Photodegradation of methoxy substituted curcuminoids

Abstract

Dedicated to the memory of Prof. Dr. Jurij V. Bren~i~. AbstractPhotodegradation of dimethoxy curcuminoids in acetonitrile solution was found to depend on the position of the methoxy group bonded to the phenyl ring. The rate of decomposition was expressed as the lifetime of the decomposing substrate, being the shortest in the case of the 3,5-dimethoxy and the longest for the 2,5-dimethoxy derivative. For the 3,5-dimethoxy curcuminoid, the major degradation products were 3,5-dimethoxybenzaldehyde, 3,5-dimethoxybenzoic acid and the Z and E isomers of dimethoxycinnamic acid, together forming about 90% of the reaction mixture. Minor products found were 4,5-bis(3,5-dimethoxyphenyl)hex-2-endionic acid, products with the molecular formula C 23 H 24 O 6 and C 23 H 22 O 6 attributed to the reaction of intramolecular [2 + 2] cycloaddition of the dimethoxy curcuminoid, and the dioxygenated bicyclopentadione derivative (C 23 H 24 O 8 ) derived from autoxidative transformation of the dimethoxy curcuminoid.

UV-A-Induced Photoisomerization and Photodimerization of Curcumin: An Ion Mobility Mass Spectrometry Study

“…58 Formation of a similar photoisomer by intramolecular [2 + 2]-cycloaddition upon UV irradiation of the dimethoxy curcuminoids in acetonitrile solution was proposed by Galer and S ̌ket. 28 For further authentication of the structure of the isomeric species S7, the CCS value for its optimized protonated geometry (Figure S5) is calculated theoretically, and the value predicted, 197.2 Å 2 , correlates well with the experimentally measured CCS value of 187 Å 2 (Figure 7C) for the isomeric curcumin monomer produced upon fragmentation of the photodimers. Thus, it is inferred that isomer S7 is produced via dissociation of the dimer, and this isomer acts as the precursor of the fragment ion of m/z 273.…”

“…It was also proposed that the species could be formed via fragmentation of a cyclobutyl­cyclopentadione type isomer (isomer S7, Scheme ) of curcumin produced by the intramolecular [2 + 2]-cycloaddition reaction . Formation of a similar photoisomer by intramolecular [2 + 2]-cycloaddition upon UV irradiation of the dimethoxy curcuminoids in acetonitrile solution was proposed by Galer and Šket . For further authentication of the structure of the isomeric species S7, the CCS value for its optimized protonated geometry (Figure S5) is calculated theoretically, and the value predicted, 197.2 Å 2 , correlates well with the experimentally measured CCS value of 187 Å 2 (Figure C) for the isomeric curcumin monomer produced upon fragmentation of the photodimers.…”

“…The other fragment ion at m/ z 217 was previously reported to be the most abundant fragment ion of an H 2 -loss photoproduct formed by visible light induced degradation of curcumin. 28 In the following section, the structure of this H 2 -loss photoproduct is suggested based on the results of ion mobility mass spectrometric analysis.…”

“…26 Recently, photostability of curcuminoids has been studied by Galer and S ̌ket using a series of dimethoxy curcuminoids in acetonitrile solution upon irradiation with 360 nm light. 28 For the 3,5-dimethoxy curcuminoid, the major photoproducts were identified as 3,5-dimethoxybenzaldehyde, 3,5-dimethoxybenzoic acid, and dimethoxycinnamic acid by means of high-resolution mass spectrometric analysis. A cyclized derivative, 4,5-bis(3,5-dimethoxyphenyl)hex-2-endionic acid, formed by intramolecular [2 + 2] cycloaddition reaction, was also identified.…”

“…It has been reported also that upon prolonged exposure to sunlight, curcumin undergoes degradation into smaller fragments like vanillin, ferulic aldehyde, ferulic acid, and vanillic acid . Recently, photostability of curcuminoids has been studied by Galer and Šket using a series of dimethoxy curcuminoids in acetonitrile solution upon irradiation with 360 nm light . For the 3,5-dimethoxy curcuminoid, the major photoproducts were identified as 3,5-dimethoxy­benzaldehyde, 3,5-dimethoxy­benzoic acid, and dimethoxycinnamic acid by means of high-resolution mass spectrometric analysis.…”