Umang Singh

Reactions of reactive oxygen species (ROS) with curcumin analogues: Structure–activity relationship

Abstract Three curcumin analogues viz., bisdemethoxy curcumin, monodemethoxy curcumin, and dimethoxycurcumin that differ at the phenolic substitution were synthesized. These compounds have been subjected for free radical reactions with DPPH radicals, superoxide radicals (O2•−), singlet oxygen (1O2) and peroxyl radicals (CCl3O2•) and the bimolecular rate constants were determined. The DPPH radical reactions were followed by stopped-flow spectrometer, 1O2 reactions by transient luminescence spectrometer, and CCl3O2• reactions using pulse radiolysis technique. The rate constants indicate that the presence of o-methoxy phenolic OH increases its reactivity with DPPH and CCl3O2•, while for molecules lacking phenolic OH, this reaction is very sluggish. Reaction of O2•− and 1O2 with curcumin analogues takes place preferably at β-diketone moiety. The studies thus suggested that both phenolic OH and the β-diketone moiety of curcumin are involved in neutralizing the free radicals and their relative scavenging ability depends on the nature of the free radicals.

Interaction of a Curcumin Analogue Dimethoxycurcumin with DNA

Dimethoxycurcumin (Dimc), a synthetic analogue of curcumin, that has been reported to exhibit better in vivo stability and anti‐tumour activity, was investigated for its interaction with DNA, employing spectroscopic methods based on absorption, fluorescence, circular dichroism (CD), ethidium bromide (EtBr) competitive binding assay, 4′‐6‐diamidino‐2‐phenylindole (DAPI) displacement assay and fluorescence resonance energy transfer (FRET) assay. The mean binding constant for its interaction with calf thymus DNA (ct‐DNA) was estimated to be 4.4 ± 0.8 × 104 m−1. The studies using CD revealed that Dimc did not cause unwinding of the ct‐DNA helix or induce major conformational changes. The EtBr and DAPI assays indicated that Dimc is not an intercalator but a minor groove binder. FRET assay also confirmed that Dimc interacts with DNA strands. Furthermore, viscosity measurements of ct‐DNA solutions in the presence of Dimc supported these spectroscopic observations. Addition of Dimc to MCF‐7 cells showed nuclear localization as visualized by confocal microscopy. In conclusion, the present studies addressed the mode of interaction of Dimc with biomolecules, which may have implications in developing Dimc as a DNA‐targeted drug.

Reactions of hydroxyl radical with bergenin, a natural poly phenol studied by pulse radiolysis

Reactions of pulse radiolytically generated hydroxyl (()OH) radicals and one-electron specific oxidants, Br(2)(-) radicals with bergenin, a polyphenolic tannin derivative, were studied and the transients detected by absorption spectrometry. The transient absorption spectrum produced during the reaction of ()OH radicals with bergenin was broad, and pH dependent. Different modes of reactions of ()OH radicals with bergenin, viz., addition to the aromatic ring adduct and hydrogen abstraction was established by time resolved (5-400micros) transient absorption studies and also by the reaction of Br(2)(-) radicals. Comparing the transient spectra with ()OH radicals and Br(2)(-) radicals at pH 4.5 and 8.5, the absorption maximum of the phenoxyl radical was found to be at 440nm at pH 4.5 and 480nm at pH 8.5. Phenoxyl radicals are produced during ()OH radical reaction through the formation of ()OH radical adduct followed by water/OH(-) elimination. While the phenoxyl radicals of bergenin are oxidizing in nature, the hydroxyl radical adducts and the radicals produced from hydrogen abstraction are of reducing nature. The yield of the oxidizing radicals produced from the ()OH radical reaction with bergenin was determined to be 26.2% by secondary electron transfer reaction from TMPD. On the other hand the yield of reducing radicals produced from the ()OH radical reaction with bergenin was determined to be 74.1% by secondary electron transfer reaction to MV(2+). ()OH radical reactions with bergenin under oxygenated conditions and reaction with trichloro methyl peroxyl radicals with bergenin produced a new transient absorbing at 400nm, which is attributed to peroxyl type of radicals. The one-electron reduction potential for the formation of phenoxyl radical from bergenin was determined to be 0.938V versus NHE at pH 7, by electron transfer equilibrium between bergenin and chlorpromazine. The above results confirmed that reaction of ()OH radicals with bergenin, mainly produced radical adducts and one-electron oxidation accounts to only a minor process. The radical adducts may be converted to peroxyl radicals in presence of oxygen. Based on these results it can be concluded that although bergenin is a polyphenol, it may not act as a potent antioxidant, but may be act as pro-oxidant.