Induced circular dichroism and mode of binding of acridine orange adsorbed on β‐form poly(S‐carboxyethyl‐L‐cysteine) in aqueous solutions

Abstract

AbstractThe absorption spectra and circular dichroism (CD) have been measured for aqueous solutions of acridine orange of a constant concentration, [D] = 5 × 10−5M, mixed with poly(S‐carboxyethyl‐L‐cysteine) in various mixing ratios, [P]/[D], ranging from 330 to 11, at different pH. The absorption spectra of the dye–polymer solutions are hypochromic, and the main band is located at 470 nm, accompanying a shoulder at 500 nm. At alkaline pH, no CD is induced in the visible region. At neutral and acidic pH, where the polymer is in the β‐conformation, CD is induced in the visible and near‐uv regions. A pair of CD bands is located at the region around 450 nm, when the pH is around the neutrality, while it appears at the region around 500 nm at acidic pH. Thus, the optically active species of bound dye changes from dimer to monomer on lowering the pH. These species form dissymmetric arrays along a polypeptide chain. The fraction of bound dye forming dissymmetric sequences is not high, but most of bound dye is adsorbed randomly on the ionized carboxyl groups of polypeptide chain and gives rise to hypochromism only. A dissymmetric structure of dye–polymer complexes is presented, in which the polymer has the β‐conformation and the dye cations, either dimeric or monomeric, bind to its side chains, in such a way that the longer axes of molecular planes of bound dye form a two‐fold, right‐handed helix along the extended polypeptide chain. A zeroth‐order calculation of CD based on the coupled oscillator model leads to the result that each dissymmetric array of dye consists, on the average, of two dimeric or monomeric cations. This low number of bound cations in a dissymmetric array and the large fraction of randomly adsorbed dye suggest that the hydrophobic interaction of dye with the polymer is strong, so that dye cations are adsorbed sparsely on both sides of the extended polypeptide chain.