusion-ordered spectroscopy and titration experiments indicate that the anion-sealed species have a diameter of >26 Å and suggest stoichiometry (M)₆(X^–)₂₄ and tight ion pairing with cations. This self-assembly is effective in a nonpolar environment (THF and benzene but not in chloroform), however, requires initiation by mechanochemistry (dry milling) in the case of non-compatible solubility. Notably, it is common among various polyphenolic macrocycles (M) having diverse geometries and various conformational lability.

Temperature- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) microgels as a carrier for controlled protein adsorption and release

Herein, we report controlled protein adsorption and delivery of thermo- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels at different temperatures, pH values and ionic strengths by employing bovine serum albumin (BSA) as a model protein. For these dual-responsive microgels, we found that the BSA adsorption was driven by several of six competing contributions, viz., physical diffusion (PD), hydrophobic interactions (HI), electrostatic attraction (EA), hydrogen bonding (HB) and temperature or pH-induced seizing action (SA_T or SA_pH), depending on the temperature and pH of the solution. Compared to the pure PNIPAM microgels, the higher swelling degree of the PNIPAM-co-MAA microgels allowed a large amount of BSA loading under any experimental conditions. A largest BSA adsorption of 45.1 μg mg^-1 was achieved at 40 °C and pH 4 due to the presence of all six contributions. The BSA adsorption and delivery could be further tuned by changing the crosslinking density within the microgels. The BSA binding onto the microgels was found to be ionic strength dependent, which could be attributed to the charge shielding of Na⁺ ions, salting out of BSA and aggregate formation of the microgels. The adsorbed BSA could be controllably released by adjusting the temperature and pH of the experiment, and with the help of sodium dodecyl sulphate (SDS) addition so as to eliminate each interaction between BSA and the microgels. Thus, this study can be useful to design a stimuli-responsive microgel-based carrier for controlled release of proteins.

Solution structure of zinc-seamed C-alkylpyrogallol[4]arene dimeric nanocapsules

The structural stability and solution geometry of zinc-seamed-C-propylpyrogallol[4]arene dimers has been studied in solution using in situ neutron scattering and 2D-DOSY NMR methods. In comparison with the structures of the analogous copper-/nickel-seamed dimeric entities, the spherical geometry of the PgC₃Zn species (R = 9.4 Å; diffusion coefficient = 1.05 × 10^-10 m² s^-1) is larger due to the presence of ligands at the periphery in solution. This enhanced radius in solution due to ligation is also consistent with the findings of model molecular dynamics simulations of the zinc-seamed dimers.

Binding Properties and Supramolecular Polymerization of a Water-Soluble Resorcin[4]arene

Controlling the self-assembly of molecules in water is difficult because the small size, polarity, and hydrogen bond donating and accepting properties of water attenuate most non-covalent interactions. Here we describe how resorcinarene 1, with pyridinium pendent groups, assembles in water to form head-to-tail assemblies. These small supramolecular polymers form because they offer greater stabilization than any latent head-to-head assembly of resorcinarenes to form dimeric (or hexameric) containers. Instead, the resorcinarene bowl - particularly if negatively charged - is a good host for the pyridinium pendent groups of a second resorcinarene. Alternatively, resorcinarene 1 is also a good host for complexing anions and cations of any added salt. In combination therefore, host 1 possesses a rich repertoire of supramolecular properties that is dependent on the ionic strength and the nature of salts, pH, and the concentration of the host. These findings provide new information about controlling the self-assembly of resorcinarenes in water.

Mechanically induced pyrogallol[4]arene hexamer assembly in the solid state extends the scope of molecular encapsulation

Ball milling mixtures of pyrogallol[4]arene and guests gives direct access to encapsulation complexes and can be monitored by solid-state NMR.

Pyrogallol[4]arene hexamers are hydrogen-bonded molecular capsules of exceptional kinetic stability that can entrap small molecule guests indefinitely, without exchange, at ambient temperatures. Here, we report on the use of a ball mill to induce self-assembly of the capsule components and the guests in the solid state. Stoichiometric amounts of pyrogallol[4]arene and a guest, which can be an arene, alkane, amine, or carboxylic acid, were milled at 30 Hz for fixed durations, dissolved, and characterization by NMR. Most of the resulting encapsulation complexes were kinetically stable but thermodynamically unstable in solution, and the yield of their formation correlates with the duration of the milling and is related to the structures of guest and host. This method extends the scope of molecular encapsulation, as demonstrated by the preparation of kinetically trapped encapsulation complexes of [2.2]paracyclophane, for which we could find no other method of preparation. To gain mechanistic insights into the solid-state assembly process, we characterized the milled powders using ¹³C CP-MAS NMR, we studied the effects of changing the alkane domain of the host, and we examined how dissolution conditions impact on the distribution of observed encapsulation complexes once in solution. The results support a mechanism comprising mechanically induced solid-state reorganization to produce a mixture rich in nearly or fully assembled guest-filled capsules.

