Methyl purple, an exceptionally sensitive monitor of chloroplast photosystem I turnover: physical properties and synthesis.

Methyl purple is a recently introduced quinone-imide redox dye that has been shown to be an exceptionally sensitive monitor of photosystem I activity in chloroplasts. This compound has a wide range of potential applications for studies of photosynthetic electron transfer and photosynthetic bioenergetics, but the physical properties of the compound must first be established rigorously. The specific molar absorption coefficients of both the anionic and protonated forms of methyl purple have been determined. The oxidation-reduction midpoint potential of methyl purple over the pH range 3 to 12 was also determined by polarographic methods, and the effect of pH on the visible absorption spectrum is reported. A detailed procedure for the synthesis of methyl purple is given.