  Arginine has been a mainstay in biological formulation development for
decades. To date, the way arginine modulates protein stability has been widely
studied and debated. Here, we employed a hydrophobic polymer to decouple
hydrophobic effects from other interactions relevant to protein folding. While
existing hypotheses for the effects of arginine can generally be categorized as
either direct or indirect, our results indicate that direct and indirect
mechanisms of arginine co-exist and oppose each other. At low concentrations,
arginine was observed to stabilize hydrophobic polymer collapse via a
sidechain-dominated direct mechanism, while at high concentrations, arginine
stabilized polymer collapse via a backbone-dominated indirect mechanism. When
adding partial charges to sites on the polymer, arginine destabilized polymer
collapse. Further, we found arginine-induced destabilization of a model virus
similar to direct-mechanism destabilization of the charged polymer, and
concentration-dependent stabilization of a model protein similar to the
indirect mechanism of hydrophobic polymer stabilization. These findings
highlight the modular nature of the widely used additive arginine, with
relevance in the design of stable biological formulations.
