The Interaction of Azo-Dyes with Proteins. II. The Ultraviolet Spectral Changes Occurring on adding Protein to Aqueous Solutions of Methyl Orange

Abstract

An examination of the ultraviolet spectra of methyl orange and related dyes in various solvents has indicated that the spectral changes occurring on adding proteins to an aqueous solution of methyl orange are not the result of strong electrostatic interactions (PNH₃⁺ . . . ^-0₂SR) as suggested by earlier workers. It is proposed that the interaction of methyl orange with protein generally produces two types of spectral effect : (i) an absorptivity increase and bathochromic shift resulting from electronic interaction of dimethylamino, azo, and/or sulphonate groups in the dye with the protein, and (ii) an absorptivity decrease and hypsochromic shift resulting from out-of-plane twisting of the dye's conjugated system by the establishment of the above electronic interactions. In most cases the second effect predominates, giving the absorptivity decrease and hypsochromic shift usually observed.