Abstract
Kaempferol (Kae) is a natural flavonoid with potent antioxidant activity, but its therapeutic use is limited by its low aqueous solubility. Here, a series of Kae derivatives were synthesized to improve Kae dissolution property in water and antioxidant activity. These compounds included sulfonated Kae (Kae-SO3), gallium (Ga) complexes with Kae (Kae-Ga) and Kae-SO3 (Kae-SO3-Ga). The compound structures were characterized by high-resolution mass spectrometry (HRMS), nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and thermal methods (TG/DSC). The results showed that a sulfonic group (-SO3) was successfully tethered on the C3’ of Kae to form Kae-SO3. And in the metal complexation, 4-CO and 3-OH of the ligand participated in the coordination with Ga(III). The metal-to-ligand ratio 1:2 was suggested for both complexes. Interestingly, Kae-SO3-Ga was obviously superior to other compounds in terms of overcoming the poor water-solubility of free Kae, and the solubility of Kae-SO3-Ga was about 300-fold higher than that of Kae-Ga. Furthermore, the evaluation of antioxidant activities in vitro was carried out for Kae derivatives by using α,α-diphenyl-β-picrylhydrazyl (DPPH) and 2,2’-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) free radical scavenging. The results showed that Kae-SO3-Ga was also optimal for scavenging free radicals in a dose-dependent manner. These data demonstrate that sulfonate kaempferol-gallium complex has a promising future as a potential antioxidant and as a potential therapeutic agent for further biomedical studies.
Highlights
The most discussed feature of polyphenols and plant phenolics is their capability to scavenge reactive oxygen species (ROS) [1,2,3]
Jabeen et al [15] studied the antioxidant activity of Cu2+ and Fe3+ complexes with three flavonoids by using ultralviolet-visible (UV-Vis) spectroscopy and cyclic voltammetry
Β-cyclodextrin (β-CD) and sulfobutylether β-CD (SBE-β-CD) were useful in overcoming poor water solubility of the flavonoids by inclusion complexation with the flavonoids in their hydrophobic holes, such as has been shown for Kae, luteolin and myricetin [40]. β-CD has a hydrophilic outer suface, and sulfobutylether is a negatively charged group. These factors led to understand that SBE-β-CD was more effective in increasing the water solubility and antioxidant activity of the flavonoids when compared with β-CD
Summary
The most discussed feature of polyphenols and plant phenolics is their capability to scavenge reactive oxygen species (ROS) [1,2,3]. Flavonoids exert a wide range of benefits to human health [9,10] They are considered as one of irreplaceable antioxidants in our daily diet because they function as singlet oxygen scavengers, free radical quencher, or chelators of metal ions, which catalyze the oxidative reactions [11]. Flavonoids are ideal antioxidants due to their radical scavenging ability and their power to chelate with metal ions [15,16,17] Their metal complexes are usually found to be more active when compared with free ligands [18]. Jabeen et al [15] studied the antioxidant activity of Cu2+ and Fe3+ complexes with three flavonoids (morin, quercetin and 5-hydroxyflavone) by using ultralviolet-visible (UV-Vis) spectroscopy and cyclic voltammetry. Because the sulfonic group was a strong electron withdrawing group, it influences electronic cloud density of nearby hydrogen atoms
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