Unique organization of solvent molecules within the hexameric capsules of pyrogallol[4]arene in solution

The hexameric capsules of pyrogallol[4]arene (2b) were prepared in nondeuterated solvents in the absence and presence of adamantane carboxylic acid (3). The small encapsulated molecules were shown to occupy different sites within the same capsule. In the presence of 3, which are also encapsulated in the hexameric capsules, one observes yet another pair of signals for the encapsulated solvent molecules. Different NMR experiments enabled assignment of the different sites within the hexameric capsules of 2b.

Solution structures of nanoassemblies based on pyrogallol[4]arenes

Nanoassemblies of hydrogen-bonded and metal-seamed pyrogallol[4]arenes have been shown to possess novel solution-phase geometries. Further, we have demonstrated that both guest encapsulation and structural rearrangements may be studied by solution-phase techniques such as small-angle neutron scattering (SANS) and diffusion NMR. Application of these techniques to pyrogallol[4]arene-based nanoassemblies has allowed (1) differentiation among spherical, ellipsoidal, toroidal, and tubular structures in solution, (2) determination of factors that control the preferred geometrical shape and size of the nanoassemblies, and (3) detection of small variations in metric dimensions distinguishing similarly and differently shaped nanoassemblies in a given solution. Indeed, we have shown that the solution-phase structure of such nanoassemblies is often quite different from what one would predict based on solid-state studies, a result in disagreement with the frequently made assumption that these assemblies have similar structures in the two phases. We instead have predicted solid-state architectures from solution-phase structures by combining the solution-phase analysis with solid-state magnetic and elemental analyses. Specifically, the iron-seamed C-methylpyrogallol[4]arene nanoassembly was found to be tubular in solution and predicted to be tubular in the solid state, but it was found to undergo a rearrangement from a tubular to spherical geometry in solution as a function of base concentration. The absence of metal within a tubular framework affects its stability in both solution and the solid state; however, this instability is not necessarily characteristic of hydrogen-bonded capsular entities. Even metal seaming of the capsules does not guarantee similar solid-state and solution-phase architectures. The rugby ball-shaped gallium-seamed C-butylpyrogallol[4]arene hexamer becomes toroidal on dissolution, as does the spherically shaped gallium/zinc-seamed C-butylpyrogallol[4]arene hexamer. However, the arenes are arranged differently in the two toroids, a variation that accounts for the differences in their sizes and guest encapsulation. Guest encapsulation of biotemplates, such as insulin, has demonstrated the feasibility of synthesizing nanocapsules with a volume three times that of a hexamer. The solution-phase studies have also demonstrated that the self-assembly of dimers versus hexamers can be controlled by the choice of metal, solvent, and temperature. Controlling the size of the host, nature of the metal, and identity of the guest will allow construction of targeted host-guest assemblies having potential uses as drug delivery agents, nanoscale reaction vessels, and radioimaging/radiotherapy agents. Overall, the present series of solid- and solution-phase studies has begun to pave the way toward a more complete understanding of the properties and behavior of complex supramolecular nanoassemblies.

Investigating structural alterations in pyrogallol[4]arene-pyrene nanotubular frameworks

Small-angle neutron scattering (SANS) and diffusion NMR studies are performed to investigate the stability and geometry of hydrogen-bonded pyrene-guest-containing C-hexylpyrogallol[4]arene (PgC(6) -pyrene) nanotubular frameworks in solution. In the solid state, hydrogen-bonded pyrogallol[4]arene tubes are formed; however, the scattering data for PgC(6) -pyrene assemblies in acetone are best modeled as dimeric spheres of PgC(6) with no pyrene guest. The result of diffusion NMR study also indicates the rearrangement of tubular entity into spherical framework in acetone. This is the first example of structural transformation of pyrogallol[4]arene nanotubes (guest-exo) in solution. Individual hydrogen-bonded spheres of PgC(6) exhibits a uniform radius of ca. 8.6 Å and a diffusion coefficient of 9.12 × 10(-10) m(2) s(-1) in acetone. The diffusion NMR measurements further gave, for the first time, insights into how the type of solvent (acetone vs. methanol vs. acetontitrile/D(2) O) governs the structural differences in these nanoassemblies. Solution-phase structural alteration observed for PgC(6) -pyrene gives evidence of enhanced stability of pyrogallol[4]arene nanocapsules over nanotubes.

Recent advances in hydrogen-bonded hexameric encapsulation complexes

Basic research in the chemistry of hexameric resorcin[4]arenes and pyrogallol[4]arenes during the last decade is reviewed. Applications of NMR methods to determine solution structures, host guest properties and exchange dynamics are discussed. The scientific issue is the behavior of molecules in small spaces; the challenge is to translate this information to practical applications in, say, catalysis or transport.

The inherent structural instability: concentration-dependent transformation of pyrogallarene to pyrogallarene lactones

Pyrogallarene shows concentration-dependent instability in dilute solutions resulting in elimination of two ketene molecules and formation of pyrogallarene lactones. This unexpected phenomenon, which is not observed with resorcinarenes, highlights the significance of the four hydroxyl groups at 2-position for the molecular characteristics of pyrogallarenes.