Interaction of merocyanine 540 with cations of physiological solutions

The effect of H+, Na+, Ca2+, Mg2+ on merocyanine 540 (MC 540) absorption and fluorescence spectra was determined. While MC 540 showed two absorption peaks in water (at 535 nm, the monomer peak; at 503 nm, the dimer peak) the addition of salts to MC 540 aqueous solutions resulted in the formation of a new absorption peak at 517 nm and the disappearance of both MC 540 “water” bands. MC 540 fluorescence spectra showed only one peak in pure water (at 573 nm) as well as in salt and HCl aqueous solutions (at 571 nm). The addition of salts did not change the shape of the fluorescence spectra but substantially quenched the fluorescence intensity. The effect of salts reflected principally differences in their cationic charge. These spectral changes were discussed in terms of interaction between the dye anion MC 540− and the aqueous proton H+ followed by dimerization of the protonated merocyanine dye. This assumption was supported by the low pH form of MC 540. A pK value of 1.6 between the neutral protonated form and the anionic form of MC 540 was determined.