Hello, I would like to ask about the exact chemical structure of pheophorbide a after saponification process of chlorophyll using KOH. Will the resulting structure carry one or two K⁺ ions? I ask because I have found two different versions of the pheophorbide a structure in the literature: one where C13¹ has a keto group (C=O) and another where it is represented as a hydroxyl group (–C–OH). In the first case, only the carboxyl group at C17 would be deprotonated and bind K⁺, while in the second case it seems possible for both the carboxyl and the enol/hydroxyl at C13¹ to be deprotonated, resulting in two K⁺ ions attached to the molecule. Which representation is considered correct after saponification with KOH?
Thank you in advance.
After saponification of chlorophyll with KOH, pheophorbide a typically carries one K⁺ ion bound to the deprotonated carboxyl group at C17, while the keto group at C13¹ usually remains a carbonyl (C=O) and is not deprotonated. Therefore, the correct structure usually has one K⁺ ion attached.
After saponification, there is hydrolysis of ester bond side chain. With the Keto-enol equilibrium, the following changes are contemplated.
1 There is Keto-enol tautomerism.
2 Formation of carboxyllic group at C17 position.
3 Methoxycarbonyl gp at C13 is hydrolyzed.
4 Removal of central magnesium atom.
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Dipak Kumar Chattopadhyay Thank you for your answer. Since keto–enol tautomerism is reversible, does that mean that after neutralizing the solution at the end of the saponification process the equilibrium will shift back toward the keto form, leaving only the carboxylate at C17 deprotonated? In that case, would the final neutralized product contain only one K⁺ ion bound to pheophorbide a?
Abdelhak Maghchiche Thank you for the clarification. That makes sense.
Out of curiosity, why do some references and databases show pheophorbide a in the enol form (–C–OH at C13¹) instead of the keto form? Including PubChem. This is a bit confusing, since PubChem is a widely used source. So I wonder why there are these two representations?
Though keyo-enol tautomerism is reversible, for most carbonyl compounds, at equilibrium keto form is the most stable. At final neutralized saponified pheophorbide, final product has two K+ ions, one at C17 carboxylate and one at enolate-oxygen.
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