Figure 4.

¹H-NMR and ¹³C-NMR of compound 3 at pH values at approximately 1.8 and 0.8, respectively (left). Observed chemical shifts (δ_obs) of the signals arising from the methylenehydrogens and the carbon in position 1 associated with the oxo and hydrated forms of 3 plotted against pH (right).The four continuous lines with a sigmoidal shape are described by Eq.14, fitting δ_obs values at different pH values.