  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.
