The thermodynamic stability of the polymorph S (salt condition) and NS (non-salt) was similar when assayed in the conditions at which they were formed (Figure ). However, the polymorph NS was more stable than polymorph S when measured in the buffer conditions in which the latter was formed (Figure ). The result was unexpected since the kinetics of fibril formation was faster at high salt, both in terms of nucleation and elongation (SI figure ). In contrast, the aggregation of αSyn in the absence of salt was slow and the reaction had to be promoted by sonication during the time course of reaction to achieve complete conversion of monomers to fibrils. Analysis of the aggregation kinetics in the presence of preformed seeds revealed that elongation saturates already at low μM concentrations of monomer, which might explain the need for sonication which promotes formation of new elongationcompetent fibril ends and greatly accelerates the overall conversion of monomer to fibrils (Error! Reference source not found. b, right panel). To rationalize all our observations, we consider that αSyn can adopt different conformations during fibril formation . High ionic strength increases the screening of charge repulsion between monomers and fibrils . Consequently, the fibril conformations with lowest energy barrier of nucleation/elongation are formed first, i.e., the structural polymorphism is dominated by kinetic factors. This might lead to structures that are not optimised to avoid electrostatic repulsion, because such avoidance is not necessary under highly screened conditions. In contrast, the high electrostatic repulsion in the absence of salt creates an energy barrier leading to slower aggregation.