NMR Reinvestigation of the Caffeine–Chlorogenate Complex in Aqueous Solution and in Coffee Brews

Caffeine complexation by chlorogenic acid (3-caffeoylquinic acid, CAS Number [327-97-9]) in aqueous solution as well as caffeine-chlorogenate complex in freshly prepared coffee brews have been investigated by high-resolution 1 H-NMR. Caffeine and chlorogenic acid self-associations have also been studied and self-association constants have been determined resorting to both classical isodesmic model and a recently introduced method of data analysis able to provide also the critical aggregation concentration (cac). Furthermore, caffeine-chlorogenate association constant was measured. For the caffeine, the average value of the self-association constant determined by isodesmic model ( K i = 7.6 ± 0.5 M −1 ) is in good agreement with the average value ( K a = 10 ± 1.8 M −1 ) determined with the method which permits the determination of the cac (8.43 ± 0.05 mM). Chlorogenic acid shows a slight decreased tendency to aggregation with a lower average value of association constants ( K i = 2.8 ± 0.6 M −1 ; K a = 3.4 ± 0.6 M −1 ) and a critical concentration equal to 24 ± 1 mM. The value of the association constant of the caffeine-chlorogenate complex (30 ± 4 M −1 ) is compatible with previous studies and within the typical range of reported association constants for other caffeine-polyphenol complexes. Structural features of the complex have also been investigated, and the complex conformation has been rediscussed. Caffeine chemical shifts comparison (monomeric, complexed, coffee brews) clearly indicates a significant amount of caffeine is complexed in beverage real system, being chlorogenate ions the main complexing agents.