#6 Different core parent structures for different protonation states of the same molecule

[Anne Morgat]

Dear InChI members,

I have a question regarding the InChI/InChIKey of different protonation states of the citrate molecule.
I naively expected that two protonation states of the same molecule would have the same first InChIKey block, this is the case for acetic acid/acetate for instance. For citrate, it is not the case since the central parent structures (+ charge layers) are different (see below).
Could you help me to understand why I get different parent structures for citrate but not for acetate?
Thank you for your support,

Best regards
Anne

https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:16947
citrate(3-)
InChI=1S/C6H8O7/c7-3(8)1-6(13,5(11)12)2-4(9)10/h13H,1-2H2,(H,7,8)(H,9,10)(H,11,12)/p-3
InChIKey=KRKNYBCHXYNGOX-UHFFFAOYSA-K

https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:132362
citrate(4-)
InChI=1S/C6H7O7/c7-3(8)1-6(13,5(11)12)2-4(9)10/h1-2H2,(H,7,8)(H,9,10)(H,11,12)/q-1/p-3
InChIKey=KSXLKRAZYZIYCZ-UHFFFAOYSA-K