Chemcial structure of <a href="https://en.wikipedia.org/wiki/phytate" class="extiw" title="w:phytate">phytate</a>, also known as inositol hexakisphosphate (IP6).<br><p><b>Protonation</b><br></p>
The pKa value of the second deprotonation of single-phosphorylated carbohydrate(-like) molecules is somewhere around 6 (see <a rel="nofollow" class="external text" href="https://www.chem.wisc.edu/areas/reich/pkatable/pKa_compilation-1-Williams.pdf">pKa data compiled by Williams</a>). In this case, I'd expect the pKa values of the second deprotonation to increase for each phosphate group, as electrostatic interactions may make double-deprotonations of all phosphates less favorable. At a cellular pH of slightly above 7, one would expect some phosphates to be double-deprotonated and some to be single-deprotonated. To make things less complicated, I decided to show all phosphates in their single-deprotonated form, although that's probably not accurate at cellular pH.
<a href="//commons.wikimedia.org/wiki/User:Hbf878" title="User:Hbf878">Hbf878</a>
2018-08-10
<span class="int-own-work" lang="en">Own work</span>