Differentiation of puerarin chelate from salt by phase solubility test

Different from salt, metal chelate is a novel state of drug constructed by more separate coordinate bonds to form a chelating circle. Due to their composition similarity, it is hard to distinguish them except identifying ionic bond (i.e., salt) or coordinate bond (i.e., chelate) in the single crystal structure. In this study, sodium chelate (CDCC No: 1865670) and lithium salt (CDCC No: 2161617) of puerarin (PUE) was prepared. In addition to difference in single crystal structure, it was found that they showed totally different phase solubility behaviors: lithium salt demonstrated a typical inverse proportion curve as other common salts, while sodium chelate exhibited disordered scatters. However, when incorporating the unit PUE-Na complex in solution state and complexation constant K₁₁ in chemical equation, the scatters in phase solubility diagram of chelate could be well fitted and the value of K₁₁ was dramatically higher with orders of magnitude than the dissociation constant K_c; while processing phase solubility curve of lithium salt by incorporating complex item, it could not well match the curve at all. PUE sodium chelate is more likely to be a weak electrolyte with partial dissociation, while PUE lithium salt acted as a strong electrolyte with complete dissociation. The phase solubility test would be served as a surrogate tool for differentiation of chelates from salts when single crystal was not available.