What are the different ways to fix hydrolyzed proteıns and peptides inside the hair?

[u/No-Faithlessness1786]

so that they last for several washes and not just 2 or 3 shampoos? I know formaldeĥyde does this but maybe it's not the only one

[u/sudosussudio]

The text Hair and Hair Care edited by Dale Johnson says you can "quaternize" them for greater substantivity (last for more washes)

Since protein molecules are too large to penetrate normal hair, proteins are partially hydrolyzed to a peptide mix or completely hydrolyzed and used as the individual amino acids. Johnsen (43) found that protein fractions with a molecular weight of 1000 or less were the most substantive to hair.

Since proteins contain both positively and negatively charged R groups, they exhibit amphoteric behavior. Commercially available proteins have isoelectric points in the pH range of 4.0 to 6.0, limiting the practical range for true cationic behavior. Also, the amount of positive charge on the protein is small.

By covalently grafting a quaternary ammonium group onto a protein, the cationic properties are greatly enhanced. Protein fractions of higher molecular weights become substantive to hair. Quaternization increases the isoelectric point considerably; thus, quaternized proteins retain cationic properties over a wider pH range increasing their versatility in formulations. Jones (44) used radioactive labeling techniques to compare deposition on hair of a quaternized protein versus its parent protein and found that twice as much of the quaternized protein deposits on hair.