Extensive fibril contacts are required to overcome electrostatic repulsion between the fibril and monomer. Our experiments demonstrate that misfolding of a monomer at the fibril end (in the linear growth regime) proceeds in a clear sequence of events. The rate-limiting step is formation of the first β-sheet contacts for amino acids in the ranges 11-15 and 49-62 and the free energy barrier largely stems from electrostatic repulsion between the fibril end and incoming monomer (See discussion in Supporting Material). Once those contacts have formed, the molecule is sufficiently constrained that the remaining amino acids lock into place as an energetic downhill process. Thus, we can interpret the TS β-sheet structure as the minimal set of interactions capable of stabilizing the encounter complex against the electrostatic repulsion between the monomer and fibril end.