Conformational Properties of Cyanogen Bromide-cleaved Glucagon

The circular dichroism of the cyanogen bromide peptide of glucagon, measured in dilute aqueous solution, in chloroethanol, and in urea is similar to that of glucagon. However, in more concentrated aqueous solutions the cyanogen bromide peptide does not rapidly aggregate to structures of higher helical content as does glucagon. Thus this property of glucagon is dependent on the presence of the two COOH-terminal amino acids. Intramolecular quenching of tryptophan fluorescence by tyrosinate is observed in the cyanogen bromide peptide. From this quenching the approximate distance between aromatic residues is calculated and it is concluded that the peptide is in a compact conformation. The lack of quenching in unmodified glucagon is interpreted as due to the fixed angle between the aromatic residues. Nuclear magnetic resonance studies at 250 MHz reveal small differences in the chemical shift of several of the protons between the cyanogen bromide peptide in water and the random coil spectrum. These differences suggest that the conformational change between a randomly coiled peptide and the glucagon monomer in water involves several sections of the molecule. The frictional coefficient calculated from sedimentation velocity results along with the fluorescence and nuclear magnetic resonance data suggests that the cyanogen bromide peptide, like glucagon, folds into a compact, globular structure in dilute aqueous solution.