Intramolecular hydrogen bonding in hydroxylated semiquinones inhibits semiquinone-Mg2+ complex formation

Complex formation between the semiquinones of 5,8-dihydroxynaphtho-1,4-quinone, NZQ(.-), 1,4-dihydroxyanthracene-9,10-dione, QNZ(.-), benzo-1,4-quinone, BQ(.-), and phenanthraquinone, PHQ(.-), and Mg2+ was studied utilizing EPR spectroscopy. Weighted average EPR spectra between those corresponding to the uncomplexed and complexed semiquinones were observed for NZQ(.-), QNZ(.-) and BQ(.-) while these species were observed simultaneously for PHQ(.-). The semiquinones NZQ(.-), QNZ(.-) and BQ(.-) behave as weak Mg2+ chelators while PHQ(.-) chelates this cation much more strongly (binding constant=(1.1 +/- 0.5)x10(3) dm(3) mol(-1)). The weak binding of Mg2+ by NZQ(.-) and QNZ(.-) is in contrast with the large complex formation constants between the parent quinones NZQ and QNZ and different metal cations. This apparent paradox is explained by the strong intramolecular hydrogen bonding existing in NZQ(.-) and QNZ(.-).