Free radical reactions of MTDQ and its effect on biological membrane

The effect of 6,6'-methylene-bis-2,2,4-trimethyl-1,2-dihydroquinoline (MTDQ) and its water soluble species (MTDQ-DA) was studied in biological membrane and model systems using EPR spectroscopy for detecting molecular motion and radical formation. Both compounds influenced the rotational mobility of maleimide spin labels attached to proteins of the nerve membrane: the addition of MTDQ or MTDQ-DA induced an increase of the rigidity of the membrane in the environment of the attaching sites. The reaction of MTDQ-DA with hydroxyl and superoxide free radicals showed that this compound was also a competitive OH and superoxide free radical scavenger. The reaction rate constant of the formation of MTDQ-DA free radical was k = (4.0 +/- 0.5) x 10(9) M-1 s-1 in the hydroxyl free radical generating system. Simulation of EPR spectra supported that MTDQ-DA free radical was very likely a stable nitroxide free radical.