Thermochemical and structural studies of gallic and ellagic acids
Graphical abstract
Introduction
Gallic (1) and ellagic (2) acids are polyphenols present in numerous fruits and vegetables, including nuts, grapes, berries and also beverages such as tea and wine. Both compounds, and their derivatives (e.g. the hydrolysable tannins), are objects of numerous investigations, since they are considered as potent and versatile antioxidants with promising therapeutic and industrial applications [1], [2], [3], [4], [5], [6].
Ellagic acid (2) is a dimeric form of gallic acid (1) (See Scheme 1), and comprised of a fused four-ring structure with four hydroxyl groups and two lactone rings representing the hydrophilic part. The antioxidant activity of these compounds is related to their molecular structure, particularly to the presence and the number of hydroxyl groups, to the conjugation and the resonance effects and also to the capacity to improve the stability of their corresponding phenoxyl radicals [7], [8], [9], [10].
Despite the fundamental and applied relevance of 1 and 2, the thermochemical data of both acids are incomplete. Herein we report a reliable experimental data of standard enthalpy of formation in the solid state at 298.15 K, (cd), of 1 and 2 by using static micro-bomb combustion calorimetry measurements. The standard enthalpy of sublimation, , of 1, was also determined experimentally by Knudsen-effusion technique. This value allowed us to determine the standard enthalpy of formation in the gas phase, (g), of 1. Unfortunately, our attempts to determine of 2, by Knudsen-effusion or Calvet-microcalorimetry techniques, were unsuccessful due to the very low volatility of this compound.
Quantum chemical calculations at the density functional theory (M05-2X) level and composite ab initio G3 and G4 methods allowed us to discuss and check the consistency of the obtained results. These calculation methodologies were also used to determine (g) of 2 by means of isodesmic reactions, which combine theoretical and available experimental data of the involved compounds.
Section snippets
Materials and DSC measurements
3,4,5-Trihydroxybenzoic acid (C7H6O5, 1, gallic acid, CAS 149-91-7) and 2,3,7,8-Tetrahydroxychromeno[5,4,3-cde]chromene-5,10-dione (C14H6O8, 2, ellagic acid, CAS 476-66-4) were purchased from Sigma-Aldrich, carefully dried under vacuum at 363.15 K and used without further purification. Table 1 summarizes relevant information on purity and provenance of samples. The thermophysical properties were derived from DCS (Perkin Elmer Pyris 1) results over the temperature ranges considered (see below).
Thermophysical properties
The onset temperature of melting point for 1 was estimated by DSC technique as Tfus = 536.6 ± 0.6 K, where the associated uncertainty is expressed as expanded uncertainty of the mean (0.95 level of confidence, coverage factor k = 2) for three experiments. This value is in good agreement with that reported by Queimada et al. [23]. It is important to mention that we have not observed a solid–solid phase transition at 351.31 K reported by Bogel-Łukasik et al. [24]. It could be because we have used
Conclusions
In this work, we report an experimental and computational study of the structural, thermochemical and thermophysical properties of gallic (1) and ellagic (2) acids. We report a reliable experimental data for:
- i)
standard enthalpy of formation in the solid state at 298.15 K, (cd), of 1 as −(985.0 ± 2.9) kJ·mol−1 and 2 as −(1377.9 ± 4.7) kJ·mol−1 determined by using static micro-bomb combustion calorimetry,
- ii)
standard enthalpy of formation in the gas phase at 298.15 K, (g) of 1 as
Acknowledgments
J.Z.D acknowledges the support funding to IQFR-CSIC (Spain) and peruvian CONCYTEC (INNÓVATE PERÚ, ECIP-1-P-030-14 Grant). We wish to thank Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and the European Social Fund (ESF) for their financial support to CIQUP, University of Porto (Projects: PEst-C/QUI/UI0081/2011, FCUP-CIQ-UP-NORTE-07-0124-FEDER-000065). Carlos F.R.A.C.L. thanks for is Research Grant SFRH/BPD/77972/2011.
Dedication
This paper is dedicated to Prof. Gennady Kabo on the occasion of his 80th birthday.
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