Cyclodextrine als Bausteine supramolekularer Strukturen und Funktionseinheiten

Water-soluble Self-assembled {Pd84 }Ac Polyoxopalladate Nano-wheel as a Supramolecular Host

Polyoxopalladates (POPs) are a class of self-assembling palladium-oxide clusters that span a variety of sizes, shapes and compositions. The largest of this family, {Pd₈₄ }^Ac , is constructed from 14 building units of {Pd₆ } and lined on the inner and outer torus by 28 acetate ligands. Due to its high water solubility, large hydrophobic cavity and distinct ¹ H NMR fingerprint {Pd₈₄ }^Ac is an ideal molecule for exploring supramolecular behaviour with small organic molecules in aqueous media. Molecular visualisation studies highlighted potential binding sites between {Pd₈₄ }^Ac and these species. Nuclear Magnetic Resonance (NMR) techniques, including ¹ H NMR, ¹ H Diffusion Ordered Spectroscopy (DOSY) and Nuclear Overhauser Spectroscopy (NOESY), were employed to study the supramolecular chemistry of this system. Here, we provide conclusive evidence that {Pd₈₄ }^Ac forms a 1 : 7 host-guest complex with benzyl viologen (BV²⁺ ) in aqueous solution.

A Pseudorotaxane System Containing γ-Cyclodextrin Formed via Chiral Recognition with an AuI 6 AgI 3 CuII 3 Molecular Cap

Solvent-mediated crystal-to-crystal transformations of [Au₆ Ag₃ Cu₃ (H₂ O)₃ (d-pen)₆ (tdme)₂ ]³⁺ (d-[1(H₂ O)₃ ]³⁺ ; pen^2- =penicillaminate, tdme=1,1,1-tris(diphenylphosphinomethyl)ethane) to form unique supramolecular species are reported. Soaking crystals of d-[1(H₂ O)₃ ]³⁺ in aqueous Na₂ bdc (bdc^2- =1,4-benzenedicarboxylate) yielded crystals containing d-[1(bdc)(H₂ O)₂ ]⁺ due to the replacement of a terminal aqua ligand in d-[1(H₂ O)₃ ]³⁺ by a monodentate bdc^2- ligand. When γ-cyclodextrin (γ-CD) was added to aqueous Na₂ bdc, d-[1(H₂ O)₃ ]³⁺ was transformed to d-[1(bdc@γ-CD)(H₂ O)₂ ]⁺ , where a γ-CD ring was threaded by a bdc^2- molecule to construct a pseudorotaxane structure. While the use of dicarboxylates with an aliphatic carbon chain instead of bdc^2- afforded analogous pseudorotaxanes, such pseudorotaxane species were not formed when crystals of [Au₆ Ag₃ Cu₃ (H₂ O)₃ (l-pen)₆ (tdme)₂ ]³⁺ (l-[1(H₂ O)₃ ]³⁺ ) enantiomeric to d-[1(H₂ O)₃ ]³⁺ were soaked in aqueous Na₂ bdc and γ-CD, affording only crystals containing l-[1(bdc)(H₂ O)₂ ]⁺ .

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings*

Poly(ethylene oxide)s (PEOs) are useful polymers with good water solubility, biological compatibility, and commercial availability. PEOs with various end groups were threaded into pillar[5]arene rings in a mixture of water and methanol to afford pseudopolyrotaxanes. Corresponding polyrotaxanes were also constructed by capping COOH-terminated pseudopolyrotaxanes with bulky amines, in which multiple hydrogen bonds involving the pillar[5]arene OH groups were critically important to prevent dethreading. The number of threaded ring components could be rationally controlled in these materials, providing a simple and versatile method to tune the mechanical and thermal properties. Specifically, a polyrotaxane with a high-molecular-weight axle became elastic upon heating above the melting point of PEOs and exhibited temperature-dependent shape memory property because of the topological confinement and crosslinked hydrogen bonds.

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Control of Guest Inclusion and Chiral Recognition Ability of 6-O-Modified β-Cyclodextrins in Organic Solvents by Aromatic Substituents at the 2-O Position

The host-guest chemistry and applications of cyclodextrins in aqueous media is well established. However, a comprehensive study in organic solvents is lacking. Here, we report the design and synthesis of 6-O-tert-butyldimethylsilylated β-cyclodextrin (TBDMS-β-CD) bearing various aromatic substitutions at the 2-O position and their inclusion complex formation with aromatic guests in nonpolar organic solvents. Compared to the parent TBDMS-β-CD, these derivatives exhibit at least a 10-fold increase in inclusion ability toward pyrene through cooperative guest binding with the CD cavity and the aromatic substituents at the 2-O position. The type of the aromatic substituent largely affects the chiral recognition ability of TBDMS-β-CD toward 1-(1-naphthyl)ethylamine in cyclohexane. A TBDMS-β-CD derivative with a p-tolyl substituent has a remarkable chiral selectivity for the (S)-1-(1-naphthyl)ethylamine over the corresponding (R)-isomer (K_S /K_R =4.1±0.5), whereas a TBDMS-β-CD derivative with a 2-picolyl substituent shows the inverse chiral selectivity (K_R /K_S =8.7±0.6).

