What levels of protein structure are affected when chemically straightening hair?

Why doesn't hair stay straight forever after this treatment?

The levels of protein structure that are most affected by chemically straightening the complex of protein that makes hair would be the quaternary and tertiary levels. This is because in this process, one would be disrupting many of the interactions that would be occurring between the R groups of the protein, including hydrogen bonding and other van der waals forces.

These interactions can affect more than 1 polypeptide chain as well and thus the quaternary is also affected.

Hair will not remain straight as after the reagent or other substance is removed the hydrogen bonds will again reform and the R groups will orient themselves to maximize interactions.