Determination of acid dissociation constant of 20 coumarin derivatives by capillary electrophoresis using the amine capillary and two different methodologies

• Published in: Journal of Chromatography A Vol. 1446; pp. 149 - 157
• Main Authors: Nowak, Paweł Mateusz, Woźniakiewicz, Michał, Piwowarska, Monika, Kościelniak, Paweł
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 13-05-2016
• Subjects: Acid dissociation constant; Amines; Amines - chemistry; Capillarity; Capillary coating; Capillary electrophoresis; Coating effects; Constants; Coumarin; Coumarin derivatives; Coumarins - chemistry; Derivatives; Electrophoresis, Capillary - instrumentation; Electrophoresis, Capillary - methods; Electrophoretic mobility; Hydrogen-Ion Concentration; Mathematical analysis; Migration; Reproducibility of Results; Silicon Dioxide; Capillary electrophoresis; Electrophoretic mobility; OVM; Acid dissociation constant; Capillary coating; TVM; Coumarin derivatives; EOF; IS-CE
• ISSN: 0021-9673; 1873-3778
• DOI: 10.1016/j.chroma.2016.03.084

•PKa of 20 coumarin derivatives has been determined by capillary electrophoresis.•Coumarins’ acidity strongly depends on chemical structure (pKa between 4.16–9.10).•A permanently coated amine capillary ensures more repeatable and faster analysis.•A novel calculation method has been proposed, requiring less measurements.•Our work shows a new attractive way for fast pKa estimation by CE.
In this work capillary electrophoresis has been used to determine acid dissociation constant of 20 structurally diverse coumarin derivatives. For a majority of compounds pKa value has been determined for the first time. The obtained values vary between 4.16–9.10pH unit, pointing to the interesting structure-acidity relationships. The amine permanently coated capillary has been applied for that purpose, because it has turned out to be more effective in pKa determination than the bare silica and other coated capillaries, ensuring good precision and shorter migration times. A traditional methodology relying on measurements in a broad pH range and fitting of a sigmoidal function has been compared to an alternative simplified approach, reported for the first time, where only two electrophoretic mobility values suffice for pKa estimation. The first value corresponds to the partially ionized form and it is measured experimentally, while the second one to the totally ionized form − it is measured experimentally (two-values method) or estimated directly from molecular mass (one-value method). We show that despite a limited measurements number, the alternative approach may be consistent with the traditional methodology, yielding the relatively low pKa deviation. Its reliability has also been confirmed by the analytical predictions, comprising resolution, migration order, migration times and peaks overlapping. Therefore, combination of the amine capillary with the simplified calculation method is an attractive tool for fast and reliable pKa estimation.