Cobalt probing of structural alternatives for insulin in solution

Inorganic anions and phenolic compounds make the subunits of insulin hexamers undergo the T----R transition whereby the extended N-terminal B chain becomes helical and the octahedral metal coordination tetrahedral. The role of the metal ions is permissive. With cresol the transition is also undergone by metal-free hexamers. For coordinative reasons only zinc insulin can be transformed by moderate concentrations of inorganic anions. At higher concentrations and particularly with cresol transformation is also possible if Zn2+ is replaced by other metal ions. Owing to its d--d transitions in the visible cobalt lends itself as a spectroscopic probe for studying the interdependence of transformation and coordination. The transformation-related change in coordination is reflected in both the isotropic absorption and the CD spectrum. Cresol achieves T6----R6 transformation whereas that induced by SCN- ions is T6----T'3R3 with only the axial metal-binding site being realized in the R3 trimer. The spectral effects of the transformation of the two trimers are not additive; an extra contribution seems to be indicative of trimer/trimer interaction. Oxidation of 2 Co2+ insulin to a certain extent affects the structure of insulin; a characteristic positive band appears at 251 nm. Because of its extremely stable and exclusively octahedral complexes the Co3+ ion most strongly withstands transformation. The oxidation of tetrahedrally liganded Co2+ ions in R3 trimers proceeds with reduced velocity. Independent transformation of the Zn2+ trimers is possible in Zn2+/Co3+ metal hybrids of insulin.