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 are polyphenolic molecules which can be isolated from black tea infusions. The theaflavin family of polyphenols contribute to the taste and colour of tea, and their complexation with caffeine is thought to be largely responsible for the formation of tea cream, a precipitate that forms as tea cools. The self-association of theaflavin and caffeine was studied using nuclear magnetic resonance methods (chemical shift changes and self-diffusion constants 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 ring system, with the two flavan rings approximately orthogonal to this plane, and stacked against each other. In the dimer, two benzotropolone rings align with an antiparallel geometry. Two molecules of caffeine bind to one molecule 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 proposed that the first caffeine inserts between the two flavan rings, and the second then binds to the newly liberated flavan surface.