Bibliographical note

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The results presented in this section show that flavone, diosmetin and hesperetin are the least reactive flavonoids at the conditions of the algae growth inhibition test. Interestingly, two of this three compounds (diosmetin, hesperetin) carry a methoxy group at the B ring indicating that this could be a structural feature lowering the reactivity. This is consistent with increased half-lives of isorhamnetin and kaempferide in comparison to their non-methylated counterparts quercetin and kaempferol in DMEM cell culture medium (Xiao and Högger, 2015). In line with this, tamarixetin (also carrying a methoxy group) was found slightly more stable than quercetin in our study, but still degraded below LOD within 2 days. In comparison to tamarixetin, diosmetin, and hesperetin both lack the 3-OH. The influence of a 3-OH was investigated previously in regard to degradation rates and antioxidant activity. Concurrent with decreasing half-lives (Xiao and Högger, 2015), this structural feature was also found to increase the antioxidant activity of flavonoids (Rice-Evans et al., 1996). Thus, the 3-OH group could counteract the effect of the methoxy group in tamarixetin. The fast degradation of quercetin and taxifolin, also carrying a 3-OH, is in further support of the reactivity enhancing effect of a 3-OH. In comparison to their counterparts lacking the 3-OH, quercetin (below LOD after 24 h) degrades faster than luteolin (below LOD after 48 h), while the concentration of taxifolin and eriodictyol showed a similar decrease. The slower degradation of taxifolin and eriodictyol could be due to a C2–C3 single bond. This structural feature was reported to increase degradation half-lives (Xiao and Högger, 2015) and decrease antioxidant activity (Plaza et al., 2014). Furthermore, the very fast degradation of myricetin and dihydromyricetin supports the proposed rule that an increasing number of hydroxy groups increases degradation rates (Maini et al., 2012; Xiao and Högger, 2015).This research was funded by the EU within the European Regional Development Fund (ERDF), support measure INTERREG V in the Upper Rhine as part of the NAVEBGO project 5.3 (sustainable reduction of biocide inputs to groundwater in the Upper Rhine region; grand agreement number: 66300015). The authors thank Jonas R. L. Schneider (https://orcid.org/0000-0003-1645-8430) from the Laboratory for Environmental Analysis, Institute for Hygiene and Environment, Free and Hanseatic City of Hamburg for his extensive and friendly support with HPLC-HRMS measurements and Magnus Winkelmann from the Institute of Sustainable Chemistry, Leuphana University for his initial training at the HPLC-UV/vis and careful maintenance of it.

Funding Information:
This research was funded by the EU within the European Regional Development Fund (ERDF) , support measure INTERREG V in the Upper Rhine as part of the NAVEBGO project 5.3 (sustainable reduction of biocide inputs to groundwater in the Upper Rhine region; grand agreement number: 66300015 ). The authors thank Jonas R. L. Schneider ( https://orcid.org/0000-0003-1645-8430 ) from the Laboratory for Environmental Analysis, Institute for Hygiene and Environment, Free and Hanseatic City of Hamburg for his extensive and friendly support with HPLC-HRMS measurements and Magnus Winkelmann from the Institute of Sustainable Chemistry, Leuphana University for his initial training at the HPLC-UV/vis and careful maintenance of it.

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