A spectral investigation of the copper(II) complex of the antitumor compound bleomycin

A thorough spectral investigation of the copper(II) complex of the antitumor compound, bleomycin, has been carried out in solution employing optical, difference optical, electron spin resonance, and circular dichroism techniques. The optical spectrum of a pH = 7 solution of the 1:1 complex between copper(II) and bleomycin is characterized by a broad weak band in the visible region (λ_max = 610 nm) that cannot be resolved and intense ultraviolet bands at 317 (? = 2800), 327 (shoulder), 250 (? = 4700), and 257 nm (shoulder). The circular dichroism spectrum in the visible region shows the broad and weak visible absorption band contains at least three components (558, 675, and 880 nm) that are likely to be “d-d” in origin. The electron spin resonance spectrum is characteristic of a tetragonal d⁹ copper(II) system showing no rhombic distoritions at X-band frequencies (g_x = g_y ± 0.002). The spin Hamiltonian parameters for the pH = 7.0 solution corrected for second order effects are A_∥ = 177 × 10^?4 cm^?1, A_⊥ ? 15 × 10^?4 cm^?1, g_∥ = 2.214, g_⊥ = 2.039. Most interesting was the observation of extra hyperfine splitting due to endogenous nitrogen coordination in a 30% glycerol glass (A^N = 12.0 × 10^?4 cm^?1). That pattern is best interpreted as a seven-line sequence associated with three liganded nitrogens. A dramatic change in all spectral properties occurs when the pH of the copper(II)-bleomycin complex is lowered to 2.5. All these data taken together suggest a CuN₃O coordination complex in solution. Details and justifications as well as a discussion of the limitations of the interpretations are presented.