The Aromaticity of Aromatic Rings in Pyrroloquinoline Quinone Isomers

Pyrroloquinoline quinone (PQQ) is a small, redox active molecule that serves as a cofactor forseveral bacterial dehydrogenases.PQQ has four isomers with three aromatic rings (1-3 in Fig1.) [1]. The aromaticity of rings can affect the biological properties of compounds. In thiswork, the aromaticity of rings were estimated by the aromatic fluctuation index (FLU). Thegeometries of compounds were optimized at the B3LYP/6-31G(d,p) level of theory using theGaussian 09 program [2]. The FLU index is in a sense based on the ρ values calculated usingthe AIM analysis. The results show that the trend in the aromaticity of rings 1–3 in PQQisomersis3 > 1 > 2. The rings 3 and 1 are six and five membered rings, respectively and generally,where the former is more aromatic than latter. The ring 2 is less aromatic than other tworings, because two electron-withdrawing group, C=O, disturb the charge distribution of ring 2.