Solvatochromism of (-)-Epigallocatechin 3-O-Gallate (EGCG) Fluorescence: Dependence on Solvent Protonicity

Health benefits of EGCG are well established; however, the mechanisms of EGCG action are not completely understood. In our previous study we discovered solvatochromism of EGCG fluorescence and described the dependence of fluorescence maxima on solvent polarity. We also noted that fluorescence intensity (FI) depends on solvent. The goal of this study was to gain insights into how the protonic properties of the environment affect FI. We demonstrate that 1) FI of EGCG inversely correlates with the autoprotolysis constant (Kap) of the solvents, 2) HCl decreases, and NaOH increases FI of EGCG in water, and 3) NaOH evokes slow (~10 min time scale) transient changes in FI of EGCG. We conclude that EGCG fluorescence depends on Kap, i.e. protonicity of the environment, that is useful for differentiating EGCG in protic aqueous environment at physiological pH~7.0{\div}7.4, where EGCG fluorescence is substantially quenched, from EGCG in aprotic environments, where EGCG fluorescence significantly increases. Thus, this property of EGCG fluorescence is useful for investigation of specific EGCG interactions within more aprotic protein binding sites.