Fig. 2. Dark regrowth of TBA metabolites and the influence of pH and temperature on reversion.

(A) Regeneration of 17α-TBOH, 17β-TBOH, and TBO in photoproduct mixtures stored in the dark (pH 7, 25°C). hν, light energy. The inset reports the percent recovery (mean and standard deviation of triplicate analyses) when photoproduct mixtures were adjusted to pH 2 or pH 12. Also shown is the regeneration of 17α-TBOH as a function of (B) pH (5, 7, and 9 at 25°C) and (C) temperature (5, 15, 25, and 35°C at pH 7) from photoproduct mixtures. In (B), the initial rate of regrowth obtained from linear regression analysis was enhanced at pH 9 (0.3 μM hour⁻¹) and, at least initially [time (t) < 2 hours], at pH 5 (0.6 μM hour⁻¹) relative to pH 7 (0.17 μM hour⁻¹). The slower rate observed over longer time scales at pH 5 (0.07 μM hour⁻¹) probably reflects differences in the dehydration rates of 5-and 12-hydroxy-17α-TBOH.