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

Abstract

Femtosecond fluorescence upconversion experiments were performed on the naturally occurring medicinal pigment, curcumin, in anionic, cationic, and neutral micelles. In our studies, the micelles are composed of sodium dodecyl sulfate (SDS), dodecyl trimethyl ammonium bromide (DTAB), and triton X-100 (TX-100). We demonstrate that the excited-state kinetics of curcumin in micelles have a fast (3−8 ps) and slow (50−80 ps) component. While deuteration of curcumin has a negligible effect on the fast component, the slow component exhibits a pronounced isotope effect of ∼1.6, indicating that micelle-captured curcumin undergoes excited-state intramolecular hydrogen atom transfer. Studies of solvation dynamics of curcumin in a 10 ps time window reveal a fast component (≤300 fs) followed by a 8, 6, and 3 ps component in the solvation correlation function for the TX-100, DTAB, and SDS micelles, respectively. October 23, 2009; ReVised Manuscript ReceiVed: January 4, 2010 Femtosecond fluorescence upconversion experiments were performed on the naturally occurring medicinal pigment, curcumin, in anionic, cationic, and neutral micelles. In our studies, the micelles are composed of sodium dodecyl sulfate (SDS), dodecyl trimethyl ammonium bromide (DTAB), and triton X-100 (TX-100). We demonstrate that the excited-state kinetics of curcumin in micelles have a fast (3-8 ps) and slow (50-80 ps) component. While deuteration of curcumin has a negligible effect on the fast component, the slow component exhibits a pronounced isotope effect of ∼1.6, indicating that micelle-captured curcumin undergoes excited-state intramolecular hydrogen atom transfer. Studies of solvation dynamics of curcumin in a 10 ps time window reveal a fast component (e300 fs) followed by a 8, 6, and 3 ps component in the solvation correlation function for the TX-100, DTAB, and SDS micelles, respectively. Keywords

Nanoprecipitation and Spectroscopic Characterization of Curcumin-Encapsulated Polyester Nanoparticles

“…12,15 In addition, the shoulder in the absorption band observed around 450 nm in all three Cur-polyester NPs (as indicated by the arrows in Figure 2) is a feature that is absent in the absorption band of curcumin in the aqueous environment. 20 It has been suggested that curcumin is located at the palisade layer of the neutral and cationic micelles, in which the surrounding environment is similar to protic solvents.…”

“…It is well known that bulk-like or confined water is present in the TX-100 and DTAB micelles. 15 Therefore, it follows that the local environment of curcumin in the polyester NPs has a significant level of bulk-like water.…”

“…The fl are 5 % for the Cur-PLA and Cur-PCL, and 4 % for the Cur-PLGA NPs, which are higher than that of curcumin in micelles 19 ( fl = 0.007 3 %). 11,12,15 The average distance between adjacent curcumin molecules in all three polyester NPs is approximately 1.5 nm (Supporting Information). The short distance between adjacent curcumin indicates that self-quenching of fluorescence by nonradiative relaxation is likely.…”

“…[14][15][16][17] In particular, previous work has demonstrated that solvent reorganization and excited-state intramolecular hydrogen atom transfer (ESIHT) in the keto-enol moiety of curcumin are major relaxation pathways of its excited state. [14][15][16][17] The ESIHT of curcumin exhibits a deuterium isotope effect while solvation remains unaffected. 14,15 In addition to having a low solubility in water, curcumin undergoes rapid degradation.…”

“…[14][15][16][17] The ESIHT of curcumin exhibits a deuterium isotope effect while solvation remains unaffected. 14,15 In addition to having a low solubility in water, curcumin undergoes rapid degradation. 18,19 The degradation of curcumin has been also studied in a pH 7.4 buffer and more than 50 % of curcumin is degraded within 30 min.…”