New synthetic strategies for xanthene-dye-appended cyclodextrins

Differential interaction of Fluorescein-β-cyclodextrin conjugate to quadruplex kit22 DNA: Inclusion of Berberine and modulation of binding

Clinical applicability of G-quadruplexes as anticancer drugs is an area of current interest. Identification of supramolecular systems for selective targeting G-quartets is particularly intriguing. In this work, the DNA binder Berberine is encapsulated inside the molecular cavity of the synthesised host structure, Fluoresecein-β-cyclodextrin conjugate. The host: guest complex is characterized and the mode of binding is optimized using two dimensional rotating-frame Overhauser effect spectroscopy. The conjugate is examined for its binding to quadruplex DNAs viz., kit22, myc22, telo24 and the duplex calf-thymus DNA before and after Berberine encapsulation. UV-vis and fluorescence spectroscopic methods were employed to determine the strength of binding of the complex with the DNAs. The binding strength and the stoichiometry of the host: guest complex are 1.9 × 10⁶ mol^-1 dm³ and 1:1, respectively. A quenching of fluorescence of the quadruplex kit22 and duplex ctDNA is observed on binding to the Fluorescein-β-cyclodextrin conjugate. The quadruplexes of myc22 and telo24 display an enhanced fluorescence on binding to the modified cyclodextrin. The Stern-Volmer quenching constants are 1.4 × 10⁶ mol^-1 dm³ and 3.8 × 10⁵ mol^-1 dm³ for binding to kit22 and ctDNA respectively. kit22 shows a different emission profile on interacting with the Berberine encapsulated conjugate, whereas all the other quadruplexes and duplex exhibit similar emission profiles. The results indicate a variation in the binding mode and strength of the ligand-quadruplexes and depend on the conformation of the quadruplexes.Communicated by Ramaswamy H. Sarma.

A Self-locked β-Cyclodextrin-rhodamine B Spirolactam with Photoswitching Properties

Supramolecular organization and self-assembly are the pillars of functionality of many nanosystems. The covalent conjugate (6-spirolactam rhodamine B-6-monodeoxy)-β-cyclodextrin (Rho-βCD) is assembled as a self-included, rigid nanostructure, identical in the crystal and in aqueous solution, as revealed by detailed X-ray and NMR analyses. Rho-βCD self-assembly is the result of an interesting reaction pathway, which partially de-aggregates Rho and disturbs the zwitterion↔spirolactone equilibrium. Rho-βCD is stable at pH 4.6, but displays controllable photoswitching between the colored, fluorescent, zwitterionic and the colorless, non-fluorescent closed structures, during several iterative cycles. After an initial drop in absorbance, the on-off process continues without further changes under our irradiation conditions, a consequence of the specific self-locked arrangement of Rho in the cavity. Rho-βCD exemplifies a water soluble photoresponsive nanosystem with improved photostability suggesting promising applications in super resolution bioimaging.

Mono-6-Substituted Cyclodextrins-Synthesis and Applications

Cyclodextrins are well known supramolecular hosts used in a wide range of applications. Monosubstitution of native cyclodextrins in the position C-6 of a glucose unit represents the simplest method how to achieve covalent binding of a well-defined host unit into the more complicated systems. These derivatives are relatively easy to prepare; that is why the number of publications describing their preparations exceeds 1400, and the reported synthetic methods are often very similar. Nevertheless, it might be very demanding to decide which of the published methods is the best one for the intended purpose. In the review, we aim to present only the most useful and well-described methods for preparing different types of mono-6-substituted derivatives. We also discuss the common problems encountered during their syntheses and suggest their optimal solutions.

Photocross-Linked Antimicrobial Amino-Siloxane Elastomers

Mechanically robust bulk antimicrobial polymers are one way to address disease transmission via contaminated surfaces. Here, we demonstrate the visible light photo-oxidative cross-linking of amine-containing PDMS using a single-component, solvent-free system where amines have a dual role as antimicrobial functionalities and cross-linking sites. Rose Bengal, a xanthene dye used as a fluorescent stain, is thermally reacted with the polymer to give a solvent-free liquid siloxane that can generate reactive singlet oxygen upon aerobic green light irradiation, coupling the amine functionalities into imine cross-links. Photorheological experiments demonstrate that light intensity is the largest kinetic factor in the photo-oxidative curing of these polymers. Room temperature irradiation under an ambient atmosphere results in free-standing elastic materials with mechanical properties that depend on the amount of Rose Bengal present. An ultimate elongation strain of 117% and Young's modulus of 2.15 MPa were observed for the highest dye loading, with both mechanical properties found to be higher than those for the same solution-based dye amounts. We demonstrate that the solvent-free nature of the material can be exploited to generate 3D structures using low-temperature deposition as well as direct-write patterning and photolithography on glass substrates. The antimicrobial activity was investigated, with the cross-linked material demonstrating greater efficacy against E. coli (Gram negative) compared with MRSA (Gram positive) bacterial strains and inducing complete cell lysis of incubated CHO-K1 mammalian cells, demonstrating applicability as a mechanically robust single-component antimicrobial elastomer.

