Robert K. Boggess

Fe(III) and Cu(II) conalbumin visible circular dichroism spectra.

The single polypeptide chain of conalbumin strongly binds two Fe(III) or two Cu(II) ions to yield intense absorption in the visible region similar to that shown by the related protein transferrin. Comparison of the metal-ion-binding sites in the two proteins is made by exploiting the sensitivity to ligand geometry of circular dichroism (CD). For the Fe(III) proteins strong similarities of the CD spectra outweigh marginal differences. For Cu(II) conalbumin an additional negative extremum near 506 nm appears between two positive ones at 634 and 410 nm suggesting greater subtraction of oppositely signed CD components leading to lesser magnitudes for the two positive peaks than are found in Cu(II)-transferrin. The two Fe(III)-binding sites within conalbumin are compared by noting the strong similarities of the CD and MCD of proteins with Fe(III) in one site and Ga(III) in the other site, and vice versa, with the protein containing Fe(III) in both sites. Due to features of the amino acid sequences of the single protein chains, the four strong metal ion binding sites in conalbumin and transferrin cannot be identical in all particulars, yet CD spectra of their metal ion complexes are closely similar. From a study of model phenolate complexes and the wavelength maxima of visible absorption in the Fe(III), Cu(II), and Co(III) proteins near 465, 440, and 405 nm, respectively, these strong absorption bands are identified as ligand to metal ion electron-transfer transitions. It is suggested that tyrosyl residues are the donors in the electron transfer transitions and that they lock in the metal ions after being keyed into position by binding of bicarbonate or other anions.

Co(II) interactions with penicillamine and cysteine

Abstract Penicillamine, even in excess, forms only bis complexes with hexacoordinate Co(II) according to potentiometric and spectrophotometric results. L -cysteine appears to behave similarly. Extensive visible circular dichroism results are reported for mixed ligand complexes of Co(II), histidine, and either penicillamine or cysteine with the ligands in a variety of configurations. The CD sign pattern and intensities are determined by the sulfhydryl compound with the histidine enantiomer having a secondary effect. It is shown that the geometry about Co(II) in yeast aldolase cannot be tetrahedral. The possibility that the Co(II) in the enzyme is hexacoordinate and bound to a sulfhydryl group is considered.