Extraordinary Control of Photosensitized Singlet Oxygen Generation by Acyclic Cucurbituril-like Containers

Supramolecular control of singlet oxygen generation is incredibly valuable for several fields with broad applications and thus still challenging. However, macrocyclic inclusion complexes inherently restrict the interaction of photosensitizers with surrounding oxygen in the media. To circumvent this issue, we turned our attention in this work to acyclic cucurbituril-like containers and uncover their properties as supramolecular hosts for photosensitizers with extraordinary control of their photophysics, including singlet oxygen generation. Thermodynamic and photophysical studies were carried out showing that these acyclic containers compare very favorably to benchmark macrocycles such as cucurbiturils and cyclodextrins in terms of their binding affinities and supramolecular control of singlet oxygen generation. Acyclic container with terminal naphthalene walls offers a similar cavity to cucurbit[7]uril and the same carbonyl-lined portals for a tight binding of phenothiazinium dye methylene blue and stabilizing its singlet and triplet excited states. Thus, generation of singlet oxygen for this container is higher than for other macrocycles and even higher than the free photosensitizer. While the acyclic container with smaller terminal benzene walls, stacks over the dye through sulfur-π and π-π interactions deactivating the singlet and triplet excited states, thus showing the lowest generation of singlet oxygen out of all of the studied systems. Due to the great water solubility and biocompatibility of these systems, they possess great potential for novel applications in photocatalysis, synthesis, and biomedical fields, among others.

Binding of toluidine blue-myristic acid derivative to cucurbit[7]uril and human serum albumin: computational and biophysical insights towards a biosupramolecular assembly

A new toluidine blue-myristic acid photosensitizer derivate (TBOMyr) was investigated as a design molecule to bind simultaneously to cucurbit[7]uril (CB[7]) and human serum albumin (HSA) with the aim of constructing a biosupramolecular assembly. Molecular docking and dynamics calculations revealed the main supramolecular and bio-molecular interactions of TBOMyr with the macrocycle or the protein, respectively. The addition of the negatively charged myristic acid-like tail resulted in a unique conformation of the CB[7] complex where the phenothiazine core was included in the cavity of CB[7], leaving the fatty acid portion free to interact with the protein. A favorable ternary interaction between TBOMyr, CB[7] and HSA was suggested by the calculations, and an experimental binding affinity in the order of 10⁵ M^-1 was determined for the TBOMyr@CB[7] complex with HSA. The new TBOMyr derivative could find applications in photodynamic therapy benefiting from the biosupramolecular interactions as a transport system.

Biosupramolecular complexes of amphiphilic photosensitizers with human serum albumin and cucurbit[7]uril as carriers for photodynamic therapy

In the present work, we evaluated the supramolecular interactions between three photosensitizers, namely toluidine blue O (TBO, positively charged) and two fatty acid conjugates of 6 and 14 carbon atoms chain lengths (TBOC6 and TBOC14), with human serum albumin (HSA) and the macrocycle cucurbit[7]uril (CB[7]), alone or in combination within a biosupramolecular system as potential carriers of photosensitizers for Photodynamic therapy (PDT). Binding studies were carried out using photophysical and calorimetric techniques and accompanied with molecular docking simulations. Amphiphilic photosensitizers, particularly TBOC14, showed stronger binding to HSA and (CB[7]). Comparing the different delivery systems, (CB[7]) had a marginal effect on cell uptake and phototoxicity in HeLa cells, while HSA showed enhanced cell uptake with phototoxicities that depended on the photosensitizer. Despite low cell uptake, the combination of both (CB[7]) and HSA was the most phototoxic, which illustrates the potential of combining these systems for PDT applications.

Encapsulation of Chemotherapeutic Drug Melphalan in Cucurbit[7]uril: Effects on Its Alkylating Activity, Hydrolysis, and Cytotoxicity

The formation of inclusion complexes between drugs and macrocycles has proven to be an effective strategy to increase solubilization and stabilization of the drug, while in several cases improving their biological activity. In this context, we explored the formation of an inclusion complex between chemotherapeutic drug Melphalan (Mel) and cucurbit[7]uril (CB[7]), and studied its effect on Mel alkylating activity, hydrolysis, and cytotoxicity. The formation of the inclusion complex (Mel@CB[7]) was proven by absorption and fluorescence spectroscopy, NMR, docking studies, and molecular dynamics simulations. The binding constant for Mel and CB[7] was fairly high at pH 1 ((1.7 ± 0.7) × 10⁶ M^-1), whereas no binding was observed at neutral pH. The Mel@CB[7] complex showed a slightly decreased alkylating activity, whereas the cytotoxicity on the HL-60 cell line was maintained. The formation of the complex did not protect Mel from hydrolysis, and this result is discussed based on the simulated structure for the complex.