The most important step on a reaction pathway is the transition state (TS) of the ratelimiting step. This state, by definition, is high energy and short-lived, which makes structural characterization difficult. Φ-value analysis, a technique which combines protein engineering, and kinetic and thermodynamic experiments, enables characterization of the TS and folding mechanisms and pathways of proteins . Amyloid elongation, as opposed to folding of small proteins, is a multi-step process involving diffusion to the end of the fibril, formation of an encounter complex, loss of the native fold, and conformational sampling to adopt the final amyloid fold . The processes may occur sequentially or in parallel and can be associated with individual free energy barriers. The elongation kinetics will be determined by the highest barrier, which will depend on the reaction conditions . For example, the aggregation rate initially rises linearly with concentration (linear growth regime) before reaching a concentration-independent plateau (saturated growth regime), indicating the transition to conformational sampling-limited kinetics.