Electronic spectrum, optical activity, and structure of the acridine orange complex with poly‐α,<scp>L</scp>‐glutamic acid

Abstract

Abstract The electronic absorption and circular dichroism spectra of the complex formed by acridine orange with poly‐α, L ‐glutamic acid in the α‐helix conformation have been measured in aqueous solution over a range of glutamate residue‐to‐dye ratios. Three Cotton effects (circular dichroism bands) associated with the long wavelength absorption band of acridine orange at 4950 A. are induced by complex formation between the dye and the polypeptide, and further circular dichroism bands are observed in the ultraviolet region associated with the 2700 A., but not with the 2950 A. absorption band of the dye. The induced optical activity is found to be relatively insensitive to the glutamate residue‐to‐dye ratio and to be more dependent upon the ionic strength of the solution. By Measuring the circular dichroism spectrum of the complex in aqueous solution under streaming conditions with the light propagated along the direction of flow the observed circular dichroism bands are assigned to electronic transitions polarized parallel or perpendicular to the axis of the polypeptide α‐helix. From the spectroscopic data it is inferred that the dye aggregate in the L ‐PGA–AO complex has the form of a left‐handed superhelix bound to the core of the right‐handed α‐helix of poly‐α, L ‐glutamic acid. It is shown that the longer and the shorter of the in‐plane axes of the dye molecule are probably orientated respectively at a small angle, and radially, with respect to the axis of the α‐helix in the complex.