## 3 Results  
Firstly, the iron-chelating properties of all the tested compounds were determined under different pH conditions. In general, the iron chelation properties did not vary substantially among the compounds, and became weaker with a lowering of the pH (major representatives are shown in Figure 2, other compounds in Figure S1). At pH 7.5, the chelation activity of the majority of the compounds likely corresponded with the chelation stoichiometry of 1:1 (roughly 100% iron was chelated at pH 7.5 at the ratio of 1:1, see e.g., Figure 2B,C) with the exception of 2'-hydroxy-3-methoxy derivative (**2**, Figure S2A), which apparently formed complexes with ferrous ions at the stoichiometric ratio of 2:1 (see Figure 2E; at the ratio of 1:1, about 50% of the iron was chelated). At pH 6.8, the 4'-dimethylamino derivative (**8**) and 4'-trifluoromethylderivative (**9**) were the most efficient, while other compounds had lower effects than these two congeners (Figure S2B). At pH 5.5, the chelating effects of all tested compounds were similar, and it appears that all formed complexes had a stoichiometry of 2:1, compound to iron. The only exception was **8**, which was clearly less efficient (Figure S2C). At pH 4.5, again, there were generally no differences between ferrous chelation, with the exception of **9**, which was the most efficient (Figure S2D). In the case of ferric chelation at the same pH, there were, however, apparently three subgroups. The largest group containing the majority of the compounds comprised the most effective xanthones with a probable stoichiometric ratio of 2:1. A lower effect was observed in the 2'-hydroxy-3'-methoxy, 4'-ethoxy and 3'-bromo derivatives, and 2'-chloro-6'-fluoro (**2**, **7**, **11** and **12**). The 4'-dimethylaminocongener (**8**, Figure S2E) was even less efficient. A comparison of the chelation effect of all tested compounds is summarized in Figure 3.  
All tested compounds were able to reduce iron at pH 4.5 and partly at pH 5.5. None of them reduced iron at pH 6.8 or 7.5. The reduction curves were bell-shaped in all cases. With the exception of the 4'-dimethylamine derivative, the reducing properties were observed only at low ratios, and were negligible or were abruptly attenuated at ratios higher than 1:1. The maximal reduction reached 50-70% of added iron in most cases. However, the 4'-hydroxy-3'-methoxy-5'-nitro derivative (**6**) and the acetylamino (**10**) derivatives reached only approximately 30%. Representative compounds are shown in Figure 4, and all other compounds in Figure S3. The screening of copper chelation activity with hematoxylin showed that all compounds were able to chelate cupric ions at pH 5.5-7.5 approximately, with the same potency (Figure S4). However, under more competitive conditions, their chelation activity towards both cupric and cuprous ions was negligible (Figures S5 and S6), suggesting a low affinity for copper. Similar to iron, all compounds were able to reduce cupric ions. In contrast to iron, all xanthones were able to reduce 100% of the added copper, and their reduction properties did not decrease at higher ratios. There was significant difference in the efficacy of cupric ion reductions at lower ratios. Comparison was performed via 95% confidence intervals of reduction lines (see an example in the Supplementary Figure S7). The most efficient compound was compound **9** (Figure S4) followed by derivative **8**, which was less efficient at pH 4.5, 5.5 and 6.8. The 3',4'-dihydroxy derivative (**3**) was less efficient at pH 5.5 and 7.5 when compared with the 4'-dimethylamine derivative (**8**). There were no differences between the 3',4'-dihydroxy derivative (**3**) and the 4'-hydroxy-3',5'-dimethoxy (**5**) derivative, which were followed by the less active 4'-ethoxy derivative (**7**) and 3'-bromo (**11**) congeners. The acetamino derivative (**10**) had the same activity as the previous two compounds, with the exception of pH 4.5, where it was less efficient.