Unusual Ionic Bond and Solubility Mechanism of NanPQQ (n = 0–4) Crystals

A comparative study of van der Waals and ionic crystals can provide vital information for the medical and food industries. In this work, we investigated the coenzyme pyrroloquinoline quinone (PQQ), which contains three carboxyl groups coupled to imidazole, pyridine, and quinone. Whole-crystal analysis (crystal-ome) was attempted for NanPQQ (n = 0–4) crystals. All deprotonation sites were found to be dependent on pKa except for the Na sites, which cannot be explained by pKa. The Na1PQQ crystal exhibited an unusual ionic bond, forming COOH–Na+ at one of the carboxyl sites in the structure. The difference in the solubility of the van der Waals and ionic crystals was also investigated, with a focus on the dissolution processes of Na0PQQ and Na2PQQ, by combining molecular dynamics simulations with experiments that define the crystal surfaces. This study is the first step toward developing a general rule to link the different types of crystal structures with different dissolution mechanisms and rates.