Formation and Structure of Gels and Fibrils from Glucagon

Glucagon in acid solution aggregates to produce gels and ultimately fibrils of β‐chains. The kinetics of the aggregation have been followed by viscometry, and are found to exhibit a sigmoidal profile with a long lag phase, associated with the formation of nuclei for polymerisation. This has been demonstrated by the elimination of the lag when fresh solutions are seeded with a preformed glucagon gel. The aggregation is promoted by salt, increased pH (within the acid range of solubility) and increased temperature up to 30°. The addition of 5% of dioxan completely inhibits the aggregation. The interaction has evidently a large hydrophobic driving force, and is opposed by coulombic repulsion. The latter effect can be minimised by acetylation of the amino groups, when the polymerisation is greatly accelerated. This also occurs when only the terminal amino group is blocked by carbamylation. By contrast esterification of the carboxyl groups completely prevents aggregation. At the working pH this does not involve a change in charge, and thus appears to be a specific steric effect. Optical rotatory dispersion shows no conformational difference between native glucagon and the derivatives. The transition from a gel to a fibrillar form is promoted by salt and increased temperature. A possible explanation for the appearance of the fibrillar form in the electron microscope is that it contains tubular structures 50–70 Å in diameter, with about 40 extended chains around the circumference lying in antiparallel manner along the axis.