The effects of the trinitrophenylation of the amino groups of glucagon on its conformational properties and on its ability to activate rat liver adenylyl cyclase.

Trinitrophenyl groups have been specifically introduced into the alpha- and/or the epsilon-NH2 groups of glucagon by reaction with trinitrobenzenesulfonic acid. Introduction of this group into the epsilon-NH2 position of the hormone leads to an apparant increase in the helical content as measured by circular dichroism, while substitution on the alpha-NH2 position causes little change in this property. The usefulness of the trinitrophenyl group for the study of intramolecular singlet excitation transfer from tryptophan is suggested. The pK and reactivity of the amino groups, as measured by the pH dependence of the rate of reaction with trinitrobenzenesulfonic acid, showed that the two amino groups of glucagon have similar properties to those of small model peptides. The trinitrophenyl-glucagon derivatives have little or no activity in stimulating adenylyl cylase of rat liver. By comparison with previously reported results, this demonstrates that the effect of chemical modifications of the amino group on the biological activity of glucagon depends critically on the group which is introduced.