Cystine-Alkali Reactions in Relation to Protein Crosslinking

Abstract

The formation of lysinoalanine, lanthionine and β-aminoalanine in proteins, through the alkaline degradation of cystine residues to dehydroalanyl residues, is discussed. The formation of these amino-acids, during the dry heating of proteins, is also reported. It is shown that, during alkaline degradation of protein-bound cystine, amino or thiol-containing compounds present in the alkaline liquor can compete with protein-bound amino and thiol groups for addition to the dehydroalanyl residues formed as intermediates in the reaction. Results showing the relative importance of such competitive reactions are given. Evidence given indicates the one protein-bound cystine residue can, on alkaline degradation, yield two dehydroalanyl residues. The extent of addition of a series of alkylamines, to the transient dehydroalanyl residues, is shown to be dependent on the structure of the pendant alkyl group. Addition is governed by two factors, activity of the addendum, and the positioning of possible addenda in relation to the protein-bound dehydroalanyl residues, when the latter are formed. Dyeing experiments with alkylamine-treated wools show that the pendant alkyl groups influence dyeability. The results indicate that hydrophobic interactions in the modified protein have as great an effect on dyeability as do covalent crosslinks and ionic groups. Dye uptake is determined by the shape and size of the pendant alkyl groups introduced during the protein modification, rather then the enhanced basicity caused by introduction of the basic sidechains. The possibility of introducing hydrophobic sidechains into proteins, as amide derivatives of protein-bound lysine residues, by simple dry heating, is speculated upon, in relation to dry-heating of a fat-containing protein.