Crystal structure of a calcium(II)–pyrrolo­quinoline quinone (PQQ) complex outside a protein environment

Pyrrolo­quinoline quinone (PQQ) is an important cofactor of calcium- and lanthanide-dependent alcohol de­hydrogenases, and has been known for over 30 years. Crystal structures of Ca–MDH enzymes (MDH is methanol de­hydrogenase) have been known for some time; however, crystal structures of PQQ with biorelevant metal ions have been lacking in the literature for decades. We report here the first crystal structure analysis of a Ca–PQQ com­plex outside the protein environment, namely, poly[[undeca­aqua­bis­(μ-4,5-dioxo-4,5-di­hydro-1H-pyrrolo­[2,3-f]quinoline-2,7,9-tri­carboxyl­ato)tricalcium(II)] dihydrate], {[Ca(3)(C(14)H(3)N(2)O(8))(2)(H(2)O)(11)]·2H(2)O}(n). The com­plex crystallized as Ca(3)PQQ(2)·13H(2)O with Ca(2+) in three different positions and PQQ(3−), including an extensive hydrogen-bond network. Similarities and differences to the recently reported structure with biorelevant europium (Eu(2)PQQ(2)) are discussed.