Enhancing the T -> R transition of insulin by helix-promoting sequence modifications at the N-terminal B-chain

Structurally, the T-->R transition of insulin mainly consists of a rearrang ement of the N-terminal B-chain (residues B1 - B8) from extended to helical in one or both of the trimers of the hexamer, The dependence of the transi tion on the nature of the ligands inducing it, such as inorganic anions or phenolic compounds, as well as of the metal ions complexing the hexamer, ha s been the subject of extensive investigations. This study explores the eff ect of helix-enhancing modifications of the N-terminal B-chain sequence whe re the transition actually occurs, with special emphasis on hi-capping, In total 15 different analogues were prepared by semisynthesis, 80% of the hex amers of the most successful analogues with zinc were found to adopt the T3 R3 state in the absence of any transforming ligands, as compared to only 4% of wild-type insulin. Transformation with SCN- ions can exceed the T3R3 st ate where it stops in the case of wild-type insulin. Full transformation to the R-6 state can be achieved by only one-tenth the phenol concentration r equired for wild-type insulin, i.e. almost at the stoichiometric ratio of 6 phenols per hexamer.