Water-Soluble Photoinitiators in Biomedical Applications

Light-initiated polymerization processes are currently an important tool in various industrial fields. The advancement of technology has resulted in the use of photopolymerization in various biomedical applications, such as the production of 3D hydrogel structures, the encapsulation of cells, and in drug delivery systems. The use of photopolymerization processes requires an appropriate initiating system that, in biomedical applications, must meet additional criteria such as high water solubility, non-toxicity to cells, and compatibility with visible low-power light sources. This article is a literature review on those compounds that act as photoinitiators of photopolymerization processes in biomedical applications. The division of initiators according to the method of photoinitiation was described and the related mechanisms were discussed. Examples from each group of photoinitiators are presented, and their benefits, limitations, and applications are outlined.

Effective Guest Inclusion by a 6-O-Modified β-Cyclodextrin Dimer in Organic Solvents

A 6-O-tert-butyldimethylsilylated β-cyclodextrin (TBDMS-β-CD) dimer, in which two TBDMS-β-CD rings are connected in a head-to-head fashion by a m-xylylene linker, effectively forms inclusion complexes with pyrene and naphthalene in nonpolar organic solvents such as cyclohexane and benzene. This TBDMS-β-CD dimer shows a higher inclusion ability toward these guests than a TBDMS-β-CD dimer bearing a p-xylylene linker due to the greater cooperation of the two TBDMS-β-CD rings for the guest inclusion. Unlike the corresponding monomer, the TBDMS-β-CD dimer bearing a m-xylylene linker is also a good host even in polar organic solvents such as tetrahydrofuran. High chiral recognition of aromatic amines and alcohol is realized by utilizing inclusion within the cavity of the TBDMS-β-CD dimer in cyclohexane. In particular, an extremely high binding selectivity for (S)-1-(1-naphthyl)ethylamine and (S)-1-(1-naphthyl)ethanol over the corresponding (R)-isomers is achieved. Moreover, by utilizing the high chiral recognition with the TBDMS-β-CD dimer in cyclohexane, a non-enzymatic kinetic resolution of racemic 1-(1-naphthyl)ethylamine via enantioselective N-benzoylation is attained with an enantiomer excess of up to 87 % and an s-factor of 15.

Chlorhexidine Loaded Cyclodextrin Containing PMMA Nanogels as Antimicrobial Coating and Delivery Systems

Antimicrobial nanogels, aggregates, and films are prepared by complexation of the antiseptic and bacteriostatic agent chlorhexidine (CHX) for medical and dental applications. A series of α-, β-, and γ-cyclodextrin methacrylate (CD-MA) containing hydrophobic poly(methyl methacrylate) (PMMA) based nanogels are loaded quantitatively with CHX in aqueous dispersion. The results show that CHX is enhancedly complexed by the use of CD-MA domains in the particles structure. β-CD-MA nanogels present the highest uptake of CHX. Furthermore, it is observed that the uptake of CHX in nanogels is influenced by the hydrophobic PMMA structure. CHX acts as external cross-linker of nanogels by formation of 1:2 (CHX:CD-MA) inclusion complexes of two β-CD-MA units on the surfaces of two different nanogels. The nanogels adsorb easily onto glass surfaces by physical self-bonding and formation of a dense crosslinked nanogel film. Biological tests of the applied CHX nanogels with regard to antimicrobial efficiency are successfully performed against Staphylococcus aureus.

End group functionalization of poly(ethylene glycol) with phenolphthalein: towards star-shaped polymers based on supramolecular interactions

The synthesis of a new phenolphthalein azide derivative, which can be easily utilized in polymer analogous reactions, is presented. The subsequent cycloaddition reaction with propargyl-functionalized methoxypoly(ethylene glycol) yielded polymers bearing phenolphthalein as the covalently attached end group. In presence of per-β-cyclodextrin-dipentaerythritol, the formation of stable inclusion complexes was observed, representing an interesting approach towards the formation of star shaped polymers. The decolorization of a basic polymer solution caused by the complexation was of great advantage since this behavior enabled following the complex formation by UV-vis spectroscopy and even the naked eye.

Tenside-free preparation of nanogels with high functional β-cyclodextrin content

We present the preparation of ultrafine (R(h), 50 -150 nm) nanogels through tenside-free condensation of reactive prepolymers with β-cyclodextrin (β-CD) in water. These nanogels possess a maximum content of 60 wt % functional β-CD that can form inclusion complexes as demonstrated by dye sorption with phenolphthalein. Aside of this extremely high uptake capacity to hydrophobic molecules, the nanogels also show good adhesion to surfaces in homogeneous distribution with size of R(h) of 25 nm under dry conditions.