Enzymatic rhamnosylation of anticancer drugs by an α-L-rhamnosidase from Alternaria sp. L1 for cancer-targeting and enzyme-activated prodrug therapy

The synthesis of rhamnosylated compounds has gained great importance since these compounds have potential therapeutic applications. The enzymatic approaches for glycosylation of bioactive molecules have been well developed; however, the enzymatic rhamnosylation has been largely hindered by lacking of the glycosyl donor for rhamnosyltransferases. Here, we employed an α-L-rhamnosidase from Alternaria sp. L1 (RhaL1) to perform one-step rhamnosylation of anticancer drugs, including 2'-deoxy-5-fluorouridine (FUDR), cytosine arabinoside (Ara C), and hydroxyurea (Hydrea). The key synthesis conditions including substrate concentrations and reaction time were carefully optimized, and the maximum yields of each rhamnosylated drugs were 57.7 mmol for rhamnosylated Ara C, 68.6 mmol for rhamnosylated Hydrea, and 42.2 mmol for rhamnosylated FUDR. It is worth pointing out that these rhamnosylated drugs exhibit little cytotoxic effects on cancer cells, but could efficiently restore cytotoxic activity when incubated with exogenous α-L-rhamnosidase, suggesting their potential applications in the enzyme-activated prodrug system. To evaluate the cancer-targeting ability of rhamnose moiety, the rhamnose-conjugated fluorescence dye rhodamine B (Rha-RhB) was constructed. The fluorescence probe Rha-RhB displayed much higher cell affinity and cellular internalization rate of oral cancer cell KB and breast cancer cell MDA-MB-231 than that of the normal epithelial cells MCF 10A, suggesting that the rhamnose moiety could mediate the specific internalization of rhamnosylated compounds into cancer cells, which greatly facilitated their applications for cancer-targeting drug delivery.

Homo- and hetero-difunctionalized β-cyclodextrins: Short direct synthesis in gram scale and analysis of regiochemistry

The regioselective difunctionalization of cyclodextrins (CDs) leading to derivatives amenable to further transformations is a daunting task due to challenging purification and unambiguous characterization of the obtained regioisomers with similar physicochemical properties. The primary-side homo-difunctionalization of β-CD can lead to three regioisomers, while the hetero-difunctionalization can generate three pairs of pseudoenantiomers. Previously, approaches with several synthetic steps, expensive reagents, high purification demands and low yields of the products have been employed. Herein we present direct, short and efficient primary-side difunctionalization strategies featuring reproducibility, ease of product purification, scalability of the reactions and versatility of the substituents introduced. Specifically, the prepared ditosylated β-CDs were separated using preparative reversed-phase column chromatography and their structures were elucidated by NMR experiments. Azidation led to the corresponding pure diazido regioisomers. Direct monotosylation of 6-monoazido-β-CD or monoazidation of the single regioisomers 6^A,6^X-ditosyl-β-CDs afforded hetero-difunctionalized 6^A-monoazido-6^X-tosyl-β-CDs in significant yields. Overall, the single regioisomers, 6^A,6^X-ditosyl-, 6^A,6^X-diazido- and 6^A-monoazido-6^X-monotosyl-β-CD were prepared in one or two steps and purified in multigram scale thus opening the way towards further selective and orthogonal functionalizations of β-CD hosts.

Novel β-cyclodextrin-eosin conjugates

Eosin B (EoB) and eosin Y (EoY), two xanthene dye derivatives with photosensitizing ability were prepared in high purity through an improved synthetic route. The dyes were grafted to a 6-monoamino-β-cyclodextrin scaffold under mild reaction conditions through a stable amide linkage using the coupling agent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride. The molecular conjugates, well soluble in aqueous medium, were extensively characterized by 1D and 2D NMR spectroscopy and mass spectrometry. Preliminary spectroscopic investigations showed that the β-cyclodextrin–EoY conjugate retains both the fluorescence properties and the capability to photogenerate singlet oxygen of the unbound chromophore. In contrast, the corresponding β-cyclodextrin–EoB conjugate did not show either relevant emission or photosensitizing activity probably due to aggregation in aqueous medium, which precludes any response to light excitation.