Interaction of α‐poly‐<scp>L</scp>‐lysine and ε‐poly‐<scp>L</scp>‐lysine with methyl orange and its homologs in aqueous solution: Results from dialysis and spectroscopic measurements

Abstract

Abstract The interaction of α‐poly‐ L ‐lysine and ε‐poly‐ L ‐lysine with methyl and ethyl orange was studied by equilibrium dialysis and spectroscopic methods. The results of the dialysis measurements indicated that the extent of binding by ε‐polypeptide is substantially higher than that by α‐polypeptide, despite the much greater molecular weight of the latter. This difference in binding affinity was interpreted in terms of the increased conformational adaptability of ε‐polypeptide because of its highly flexible structure. Furthermore, ε‐polypeptide exhibited strong cooperative binding. In addition, the effect of the successive addition of α‐ and ε‐polypeptides on the absorption spectra of methyl and ethyl orange was investigated. The addition of α‐polypeptide with a molecular weight of 400,000 produced a new absorption peak at a shorter wavelength, due to the stacked dye molecules on the polypeptide chain, whereas that of ε‐polypeptide did not. From the results of spectroscopic measurements a possible mode of interaction between these two polypeptides and the small molecule is discussed.