Crosslink precursors for the dipteran puparium

During sclerotization of puparial proteins, tyrosine, lysine, and histidine were converted to highly basic aromatic metabolites. Peptides generated from the sclerotized cuticle with N-bromosuccinimide included the basic derivatives among the hydrolysis products. The absorbance maxima of the aromatic metabolites were 25 nm lower than those of the conventional tyrosyl peptides, with phenolic character poorly expressed or absent. Post-translational modification of the structural proteins preceded visual expression of tanning because aromatic conjugates also were present prior to pupariation. These results are consistent with a crosslinking mechanism favoring covalent bonding between protein chains.