Multivariate analysis of protolytic and tautomeric equilibria of Erythrosine B and its ester derivatives in ionic and non-ionic micelles Camila Fabiano de Freitas a , Douglas Vanzin a , Thais Lazzarotto Braga a , Diogo Silva Pellosi b , Vagner Roberto Batistela c , Wilker Caetano a , Noboru Hioka a, ⁎ a Department of Chemistry, State University of Maringá, 5790 Colombo Avenue, 87020-900 Maringá, Paraná, Brazil b Department of Chemistry, Federal University of São Paulo, Rua São Nicolau 210, 09972-270 Diadema, São Paulo, Brazil c Department of Technology, State University of Maringá, Dr. Ângelo Moreira da Fonseca Avenue, 1800, 87506-370 Umuarama, Parana, Brazil abstract article info Article history: Received 14 January 2020 Received in revised form 2 May 2020 Accepted 6 May 2020 Available online 11 May 2020 Keywords: Erythrosine Xanthene Protolytic equilibrium Tautomeric equilibrium Multivariate analysis Erythrosine B (ERY) is a xanthene dye that has been widely used in recent years as a photoactive drug for Pho- todynamic Therapy. However, application of ERY and its derivatives as photosensitizer drugs depend not only on their singlet oxygen quantum yield, but also on their hydrophobicity, and photoactive protolytic and tauto- meric species. In this work, the objective was to evaluate the acid-base equilibria of ERY and its derivatives: eryth- rosine methyl ester (ERYMET); erythrosine butyl ester (ERYBUT) and erythrosine decyl ester (ERYDEC), in biomimetic media (micelles of SDS and CTAB) as well as in some drug delivery systems of triblock copolymer Poloxamer (P-123 and F-127). All protolytic chemical equilibria were studied by employing chemometric multi- variate analysis based on the Imbrie's Q-Mode Factor Analysis and k-Matrix methods. Tautomeric equilibria were evaluated taking into account of possible chemical structures and electronic spectroscopic approximations. Re- sults from ester derivatives presenting only one protolytic equilibrium (at the phenolate group) and two possible tautomers, allow us to determine that protolytic equilibria follow the sequence pKa COOH b pKa OH in SDS media for ERY by structure comparison. On the other hand, CTAB and polaxamers inverted the acidity of ERY protolytic groups (pKa OH b pKa COOH ) due to changes in the tautomeric equilibrium due to a lactone formation for the dyes neutral protolytic species inside the micelles. Chemometric approaches provided detailed analyses of protolytic/tautomeric equilibria of ERY. Through this, we achieved a deeper understanding of how ERY and its de- rivatives are affected by microenvironments in biomimetic and drug delivery systems. © 2020 Published by Elsevier B.V. 1. Introduction Halogenated xanthenes are a class of molecules that present several applications. They are used as dyes for LASER [1], initiators in photopolymerization reactions [2], solar energy conversions [3], and Photodynamic Therapy (PDT) when correctly combined with visible light [4–9]. The latter is a procedure that uses a light-activated photo- sensitizer (PS) to produce reactive oxygen species as singlet oxygen ( 1 O 2 ), which when generated in situ can lead to cell death [4,6]. Among halogenated xanthene dyes, Erythrosine B (ERY), also referred to as Acid Red 51, C.I. 454,300 or 2-(2,4,5,7-tetraiodo-6-oxido-3-oxo- 8a, 10a-dihydroxanthen-9-yl) benzoic acid, is commonly used as a pink sodium powder salt (Fig. 1). With a physiological pH of 7.4, ERY is a dianionic dye and presents high molar absorptivity (ε = 96.6 × 10 3 L mol -1 cm -1 at λ max = 517 nm [10,11]) and high singlet ox- ygen generation (ΔФ 1 O 2 = 0.63 [12]). ERY has the advantage of being approved by the Food and Drug Administration (FDA) for human use. It is, as two examples, used as a food colorant [13], and to identify dental plaque and caries [6]. ERY's photodynamic effect has been successfully tested as a PS in dental biofilm treatments against Streptococus mutans [6] and against several bacterial strains, following the order of activity: S. au- reus b A. hydrophila b E. coli b S. typhimurium b P. aeruginosa [5]. ERY has also been used on the C. albicans fungi strain [4]. Other studies have reported ERY activity in malignant (H357) and premalignant (DOK) oral lesions, and in colorectal Caco-2 cancer cells [9,14,15]. However, ERY is a hydrophilic compound that presents two phenolic groups in the xanthene and benzoic rings (Fig. 1). This hampers their in- ternalization in cells and decreases photodynamic activity. In order to overcome this problem, our research group developed hydrophobic ester derivatives of ERY via the insertion of methyl (ERYMET), butyl (ERYBUT), or decyl (ERYDEC) alkyl groups in the carboxylic group of the dyes (Fig. 1)[11,16]. Through this, the ester derivatives improve in- teraction with cell membranes by preserving photodynamic action [11,15,16]. In addition, the butyl and decyl derivatives are very Journal of Molecular Liquids 313 (2020) 113320 ⁎ Corresponding author. E-mail address: nhioka@uem.br (N. Hioka). https://doi.org/10.1016/j.molliq.2020.113320 0167-7322/© 2020 Published by Elsevier B.V. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq