The effects of varying the pH
When considering the effects of the pH of the extraction
medium, it is important to remember that, like many other
properties, the ionization constant of water (Kw) and the
dissociation constant of phosphoric acid are strongly
temperature-dependent. They also exhibit a more modest
pressure-dependence. Under the different conditions examined
in this study, the negative logarithm (base 10) of the
dissociation constant of water (pKw) takes values ranging
between 12.252 and 11.205 [39]. For this reason to avoid
confusion, all quoted values of the pH of a given extracting
phase refer to those obtained under NIST standard conditions
of temperature and pressure, i.e., 293.15 K (20 °C)
and 101.325 kPa pressure (1.013247 bar). At physiological
conditions, the importance of the tautomerism of curcumin
has been examined in a study investigating the molecular
mechanism of the observed synergy between curcumin and
water soluble antioxidants in cancer chemoprevention [40].
It has been observed that the curcumin radical preferentially
exists as a phenoxyl-type species, which is more hydrophilic
than the keto form. Being more hydrophilic, it is
preferentially moved to the external side of the cell
membrane; this effect is probably responsible of curcumin