Insulin aggregation starts at dynamic triple interfaces, originating from solution agitation

The consequences of agitation on protein stability are particularly relevant to therapeutic proteins. However, the precise contribution of the different effects induced by agitation in pathways leading to protein denaturation and aggregation at interfaces is not entirely understood. In particular, the contribution of a moving triple line, induced by the sweeping of a solution meniscus on a container wall upon agitation, has only been rarely assessed. In this article, we therefore designed experimental setups to analyze how mixing, shear stress, and dynamic triple interfaces influence insulin aggregation in physiological conditions. This has been achieved by controlling agitation speed, shear stress, and the extension of triple interfaces in order to shed light on the contribution of different agitation-induced effects on insulin aggregation in physiological conditions. We demonstrate that strong agitation is necessary for the onset of insulin aggregation, while the growth of the aggregates is sustained even under weak agitation. Kinetic insulin aggregation studies in conditions of intermittent wetting show that the aggregation rate correlates with the amount of dynamic triple interfaces that the proteins are exposed to. Finally, we demonstrate that the triple line, where the protein solution, the air, and a hydrophobic surface meet constitutes a preferential early aggregation site.