Excited-State Intramolecular Hydrogen Atom Transfer and Solvation Dynamics of the Medicinal Pigment Curcumin

Abstract

The potential use of the naturally occurring yellow-orange pigment curcumin as a photodynamic therapy agent is one of the most exciting applications of this medicinal compound. Although subnanosecond spectroscopy has been used to investigate the photophysical processes of curcumin, the time resolution is insufficient to detect important and faster photoinduced processes, including solvation and excited-state intramolecular hydrogen atom transfer (ESIHT). In this study, the excited-state photophysics of curcumin is studied by means of ultrafast fluorescence upconversion spectroscopy. The results show two decay components in the excited-state kinetics with time scales of 12−20 ps and ∼100 ps in methanol and ethylene glycol. The resulting prominent isotope effect in the long component upon deuteration indicates that curcumin undergoes ESIHT in these solvents. The short component (12−20 ps) is insensitive to deuteration, and multiwavelength fluorescence upconversion results show that this decay component is due to solvation of excited-state curcumin. KeywordsAtomic spectroscopy, deuterium, ethylene, ethylene glycol, fluorescence, free radical reactions, hydrogen, methanol, photodynamic therapy, solvation, curcumin, decay components, deuteration, excited-state, fluorescence upconversion, intramolecular hydrogens, isotope effects, photophysics The potential use of the naturally occurring yellow-orange pigment curcumin as a photodynamic therapy agent is one of the most exciting applications of this medicinal compound. Although subnanosecond spectroscopy has been used to investigate the photophysical processes of curcumin, the time resolution is insufficient to detect important and faster photoinduced processes, including solvation and excited-state intramolecular hydrogen atom transfer (ESIHT). In this study, the excited-state photophysics of curcumin is studied by means of ultrafast fluorescence upconversion spectroscopy. The results show two decay components in the excited-state kinetics with time scales of 12-20 ps and ∼100 ps in methanol and ethylene glycol. The resulting prominent isotope effect in the long component upon deuteration indicates that curcumin undergoes ESIHT in these solvents. The short component (12-20 ps) is insensitive to deuteration, and multiwavelength fluorescence upconversion results show that this decay component is due to solvation of excited-state curcumin.

Excited-State Intramolecular Hydrogen Atom Transfer of Curcumin in Surfactant Micelles

“…23 Additionally, the ESIHT time constant of curcumin reported in this study is comparable to that of 7-azaindole in methanol. 49 The good agreement indicates a substantial level of curcumin-solvent and curcumin-surfactant intermolecular hydrogen bonding in the micelle.…”

“…22 The ESIHT time constant of curcumin in this nonpolar solvent is shorter than that in methanol, which has a value of ∼70 ps. 23 The results imply that the strength of the intramolecular hydrogen bond of curcumin is the determining factor in the rate of ESIHT. These authors, furthermore, suggest that in protic solvents, such as methanol and ethanol, the keto-enol group of curcumin forms strong hydrogen bonds with the solvent molecules, in addition to the intramolecular hydrogen bond within the keto-enol group.…”

“…In our previous study of curcumin in polar organic solvents, an isotope-effectfree fast decay component was also observed, and we demonstrated that this decay component is due to solvation of curcumin in these solvents. 23 Using the previous results as a guide, we conducted a series of fluorescence upconversion experiments with a time window of 10 ps. The results are shown in Figure 4.…”

“…Although there is a general agreement that ESIHT is a major photophysical event of curcumin, [21][22][23] the exact time scale of this phenomenon was not well-established until our recent work. 23 Using deuteration of curcumin and time-resolved fluorescence upconversion as a technique for probing ESIHT, our work has shown that the time constant of ESIHT of curcumin is 70 ps in methanol and 120 ps in ethylene glycol. 23 It has been proposed that the presence of a labile hydrogen as a result of ESIHT plays a role in the medicinal effects of other naturally occurring pigments such as hypericin and hypocrellin.…”

Excited State Dynamics of Curcumin and Solvent Hydrogen Bonding

“…11,18,19 Recently, Adhikary et al studied the excited state photophysics of curcumin in protic solvent and surfactant micelles by using femtosecond fluorescence upconversion spectroscopy. 20 They found that one of the decay time constants, i.e., the longer one, changed significantly when the enolic hydrogen of curcumin was deuterated. It was thus assigned to the ESIHT process, whereas the shorter time constant that did not change was thought to be associated with the process of solvation.…”