For our proof-of-principle study, we have chosen a protein system, PI3K-SH3, which displays limited fibril polymorphism under the conditions of our study . We analyzed the elongation kinetics within the linear growth regime (Figure ). Under the studied conditions, acidic pH and moderate ionic strength, PI3K-SH3 net charge is approximately +11 and electrostatic repulsion is expected to contribute significantly to the free energy barrier.