The self-association of the black tea polyphenol theaflavin and its complexation with caffeine

Caffeine is found in both coffee and tea whilst polyphenols are present in a wide variety of foods and beverages. Theaflavin and its gallate esters ar e polyphenolic molecules which can be isolated from black tea infusions. Th e theaflavin family of polyphenols contribute to the taste and colour of te a, and their complexation with caffeine is thought to be largely responsibl e for the formation of tea cream, a precipitate that forms as tea cools. Th e self-association of theaflavin and caffeine was studied using nuclear mag netic resonance methods (chemical shift changes and self-diffusion constant s on dilution) and it is shown that caffeine forms stacks of molecules (K-s = 7.9 l mol(-1) at 300 K), while theaflavin forms stable dimers (K-s = 230 l mol(-1)). The theaflavin monomer consists of a planar benzotropolone rin g system, with the two flavan rings approximately orthogonal to this plane, and stacked against each other. In the dimer, two benzotropolone rings ali gn with an antiparallel geometry. Two molecules of caffeine bind to one mol ecule of theaflavin in a strictly sequential manner, with first and second association constants of 11.9 and 16.5 l mol(-1), respectively. It is propo sed that the first caffeine inserts between the two flavan rings, and the s econd then binds to the newly liberated flavan surface.