Stabilizing effect of amino acids on protein and colloidal dispersions.

Amino acids (AAs) have a long history of being used as stabilizers for biological media. For example, they are important components in biomedical formulations. The effect of AAs on biological systems is also starting to be appreciated. For example, it is believed that water-stressed cells increase the levels of AAs to prevent protein aggregation. Several hypotheses have been put forward regarding their function, ranging from water-structuring to hydrotropic to specific effects such as stabilization against misfolding, yet it is not known whether their stabilizing function is protein specific or a generic colloidal property. Here we deduce that AAs possess a new and broad colloidal property: they stabilize patchy nanoscale colloids by adsorbing onto their surfaces through weak interactions. We demonstrate this general property by careful experimental evaluation of the stabilizing effect of AAs on dispersions of various proteins, plasmid DNA and non-biological nanoparticles. Furthermore, we develop a theoretical framework that captures this phenomenon and experimentally corroborate several new broad theoretical implications that apply beyond AAs. In vivo experiments further demonstrate that the addition of 1 M proline to insulin doubles its bioavailability in blood. Overall, our results indicate that the role of small molecules is as important as that of ionic strength and should always be reported in biophysics experiments.