Chiral Recognition using Tea Gallated Catechins (-)-Epigallocatechin-3-O-gallate

<p>When a hot tea beverage cools down, it becomes turbid and brown-white particles deforming the original taste and flavor of tea (called creaming-down). In previous studies, the phenomenon was revealed that tea gallated catechins (-)-epigallocatechin-3-O-gallate (EGCg) captured a caffeine in hydrophobic space formed by aromatic rings, A, B and B’ rings of EGCg in aqueous solution. </p><p>It is assumed that the space formed by the three aromatic rings of EGCg is a chiral space because the C ring of EGCg has two chiral carbon atoms, and is applicable to recognize the chirality of compounds in substitution for caffeine. Thus, the chiral recognition of cyclo(L-Pro-Gly) and cyclo(D-Pro-Gly), which are the same size as caffeine, using EGCg was investigated by ¹H NMR and X ray crystallographic analysis. </p><p>Upon the formation of the complex, the upfield shift values of the proton signals for H_7a, H_7b,8a of cyclo(L-Pro-Gly) were 0.087 and 0.065 ppm, and those of cyclo(D-Pro-Gly) were 0.118 and 0.113 ppm. Judging from X ray crystallographic analysis of EGCg complexes with cyclo(L-Pro-Gly) and cyclo(D-Pro-Gly), such a difference between the upfield shift values of the proton signals were considered to result mainly from the magnitude of the magnetic anisotropic shielding by the ring current from the B ring of EGCg. Therefore, a chirality of cyclo(Pro-Gly) was recognized by EGCg.</p><p> Furthermore, using a chiral recognition of EGCg in a D₂O solution as mentioned above, recognition of a chirality of commonly-used drug propranolol used as racemic form by EGCg was investigated. In ¹H NMR spectrum of an equimolecular amount of EGCg and racemic propranolol in D₂O, propranolol H₂ proton signal observed as a doublet during propranolol alone has been separated into two doublets. The reason is speculated that (R)-propranolol and (S)-propranolol were captured in the space formed by A, B, B’ ring of EGCg and diastereomeric complexes were generated, respectively. As the result, we consider that a chirality of propranolol is recognized by EGCg in a D₂O solution.</p>