Mechanism of spectral changes of a carbocyanine dye with acidic polysaccharides and oligogalacturonides

Changes in the visible spectrum of a cationic carboeyanine dye in the presence of α (1–4) linked oligomers of d-galacturonic acid have been found to be dependent on the number of uronic acid residues in the molecule. Polygalacturonic acid caused a shift in the dye spectrum that was linearly proportional to the polymer concentration. Neither mono- nor digalacturonic acid had an effect on the dye spectrum. Tri- and tetragalacturonic acid caused spectral changes which were nonlinear with respect to oligomer concentration while penta- and hexagalacturonic acid showed concentration-dependent properties similar to polygalacturonic acid.The difference spectra with polygalacturonate and other acidic polysaccharides containing one anionic site per monosaccharide residue showed two absorption maxima in the region of 550 nm and 610 nm. All of the oligomers tested (containing 3 through 6 galacturonic acid residues) yielded only a single maxima for each in the region between 650 and 670 nm. This single maxima phenomenon was also observed with acidic polysaccharides having only one anionic site for every two monosaccharide residues (hyaluronic acid and chondroitin).