First Experimental Structure of a 1:1 Metal Complex with a PQQ Cofactor Derivative outside Dehydrogenase Enzymes

Qualitatively similar metal coordination as in PQQ-dependent bacterial dehydrogenases was observed for the PQQ triester in the model complex I, although the unusual five-coordinate copper(I) center is smaller and softer than the Ca2+ ion of the native enzymes. The ambidentate PQQ thus prefers coordination through the O(5)/N(6)/O(7') atoms even without additional support from the protein scaffold.