Table 2. Concentrations of Natural Rubrolides 1–7, Compounds 8–10, and Rubrolide Analogues 12–20 Were Able to Inhibit by 50% the Light-Dependent Ferricyanide Reduction in Isolated Spinach Thylakoids.

Compound	IC50 (μM)	Compound	IC50 (μM)
1 rubrolide B	8.62 ± 1.89	12	257 ± 273
2 rubrolide I	5.54 ± 0.98	13	35.9 ± 7.7
3 rubrolide K	8.00 ± 2.05	14	61.4 ± 54.2
4 rubrolide O	4.37 ± 0.80	15	1620 ± 340
5 rubrolide E	n.e.a	16	266 ± 272
6 rubrolide F	29.3 ± 4.80	17	n.e.a
7 3″-bromorubrolide F	56.6 ± 15.2	18	438 ± 260
8	n.e.	19	n.e.
9	n.e.	20	4.97 ± 1.05
10	2.88 ± 1.50	Diuron	0.27 ± 0.01

^an.e. – not effective over the whole range of concentrations tested (0.05–100 μM). Each treatment was performed in triplication; IC₅₀ and confidence limits were calculated by nonlinear regression analysis using GraphPad Prism 6.