Research Article
Determination of the Acidity Constants of Serotonin in the Ground and Excited States Using Spectroscopic Methods
Abdourahmane Khonte*
,
Abdou Dieng,
Coura Dione,
Latyr Dione,
Atanasse Coly
Issue:
Volume 14, Issue 2, April 2026
Pages:
19-28
Received:
15 March 2026
Accepted:
24 March 2026
Published:
14 April 2026
DOI:
10.11648/j.ajche.20261402.11
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Abstract: Serotonin (5-hydroxytryptamine, 5-HT) is a biologically important neurotransmitter whose photophysical behavior depends on its protonation state. This study investigates the acid-base properties of serotonin in aqueous solution using UV-visible absorption and fluorescence spectroscopies. The ground-state acidity constant (pKa(S0)) was determined spectrophotometrically, while the excited-state acidity constant (pKa*(S1)) was estimated using the Förster thermodynamic cycle. Absorption spectra revealed an isosbestic point, indicating a simple two-state equilibrium between the protonated (R-OH) and deprotonated (R-O⁻) forms. The results show that serotonin behaves as a weak acid in the ground state (pKa(S0) ≈ 10.5–10.6), whereas its acidity increases significantly in the excited state (pKa*(S1) ≈ 4.7) due to electronic redistribution within the indole chromophore. Excited-State Proton Transfer (ESPT) occurs efficiently, influencing both fluorescence intensity and emission wavelength. These findings provide a comprehensive understanding of serotonin’s photophysical behavior and support its use as an intrinsic fluorescent probe for monitoring local pH variations in aqueous or cellular environments. The combination of UV-Vis and fluorescence measurements, with triplicate statistical validation, ensures reproducibility and accuracy of the determined acidity constants. This work contributes to a better understanding of neurotransmitter acid-base behavior under physiologically relevant conditions and demonstrates the potential application of serotonin in fluorescence-based pH sensing and molecular studies.
Abstract: Serotonin (5-hydroxytryptamine, 5-HT) is a biologically important neurotransmitter whose photophysical behavior depends on its protonation state. This study investigates the acid-base properties of serotonin in aqueous solution using UV-visible absorption and fluorescence spectroscopies. The ground-state acidity constant (pKa(S0)) was determined sp...
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