Science in a cup of coffee: A structural study of a trigonelline aqueous solution

Bioactive trigonelline is one of the important components of coffee, contributing to its characteristic undue bitterness. In this report we looked at the structure of an aqueous solution of trigonelline by means of Neutron Diffraction with Isotopic Substitution (NDIS). This experimental approach has been coupled with a Monte Carlo simulation to get an atom-level description of the solution. The rationale behind the present study is to investigate the solvent-solute and the solute-solute interactions; these have implications relevant also to other bioactive molecules and, in addition, can provide basic information on the extraction efficiency and on the solubility of important molecules in coffee. Results indicate a limited short-ranged involvement of the solute trigonelline with the water solvent and a not negligible clustering of trigonelline molecules. The investigated solution contains both positively charged and zwitterionic trigonelline, along with Cl- and H+, with this latter ion involved in hydronium ion formation. Here it is found that the observed structures are compatible with a Zundel-like cation, thus contributing to the still on-going debate on the presence of either Zundel or Eigen complexes in acidic solution. The role of a Zundel-like cation, along with its interaction with the chloride ion, might be related to the extraction and solubility of important molecules in coffee, in order to define the best water composition resulting in the best coffee.