Abstract
The solubility of theobromine was studied both experimentally and theoretically. The solubility was determined spectrophotometrically at 25 °C in neat organic solvents, aqueous binary mixtures, Natural Deep Eutectic Solvents (NADES) and ternary NADES mixtures with water. It was found that addition of water in unimolar proportions with some organic solvents increases theobromine solubility compared to neat solvents. Additionally, using NADES results in a solubility increase of the studied compound not only in relation to water but also DMSO. The addition of water (0.2 molar fraction) to NADES is responsible for an even larger increase of solubility. The measured solubilities were interpreted in terms of three theoretical frameworks. The first one—belonging to the set of data reduction techniques—proved to be very efficient in quantitative back-computations of excess solubility of theobromine in all studied systems. The default approach utilizing the well-recognized COSMO-RS (Conductor-like Screening Model for Real Solvents) framework offered at most a qualitative solubility description. The extended search for possible contacts provided evidence for the existence of many intermolecular complexes that alter the electron density of the solute molecule, thus influencing solubility computations. Taking into account such intermolecular contacts by using the COSMO-RS-DARE (Conductor-like Screening Model for Realistic Solvation-Dimerization, Aggregation, and Reaction Extension) framework seriously increased the accuracy of solubility computations.
Highlights
Theobromine exists in only one stable crystal form [89] and crystallizes in P21/c system with a = 9.2990, b = 18.6980, c = 9.0381 Å and with Z = 8 [90]
It is expected that no changes in the form of theobromine will occur when comparing the pure compound initially used for solubility studies and the sediment obtained after experiments
In order to confirm this, a series of FTIRATR experiments were performed and selected results were summarized in Figure 2, which shows the FTIR spectra of Theobromine residues after measuring its solubility in different solvents
Summary
Methylxanthines belong to a group of compounds that are widespread in nature, residing in tea and other plant leaves, coffee and cocoa beans, as well as cola seeds [1]. Theobromine in particular can be found mostly in cocoa and chocolate [2]. It was discovered in 1841 but its synthesis was achieved as late as 1882. Methylxanthines play a crucial role in several biological processes with the stimulation of the central nervous system, increase of blood pressure, diuresis of kidneys, relaxation of smooth muscles, strengthening of the concentration of skeletal muscles or gastric acid secretion [3,4] being their most important biological activities. The mode of action of methylxanthines arises from their activities as phosphodiesterase inhibitors [5]
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