Influence of cyclodextrin on the solubility of a classically prepared 2-vinylcyclopropane macromonomer in aqueous solution

A macromonomer 5 consisting of a polymerizable vinylcyclopropane end group and a poly(N-isopropylacrylamide) (poly(NiPAAm)) chain was obtained from amidation of 1-ethoxycarbonyl-2-vinylcyclopropane-1-carboxylic acid (4) with an amino-terminated poly(NiPAAm) 3 as an example. This macromonomer 5 showed an LCST effect after complexation of the vinyl end group with ß-cyclodextrin in water. Via radical ring-opening copolymerization of 5 and NiPAAm a graft copolymer 8 with a clouding point of 32 °C was synthesized. The branched unsaturated polymer was treated with ozone to cleave the double bonds of the main chain.

Increasing Solubility of Poorly Water Soluble Drug Resveratrol by Surfactants and Cyclodextrins

Resveratrol (Res) is a polyphenolic secondary natural substance and can be used as anti-inflammatory, anti-aging, anti-heart disease and anti-cancer agent. However, Res has low water solubility which causes a low bioavailability in human body. In order to increase solubility we have prepared different inclusion complexes with native ((α, β, γ) and modified (2-hydroxypropyl-beta, dimethyl-beta) cyclodextrins (Cd) as well as solid dispersions using several surfactants (Imwitor 742, Imwitor 928, PEG 6000, Span 40, Span 60, Solutol HS15) with Res. The solubility of all mixtures was determined by UV-VIS spectroscopy at the wavelength of 305 nm. Regarding the Cd, the complex of Res with 2-hydroxypropyl-beta-Cd at a ratio of 1:3 showed the highest water solubility (248210 g/l). Concerning the surfactants, the solid dispersion from Res and Solutol HS15 showed the highest water solubility (16140 g/l). The complexation and the solid dispersion phase were confirmed by DSC. The results will be a basis for better formulation of resveratrol with higher bioavailability.

Solubilization of polycyclic aromatics in water by γ-cyclodextrin derivatives

A series of hydrophilic per-6-thio-6-deoxy-γ-cyclodextrins (CDs) were synthesized from per-6-iodo-6-deoxy-γ-CD. These new hosts are able to solubilize polycyclic aromatic guests in aqueous solution to much higher extents than native CDs. Phase-solubility diagrams were mostly linear in accordance with both 1:1 and 1:2 CD-guest complexes in aqueous solution. The stoichiometry of the inclusion complexes was further investigated by fluorescence spectroscopy, which revealed very pronounced Stokes shifts typical for 1:2 complexes. This finding was further consolidated by quantum mechanical calculations of dimer formation of the guests and space-filling considerations by using the cross-sectional areas of the CDs and guests. The calculated dimerization energies correlated well with the binding free energies measured for the 1:2 complexes, and provided the main contribution to the driving force of complexation in the γ-CD cavity.

Electrospray ionization mass spectrometry studies of cyclodextrin-carboxylate ion inclusion complexes

Aqueous solutions containing simple model aliphatic and alicyclic carboxylic acids (surrogates 1-4) were studied using negative ion electrospray mass spectrometry (ESI-MS) in the presence and absence of alpha-, beta-, and gamma-cyclodextrin. Molecular ions were detected corresponding to the parent carboxylic acids and complexed forms of the carboxylic acids; the latter corresponding to non-covalent inclusion complexes formed between carboxylic acid and cyclodextrin compounds (e.g., beta-CD, alpha-CD, and gamma-CD). The formation of 1:1 non-covalent inclusion cyclodextrin-carboxylic complexes and non-inclusion forms of the cellobiose-carboxylic acid compounds was also observed.Aqueous solutions of Syncrude-derived mixtures of aliphatic and alicyclic carboxylic acids (i.e. naphthenic acids; NAs) were similarly studied using ESI-MS, as outlined above. Molecular ions corresponding to the formation of CD-NAs inclusion complexes were observed whereas 1:1 non-inclusion forms of the cellobiose-NAs complexes were not detected. The ESI-MS results provide evidence for some measure of inclusion selectivity according to the 'size-fit' of the host and guest molecules (according to carbon number) and the hydrogen deficiency (z-series) of the naphthenic acid compounds. The relative abundances of the molecular ions of the CD-carboxylate anion adducts provide strong support for differing complex stability in aqueous solution. In general, the 1:1 complex stability according to hydrogen deficiency (z-series) of naphthenic acids may be attributed to the nature of the cavity size of the cyclodextrin host compounds and the relative lipophilicity of the guest.