HEPES is 2-[4-(2-Hydroxyethyl)piperazin-1-yl]ethane-1-sulfonic acid. I have read (MA Al-Ghobashy, Bull of Faculty of Pharmacy, Cairo University 2014; 52: 71-78) that the sulfonic acid group is completely dissociated "over almost the whole pH range" and there are two basic nitrogens N1 and N2 stated to have pKa 3 and 7.5 in the mentioned paper and in Wikipedia article. But what is the pKa of the sulfonic acid group? And, in the opposite end we have an OH group - is this inert or does it ever give off the proton?
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$\begingroup$ Both of these values are quite extreme, not really relevant in aq. solutions. Also they probably weren't investigated. Still these are probably typical values for sulfonic acids and alcohols. Did you search for what is usual for them? $\endgroup$– MithoronCommented Nov 15, 2022 at 19:51
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$\begingroup$ @Sam202 No, it doesn't. These are pKa values for both nitrogens. To protonate sulfonic group, you'd need to first have them both protonated, and then go a lot further than that. $\endgroup$– MithoronCommented Nov 15, 2022 at 21:40
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$\begingroup$ @Mithoron This source claims otherwise. $\endgroup$– Sam202Commented Nov 15, 2022 at 21:47
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$\begingroup$ @Sam202 It's pretty impressive they make such error even in paper! This paper isn't about this compound's acidity, though, so could happen. If there weren't two nitrogens, it would be true. $\endgroup$– MithoronCommented Nov 15, 2022 at 21:52
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$\begingroup$ @Mithoron You're right. However, I'm finding conflicting information with respect to those pKa values belonging to the nitrogen atoms. According to this source the pKa of HEPES is 7.55 (at 25°C), and its conjugate base referenced there is the deprotonated form of HEPES with respect to the sulfonic group. $\endgroup$– Sam202Commented Nov 15, 2022 at 22:10
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