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. 2022 Aug 19;12:14158. doi: 10.1038/s41598-022-17913-6

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© The Author(s) 2022

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Determination of the pKa value for 7.44 by ¹H NMR. (A) ¹H NMR spectra of 200 µM of 7.44 measured from pH 2 to 13 at 298 K at 600 MHz. Sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) was used for chemical shift referencing and calibrated to a zero ppm value. (B) Titration curves obtained from the ¹H chemical shifts of the protons H2 and H3 as a function of pH. The pKa values were calculated using the Henderson-Hasselbalch equation²⁵, yielding a pKa value of 3.99 ± 0.01 and 3.93 ± 0.01 of the carboxylic group as indicated by proton H2 and H3, respectively.