Interaction of Cresol Red Dye with Some Basic Amino Acids under Different pH Conditions

Abstract

Results of a spectroscopic study of the interaction of three basic amino acids, namely lysine (Lys), arginine (Arg), and histidine (His), with the dye cresol red (CR) in an aqueous medium at four different pH values (4.0, 6.0, 8.0, and 10.0) are presented in this paper. The dye was found to be in its monomeric form. Charged amino acids affect the peak height and wavelength of oppositely charged dye spectra. With the increase in pH, the anionic form (DH − ) of the dye is converted into the dianionic form (D 2− ) through the negatively charged group of the dye ( −SO − 3 ) and ionization of the hydroxyl group (−OH). The absorbance of the dianionic form is directly proportional to the pH of the solution. Lys and His in CR solution exhibit similar behavior while Arg behaves differently. The ratios of absorbance of D 2− to DH − bands at different pH values reveal that an increase in the concentration of the amino acids results in increased formation of the D 2− form due to the increase in the basicity of the medium. The values of pKa's confirm the formation of the D 2− form. The Gibbs free energy of ionization also supports the interaction between the ionized form of CR and the amino acids.