PH-DEPENDENCE OF MEROCYANINE-540 ABSORPTION AND FLUORESCENCE-SPECTRA

The effect of pH on the absorption and fluorescence spectra of merocya nine 540 has been determined. At neutral pH, a double peaked (at 500 a nd 534 nm) absorption band and a single fluorescence peak (at 577 nm) of anionic merocyanine 540 were recorded. The spectral shape and the p eak positions were almost unchanged over the pH range 1.7-7.6 at a tem perature of 23 +/- 2 degrees C. Decreasing the pH from 5.5 to 1.7 caus ed only a reversible diminution of the absorption spectra and quenchin g of the fluorescence intensity without the appearance of any other ne w spectral band. The dye was seen to precipitate with an increase in a cidity. Increasing the pH above 7.6 in aqueous solutions of merocyanin e 540 resulted in the irreversible formation of a new, broad band in b oth the absorption (at 390 nm) and fluorescence (at 500 nm) spectra. T hese irreversible changes could be due to an attack on the merocyanine molecules by hydroxyl ions. (C) 1997 Published by Elsevier Science B. V.