Arrangement of Acridine Orange in the Poly-α,l-glutamic Acid-Acridine Orange Complex

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Abstract
The temperature dependencies of the absorption and circular dichroism spectra of the poly-α,L-glutamic acid-acridine orange complex were observed at the temperatures in the range of 23–95 °C. The circular dichroism spectra were resolved into several Gaussian components by means of a computer online system. Thus, the circular dichroism spectrum can be resolved to five components. The shorter-wavelength bands, consisting of one positive and one negative element, were assigned to the transitions polarized parallel to and perpendicular to the axis of the α-helix of polypeptide respectively. On the other hand, the longer-wavelength bands, having equal magnitudes but signs opposite to each other, were assigned to the transition of the acridine orange dimer bound to polymer. The fifth band, at the shortest wavelength was uncertain. On the basis of these results, the arrangement of acridine orange molecules bound to poly-α,L-glutamic acid was discussed. Thus, two kinds of arrangements of acridine orange molecules were postulated. One is a left-handed super-helix around the right-handed α-helix of the polypeptide in the acidic pH region, and the other is a right-handed super-helix around the core of the α-helix in the alkaline pH region when the molar ratio of glutamyl residues to the dye is kept around unity. In these two complexes, there are dimeric acridine orange molecules bound to the polymer; in the latter complex, the super-helix consists of dimeric acridine orange molecules. Two types of arrangements of acridine orange molecules around the polypeptide were offered for the elucidation of the circular dichroism spectra of the complex of acridine orange with poly-α,L-glutamic acid in an aqueous solution.