Circular dichroism of iron, copper, and zinc complexes of transferrin

Abstract

The absorption banda of copper-transferrin are optically active with well-defined positive dichroic bands in the visible range having ellipticities of [ θ] 620 = 3,620 deg cm 2 dmole −1 and [ θ] 426 = 6,700 deg cm 2 dmole −1 in terms of bound copper. In iron-transferrin there are also two bands; a negative ellipticity band with [ θ] 455 = −16,000 deg cm 2 dmole −1 and a positive ellipticity band with [ θ] 360 = 6,000 deg cm 2 dmole −1 in terms of bound iron. Additional copper binding to iron-transferrin produces an absorption band at 675 nm with an associated negative ellipticity band. Zinc binding to apotransferrin, 2 moles per mole, affects 1 mole of tyrosine per mole of Zn 2+ bound, and the generated tyrosinate spectrum is optically active. There is optical activity in apotransferrin in the 280 nm region attributable to tryptophan, and optical activities in the 270 and 230 nm regions suggest possible disulfide contributions. Neither the tryptophan optical activity nor the optical activities in the region of 270 nm and 230 nm are affected by metal binding. Apotransferrin has an estimated α-helical content of 17–18%, and there is no observable change in this value when 2 moles of Cu 2+ or Zn 2+ are bound per mole of protein.