Circular dichroism and ultraviolet studies of the cupric(II)- and nickel(II)- l-3,4-dihydroxyphenylalanine complex

Abstract

Extrinsic circular dichroism (CD) spectra in the wavelength region 200–600 nm were clearly exhibited by coexistent solutions of Cu(II) or Ni(II) and l-3,4-dihydroxyphenylalanine ( l-DOPA) in 10 mM phosphate buffer (pH 7.4) at 36°C. The Cu(II)- l-DOPA complexes in the buffer were not affected by the oxidative reaction, were slowly dissociated, and then simulataneously reformed into new optically inactive complexes in proportion to the incubation time. The addition of poly- l-lysine (PLL) to the Cu(II)- l-DOPA solution indicated the presence of the oxidative product of DOPA quinone under the same conditions. The formation of Ni(II)- l-DOPA complexes, on the other hand, was a two-step process and, coexistence with PLL in the solution showed the same result as noted in the case of Cu(II). The effect of PLL on the oxidation of l-DOPA was much more secondarily structurally selective than asymmetrically selective.