Two questions are asked: the first is if the lack/presence of methoxyl moiety at aromatic ring essentially affects the stability of curcuminoid-metal complexes, and the second is if it is possible that in the metal complexes one of the possible demethoxycurcumin structures predominates. To answer the first question, the ESI-MS/MS spectra were taken of ions containing demethoxycurcumin, curcumin, and metal cation (e.g., ion [dCurc + Curc-H + Pb]+), and in order to answer the second question the ESI-MS/MS spectra were recorded of ions containing demethoxycurcumin and metal cation (e.g., ion [dCurc-H + Zn]+). An interpretation of the mass spectra has indicated that (i) for some metals, curcumin-metal complexes are more stable than demethoxycurcumin-metal complexes and for some metals vice versa, and (ii) in demethoxycurcumin-metal complexes structure B1 [(1E,4Z,6E)-5-hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1-(4-hydroxyphenyl)hepta-1,4,6-trien-3-one] is more stable than structure B2 [(1E,4Z,6E)-5-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-7-(4-hydroxyphenyl)hepta-1,4,6-trien-3-one]. 1. Introduction Curcuminoids are natural compounds existing in turmeric, a popular Indian spice. The main curcuminoid is curcumin (Curc, 75–80%), which reveals interesting biological and pharmacological properties as reviewed recently [1–5]. Two other curcuminoids are demethoxycurcumin (dCur, 15–20%) and bisdemethoxycurcumin (bdCurc, 3–5%). These two curcuminoids, and especially dCurc, have also been studied in terms of their biological and pharmacological properties [6–12]; sometimes dCurc exhibited better properties than Curc [13–15]. Curcuminoids have a β-diketone unit; thus they are able to form stable complexes with metal cations, number of Curc-metal complexes have been reported [16–25] and formation of such complexes may have interesting pharmacological implications [26–35]. On the other hand, dCurc-metal complexes have not been studied yet. The only paper on the subject reports that Curc is a better Cu reducer than dCurc and Curc is more effective than dCurc in DNA cleavage reaction (the reaction is a consequence of Cu complexation by Curc) [10]. It is well known that even small structural changes in organic compounds may have great impact on their biological activity (as observed for curcuminoids). However, the question is if lack/presence of methoxyl moiety at aromatic ring essentially affects the stability of curcuminoid-metal complexes, since it can be taken for granted that metal cations are chelated by β-diketone unit, deprotonated enol form of curcuminoids. The
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