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Summer Research Fellowship Programme of India's Science Academies

Spectral Study on Curcumin and Synthesis of Gluocosyl-Curcumin Derivative for Targeted Photocytotoxicity

Daliya K Shajan

Assumption College, Changanassery, Kottayam, Kerala

Akhil Chakravarty

Indian Institute of Science, Bengaluru 560012

Abstract

Curcumin, an active ingredient of turmeric (Curcuma longa), was first isolated by Vogel and Pelletier in 1815 from the rhizomes of turmeric. Curcumin [(1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] consists of three functional reactive groups: two o-methoxyphenolic groups linked through an α,β –unsaturated β-diketone moiety. The later participates in keto-enol tautomerism and the enolic form is reported to be more stable in organic solvents than the keto form. Curcumin has been used as a dietary spice and in treating various health issues for thousands of years in many Asian countries. However, research has only relatively recently established the chemopreventive and therapeutic potential of curcumin.

In this work, we have studied the spectral properties of curcumin using various spectroscopic techniques viz. NMR, IR, UV–Visible and fluorescence spectroscopy. The 1H NMR spectrum of curcumin in DMSO-d6 shows characteristic peaks at 3.84, 6.06 and 9.67 δ ppm which correspond to the o-methoxy protons, methine proton [C(4)H] and phenolic OH groups, respectively. A peak observed at 16.4 δ ppm was assigned to the enolic-OH proton. These observations indicate the presence of curcumin in its enolic form. The IR spectrum of curcumin in solid phase reveals bands at 1627 cm-1 and 1503 cm-1, which are characteristic to the C=O and C=C stretching frequencies of β-diketonate group. A broad band at 3330 cm-1 indicates the presence of OH groups. The strong band observed at 1274 cm-1 was attributed to the stretching vibration of enolic C ̶ O bond. In UV-Visible absorption spectroscopy, curcumin displays an intense band at   ̴435 nm in DMSO as well as in 50% aqueous DMSO, the molar extinction coefficients (ε) being 66,000 and 60,290 M-1cm-1, respectively. The absorption maxima (λmax ̴ 435 nm) corresponds to π-π* transition. The fluorescence spectrum of curcumin in DMSO, on excitation at 435 nm, exhibits emission at   ̴520 nm (λem). However, in 50% aqueous DMSO, the intensity of emission reduces with an enhancement in the Stokes’ shift (λexc   ̴ 435 nm, λem   ̴ 536 nm).

Keywords: turmeric, curcumin, photodynamic therapy, photocytotoxic property, photosensitizing activity, spectroscopy

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