Circular dichroism of ferriprotoporphyrin IX-morphine complexes in aqueous solution

Complexes of ferriprotoporphyrin IX (FP) with (-)-morphine exhibit large optical activity in aqueous solution. Several CD bands of different signs were recorded under different conditions in the wavelength range of 300 to 450 nm. The largest CD band, centered at about 364 nm, showed molar ellipticities of about -5 x 10(5) deg.cm2.dmol-1 based on FP (rotational strength of about -3 DBM). An optimum pH range of 7.4 to 7.8 with regard to optical activity of the complex has been observed. Concomitantly with the formation of optically active complexes, large aggregates are being formed as determined by ultracentrifugation. A mole ratio of 1 between FP and morphine in the complex was estimated from both CD titration and measurement of the amounts of aggregate formed upon variation of the mole fraction of the complex components. Derivatives of morphine such as heroin or codeine do not form optically active complexes and aggregates under similar conditions. As in the analogous FP-quinine and FP-quinidine complexes investigated previously, the optical activity observed is considered to originate at least in part from optical interactions between FP molecules arrayed chirally in the FP-(-)-morphine aggregates.