Femtosecond Transient Absorption Spectroscopy of the Medicinal Agent Curcumin in Diamide Linked γ-Cyclodextrin Dimers

Abstract

Curcumin is a biologically active polyphenol and a yellow pigment extracted from turmeric. Our previous study has shown effective encapsulation of curcumin using diamide linked γ-cyclodextrin dimers, namely 66γCD2su and 66γCD2ur, through cooperative 1:1 host-guest complexation. In this study, the excited-state dynamics of curcumin complexed with either 66γCD2su or 66γCD2ur in water are investigated using femtosecond transient absorption spectroscopy. Both 66γCD2su-curcumin and 66γCD2ur-curcumin complexes in water show only an excited-state absorption (ESA) band at 530 nm without any stimulated emission (SE) signals, indicating non-radiative decays as the major relaxation pathways. The ESA dynamics of 66γCD2su-curcumin are similar to those of 66γCD2ur-curcumin, consisting of a rapid growth component and three decay components. The growth component, which has a time constant of 0.25-0.41 ps, is assigned to solvent reorganization. The relatively fast decay components with time constants of 9.3-21.8 ps show significant deuterium isotope effect, consistent with the presence of excited-state intramolecular hydrogen atom transfer (ESIHT) of curcumin. The small-amplitude and slow decay components may be attributed to the dynamics of complexed curcumin and molecular motions due to flexibility of 66γCD2su and 66γCD2ur. In addition, transient absorption anisotropy measurements reveal slow rotational motions of 66γCD2su-curcumin and 66γCD2ur-curcumin complexes. The overall results show that complexation in 66γCD2su and 66γCD2ur has pronounced effects on the photophysics of curcumin.

Excited-state dynamics of the medicinal pigment curcumin in a hydrogel

“…The presence of a deuterium isotope effect is consistent with the assignment of this major relaxation process of curcumin in the PAAC18 hydrogel to ESIHT. As established in previous studies, [42][43][44][45][46][47][48] ESIHT is an important photophysical process of excited-state curcumin. It was also shown that the interactions between the enolic hydrogen of curcumin and polar solvent molecules influence the rate of ESIHT.…”

“…It was also shown that the interactions between the enolic hydrogen of curcumin and polar solvent molecules influence the rate of ESIHT. [43][44][45][46][47][48] Curcumin in the PAAC18 hydrogel exhibits a relatively fast ESIHT process compared with curcumin in protic solvents, 42,43,[46][47][48] consistent with the presence of water in the PAAC18 hydrogel. The deuterium isotope effect of 1.7 of curcumin in the PAAC18 hydrogel is identical to that of curcumin in methanol, similar to those observed in micelles (1.6) but lower than that in ethylene glycol (2.1).…”

“…Thus far, femtosecond transient absorption spectroscopy and fluorescence upconversion spectroscopy have been employed to provide insight into the photophysical processes of excited-state curcumin in polar organic solvents, micelles and as a complex with Cu 2+ . [42][43][44][45][46][47][48][49][50] The dominant relaxation of curcumin in polar organic solvents and micelles is excited-state intramolecular hydrogen atom transfer (ESIHT) in the keto-enol moiety (Fig. 1a).…”

“…1a). [42][43][44][45][46][47][48][49][50] The studies of the ESIHT reaction form a part of the continuing effort in understanding the role of hydrogen and/or proton transfer in Chemistry using ultrafast spectroscopy. Most notably, the work of Huppert, Douhal and Nibbering paved the way for many of the following studies.…”

Photoinduced Antimicrobial Activity of Curcumin-Containing Coatings: Molecular Interaction, Stability and Potential Application in Food Decontamination

“…Curcumin ( Cu ), as one of the active components of Indian turmeric, has exhibited potential applications as therapeutic agents against several diseases because of its low toxicity, good anti-inflammatory and antibacterial capabilities, and anticancer activities. − However, the low solubility and fast degradation assigned to physiological pH and photoinduced instability in aqueous media restrained numerous applications of curcumin in therapeutic agents and photodynamic inactivation ( PDI ) against cancerous cells and pathogenic microorganisms. − In addition, the equilibrium corresponding to keto- and enol-tautomers, ultrafast excited-state processes closely associated with salvation, and excited-state intramolecular hydrogen atom transfer ( ESIHT ) have been considered important factors affecting its antibacterial and antioxidant capacities in solution. − Thus, surfactant utilization and encapsulation into delivery systems such as conjugates, nanoparticles, − molecular complexes, micelles, liposomes, hydrogels, and emulsions have been utilized to improve the solubility, stability, and biocompatibility of curcumin in aqueous media.…”