Complexation of epigallocatechin gallate (a green tea extract, egcg) with Mn ²⁺: Nuclear spin relaxation by the paramagnetic ion

Metal-binding sites in epigallocatechin gallate (egcg) were identified by measuring ¹H nuclear spin relaxation time T ₁ under the coexistence of paramagnetic Mn ²⁺. The T ₁ of the proton of gallate ring (D-ring) was shortened by the paramagnetic ion to a greater extent than the proton in gallocatechin ring (B-ring); the D-ring OH groups occupy the first coordination sphere around a metal ion to form a diolate chelate ring, while B-ring OH groups have a weak interaction with the metal ion. Comparison of changes in T ₁ of egcg and epigallocatechin (egc) indicates that the gallate ring has a higher coordination capability than that of the gallocatechin ring so that the former plays a predominant role in the complex formation of egcg.