Photochemistry in Cyclodextrin Cavities

Inclusion of Paracetamol into β-cyclodextrin nanocavities in solution and in the solid state

We report on steady-state UV-visible absorption and emission characteristics of Paracetamol, drug used as antipyretic agent, in water and within cyclodextrins (CDs): β-CD, 2-hydroxypropyl-β-CD (HP-β-CD) and 2,6-dimethyl-β-CD (Me-β-CD). The results reveal that Paracetamol forms a 1:1 inclusion complex with CD. Upon encapsulation, the emission intensity enhances, indicating a confinement effect of the nanocages on the photophysical behavior of the drug. Due to its methyl groups, the Me-β-CD shows the largest effect for the drug. The observed binding constant showing the following trend: Me-β-CD>HP-β-CD>β-CD. The less complexing effectiveness of HP-β-CD is due to the steric effect of the hydroxypropyl-substituents, which can hamper the inclusion of the guest molecules. The solid state inclusion complex was prepared by co-precipitation method and its characterization was investigated by Fourier transform infrared spectroscopy, 1H NMR and X-ray diffractometry. These approaches indicated that Paracetamol was able to form an inclusion complex with CDs, and the inclusion compounds exhibited different spectroscopic features and properties from Paracetamol.

Photoionization of psoralen derivatives in micelles: Imperatorin and alloimperatorin

The fluorescence properties of psoralen derivatives, 8-methoxypsoralen (8-MOP), imperatorin (IMP) and alloimperatorin (ALLOI), were investigated in various solvent and micellar solutions. The variation in intensity and maxima of the fluorescence in micellar solutions suggest that psoralens are located in the micelle-water interface region. Radical cations and hydrated electrons were generated by photoionization in micellar solution upon excitation at 266 nm. A nonlinear relationship between transient yield and photon fluency was obtained for each compound, indicating that a two-photon mechanism is predominant in the photoionization of the sensitizers. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) than in cationic cetyltrimethylammonium bromide (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. The photoionization efficiencies of IMP and ALLOI are similar.

Evaluation of novel alginate foams as drug delivery systems in antimicrobial photodynamic therapy (aPDT) of infected wounds--an in vitro study: studies on curcumin and curcuminoides XL

The aim of the present study was to incorporate a model water-insoluble photosensitizer, curcumin, in novel alginate foams, further to evaluate the suitability of the curcumin loaded foams in antimicrobial photodynamic therapy of infected wounds. Six foam formulations were prepared and characterized with respect to physical characteristics, in vitro release and storage- and photo-stability of curcumin. One formulation was sterilized (gamma-sterilization). The foams contained hydroxypropyl-beta-cyclodextrins or hydroxypropyl-gamma-cyclodextrins as solubilizers of curcumin. A reference foam without cyclodextrins was prepared with PEG 400 as the solubilizer. At a curcumin load of 0.153% (w/w), the water insoluble photosensitizer was uniformly distributed in the hydrophilic foams matrix. All foams were easy to handle, flexible and hydrated rapidly in a model physiological fluid. Release of curcumin in its monomeric form was demonstrated in vitro and found to be dependent on the type and amount of cyclodextrins in the formulation. Curcumin was stable during storage, but susceptible to photodegradation in the foams, especially when the formulations contain PEG 400 or hydroxypropyl-gamma-cyclodextrins. Curcumin did not degrade after gamma-sterilization, however a decrease in the in vitro release rate of curcumin and changes in the foams physical characteristics were detected.

Inclusion complex of 2-naphthylamine-6-sulfonate with beta-cyclodextrin: intramolecular charge transfer versus hydrogen bonding effects

The photophysical characteristics of the ground and excited states of 2-naphthylamine-6-sulfonate (2-NA-6-S) were investigated in different solvents and in beta-cyclodextrin (beta-CD). The spectral shifts are well correlated with Kamlet-Taft relationship. Multiple linear regression analysis indicated that both non-specific dipolar interaction and specific hydrogen bonding interactions play competitive roles in determining the position of the absorption maximum, while the dipolar interaction is the dominating parameter in determining the emission maximum. For the Stokes shift, both the nonspecific interaction and the hydrogen donation property of the solvent are participating equally. The molecular encapsulation of 2-NA-6-S by beta-CD in aqueous solution has been studied by different spectroscopic techniques. Fluorescence measurements show that the dielectric constant of beta-CD experienced by the included 2-NA-6-S is intermediate between water and methanol. The changes observed in the absorption and fluorescence spectra of 2-NA-6-S upon inclusion in beta-CD allowed the association constant to be calculated and found to be 465+/-100 and 495+/-100 M-1, respectively. The changes observed for the chemical shifts of 2-NA-6-S and beta-CD 1H NMR spectra and the corresponding 1H NMR spectra of their mixture confirmed the formation of the inclusion complex and showed that 2-NA-6-S is encapsulated in beta-CD cavity in a tilted equatorial approach.

