Spectroscopic characterization of different protonation/deprotonation states of Barbaloin in aqueous solution

Author(s)

Matos, Fernanda Lima ; Duarte, Evandro L. ; Muniz, Gabriel S. V. ; Milan-Garces, Erix Alexander ; Coutinho, Kaline ; Lamy, M. Teresa ; da Cunha, Antonio R.

• Document type: Journal article
• Source: SPECTROCHIMICA ACTA PART A-MOLECULAR AND BIOMOLECULAR SPECTROSCOPY; v. 286, p. 11-pg., 2023-02-05.

Abstract

Barbaloin (10-glucopyranosyl-1,8-dihydroxy-3-(hydroxymethyl)-9(10H)-anthraquinone: aloin A), present in Aloe species, is widely used in food, cosmetic and pharmaceutical industries. Here we characterize its optical absorption and emission spectra in aqueous solution at different pH values. Through pH titration, using both absorption and fluorescence spectroscopy, two pKa values for Barbaloin were determined: pKa1 = 9.6 +/- 0.6 and pKa2 = 12.6 +/- 0.8. These acidity constants were found to be higher than those found for Emodin, a similar molecule which lacks the sugar moiety present in Barbaloin. Performing quantum mechanical calculations for non-ionized, singly, doubly, and triply deprotonated forms of Barbaloin in vacuum and in water, we assigned the positions of the site for the first and third deprotonation in the anthraquinone group, and the second deprotonation in the glucose group. The instability of Barbaloin in high pH solutions is discussed here, and the optical absorption and fluorescence spectra due to products resulted from Barbaloin degradation at high pH is well separated from the Barbaloin original spectra. Biological fluids have specific pH values to maintain homeostasis, hence determining the pKa of Barbaloin is important to evaluate the mechanism of action of this drug in different parts of an organism as well as to predict pharmacological relevant parameters, such as absorption, distribution, metabolism, and excretion. (AU)

• FAPESP's process: 17/25930-1 - Characterization of lipid dispersions of biological relevance: structures and interactions
• Grantee: Maria Teresa Moura Lamy
• Support Opportunities: Regular Research Grants
• FAPESP's process: 14/50983-3 - INCT 2014: complex fluids
• Grantee: Antonio Martins Figueiredo Neto
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 18/20162-9 - Investigating the Pathogenesis and Drug Resistance in Microorganisms - Characterization and Control of ATP-Binding Cassette Transporters
• Grantee: Andrea Balan Fernandes
• Support Opportunities: Regular Research Grants
• FAPESP's process: 21/01593-1 - Multiple physical techniques to structurally characterize biological relevant membranes and its interactions
• Grantee: Maria Teresa Moura Lamy
• Support Opportunities: Regular Research Grants
• FAPESP's process: 17/03910-9 - The interaction of anthraquinones with a biological membrane model
• Grantee: Fernanda Lima Matos
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• FAPESP's process: 21/09016-3 - Structural and electronic characterization of biogenic molecules emitted by the Amazon flora
• Grantee: Kaline Rabelo Coutinho
• Support Opportunities: Regular Research Grants