Aggregation of acridine orange: crystal structure of acridine orange tetrachlorozincate 2C17H19N3-2HCl-ZnCl2-CH3COOH.

The crystal structure of the biological stain, "acridine orange," has been determined. This compound, when crystallized from ethanol, is shown to be a zinc chloride double salt of acridine orange, containing, in addition, acetic acid of crystallization. These additional components are residuals from the method of preparation of acridine orange. This complex, 2 acridine orange-2HCl-ZnCl2-CH3COOH, (2C17H19N3-2HCl-ZnCl2-CH3COOH) crystallizes in the monoclinic space group P21, a = 9.965 (2), b = 21.507 (6), c = 9.645 (2) A, beta = 113.98 degrees (2), V = 1888.7 (8) A3, FW = 800.0, Z = 2, DX = 1.41 g-cm-3, Dobs = 1.43 (9) g-cm-3. Three-dimensional diffraction data were collected with CuKalpha radiation, and the structure refined to R = 0.065 for 1885 observed reflections. In the crystal structure hydrogen bonds are formed, via the protonated nitrogen atom of the central rings of two acridine orange cations, to two chloride ions in a ZnCl42- tetrahedral grouping. These two acridine orange molecules are stacked in parallel planes, approximately 3.4 A apart, with the long axes of the ring systems inclined at 26.5 to each other. Thus an apparent dimerization of the acridine, orange is facilitated by the anions present, resulting in the complex studied. The two -N(CH3)2 groups of each acridine orange molecule are not protonated in this crystalline form. The mode of molecular packing found here may be relevant to models for the external stacking of acridine orange around a DNA molecule. The importance of removing any zinc salt from acridine orange preparations prior to aggregation studies is stressed.