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As a follow up, does the Carbonyl group at 4 position group get deprotonated or Hydroxyl group at 5 position get deprotonated? How does the resonance structure look? I have been looking through literature but have not been able to find any reference to this. Does the deprotonation start with the carbonyl group or does it always have to start with hydroxyl group? I may not sound clear as I am an amateur when it comes to organic chemistry.

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    $\begingroup$ It would help to have the structure in the question $\endgroup$
    – Waylander
    Commented Jun 24, 2020 at 7:56
  • $\begingroup$ Added chemical structure $\endgroup$
    – arun_kol
    Commented Jun 24, 2020 at 20:41
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    $\begingroup$ Did help all that much... How go you imagine carbonyl getting deprotonated, for example. Also title doesn't make sense, as there's no form that wouldn't be delocalised. $\endgroup$
    – Mithoron
    Commented Jun 25, 2020 at 1:07

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The first acid dissociation constant for genistein ($\mathrm{p}K_\mathrm{a1}$) is $6.51 \pm 0.20$ (Ref.1), mainly because of hydrogen bonding between $\ce{C_{(4)}}$ carbonyl oxygen and $\ce{C_{(5)}-OH}$, similar to that in acetylacetone (see the diagram):

Isoflavone backbone

Therefore, in physiological $\mathrm{pH}$ ($\mathrm{pH} \approx 7.3$) it should be partially ionized. If it is in acedic solution, $\ce{C_{(4)}}$ carbonyl oxygen would be partially or fully protonated based on the acid strength of the solution. Furthermore, the energy minimized three-dimensional structure of genistein shows the hydrogen bonding as depicted in following image (See Figure A; Ref.1):

Three-dimensional structure

References:

  1. Yoshiyuki Mizushina, Kazuaki Shiomi, Isoko Kuriyama, Yoshihiro Takahashi, Hiromi Yoshida, “Inhibitory effects of a major soy isoflavone, genistein, on human DNA topoisomerase II activity and cancer cell proliferation,” International Journal of Oncology 2013, 43(6), 1117-1124 (https://doi.org/10.3892/ijo.2013.2032).
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  • $\begingroup$ Thanks for your answer. As a follow up questions, since other isoflavones such as Daidzein (pubchem.ncbi.nlm.nih.gov/compound/5281708) and Glycitein (pubchem.ncbi.nlm.nih.gov/compound/5317750) do not have a hydroxyl group at C(5), how do they ionize? Another follow up question is that do the hydroxyl group in the C(7) position form phenoxyl radical in all isoflavones? $\endgroup$
    – arun_kol
    Commented Jun 24, 2020 at 20:40
  • $\begingroup$ This should be asked as a separate question. $\endgroup$ Commented Jun 25, 2020 at 0:45

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