Spectroscopic studies on the inclusion complex of 2-naphthol-6-sulfonate with beta-cyclodextrin

The photophysical properties of 2-naphthol-6-sulfonate (2-NOH-6-S) in various solvents and in aqueous beta-cyclodextrin solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.20+/-0.02, while in water the yield is higher. The fluorescence quantum yield in water was found to depend on the pH value of the medium and increases as the pH increase up to a pH value of 4.0 where it comes to be constant. Absorption and fluorescence measurements show 1:1 inclusion of 2-NOH-6-S in the beta-cyclodextrin cavity. The association constant of 2-NOH-6-S-beta-cyclodextrin complex based on fluorescence measurements was calculated using Benesi-Hildbrand relationship and found to be 330+/-30M(-1). (1)H NMR studies are used to confirm the inclusion and to provide information on the geometry of 2-NOH-6-S inside the cavity of beta-cyclodextrin.

Regioselective [2 + 2]-photocycloaddition reactions of chiral tetronates—influence of temperature, pressure, and reaction medium

The intramolecular [2 + 2]-photocycloaddition of the 1,3-divinyl-2-cyclopentyl tetronates was performed under various conditions to give perfect diastereoselectivity and a regioselectivity of up to 85/15 in the presence of gamma-cyclodextrin by differentiating the two chemically very similar double bonds of .

The inclusion complexes of hesperetin and its 7-rhamnoglucoside with (2-hydroxypropyl)-beta-cyclodextrin

The effect of (2-hydroxypropyl)-beta-cyclodextrin (HP-beta-CyD) on the solubility properties and spectroscopic features of hesperetin and its 7-rhamnoglucoside, hesperidin, was qualitatively and quantitatively investigated in water, by means of UV-vis absorption and fluorescence spectroscopy. The stoichiometric ratios and stability constants describing the extent of formation of the complexes have been determined by phase-solubility measurements; in both cases type-A(L) diagrams have been obtained (soluble 1:1 complexes). The higher degree of interaction showed by hesperetin may be attributed to the higher hydrophobicity and smaller size of the aglycone molecule, which therefore exhibits a greater affinity for the CyD and fits better into the cavity. The effect of molecular encapsulation on the two flavanones antioxidant activity was afterwards evaluated by means of different biological assays, concerned to the different mechanisms of in vivo action. The protection efficacy was in all cases higher for the complexed drugs, with respect to the free ones; these results are of great interest for their potential usefulness in pharmaceutics.

Temperature-dependent cage effects from triplet radical pairs generated upon irradiation of 1-(4-methylphenyl)-3-phenyl-2-propanone in polyethylene films

The photochemistry of 1-(4-methylphenyl)-3-phenyl-2-propanone (MeDBK) in polyethylene (PE) films of varying crystallinity and at different temperatures has been investigated. Specifically, the selectivity of combination of the initially formed triplet pairs of benzylic radicals derived from irradiation of MeDBK has been used to determine the natures of the PE reaction cavities and how they influence the motions of the radicals. The results indicate that in-cage combinations are favored at lower temperatures and in PE of higher crystallinity. At temperatures significantly above the glass transition of PE, the fraction of radical pairs combining in-cage (F(c)) is independent of the extent of conversion of MeDBK. The data are interpreted according to a structural model that considers the shape, free volume, wall stiffness, and permeability of the reaction cavities and a dynamic model that includes the competition between the rates of in-cage combination and cage escape by the radical pairs. Near the glass transition temperature, F(c) decreases as the temperature is reduced and the data are interpreted using a stochastic model that considers the slow relaxation of the walls of the reaction cavities. The results demonstrate that the cavities afforded by PE can be subtly 'tuned' by temperature changes and microscopic variations in the polymer structure and morphology.