Potassium glutamate (KGlu) is the primary Escherichia coli cytoplasmic salt. After sudden osmotic upshift, cytoplasmic KGlu concentration increases, initially because of water efflux and subsequently by K + transport and Glu − synthesis, allowing water uptake and resumption of growth at high osmolality. In vitro, KGlu ranks with Hofmeister salts KF and K 2 SO 4 in driving protein folding and assembly. Replacement of KCl by KGlu stabilizes protein-nucleic acid complexes. To interpret and predict KGlu effects on protein processes, preferential interactions of KGlu with 15 model compounds displaying six protein functional groups—sp 3 (aliphatic) C; sp 2 (aromatic, amide, carboxylate) C; amide and anionic (carboxylate) O; and amide and cationic N—were determined by osmometry or solubility assays. Analysis of these data yields interaction potentials ( α -values) quantifying non-Coulombic chemical interactions of KGlu with unit area of these six groups. Interactions of KGlu with the 15 model compounds predicted from these six α -values agree well with experimental data. KGlu interactions with all carbon groups and with anionic (carboxylate) and amide oxygen are unfavorable, while KGlu interactions with cationic and amide nitrogen are favorable. These α -values, together with surface area information, provide quantitative predictions of why KGlu is an effective E. coli cytoplasmic osmolyte (because of the dominant effect of unfavorable interactions of KGlu with anionic and amide oxygens and hydrocarbon groups on the water-accessible surface of cytoplasmic biopolymers) and why KGlu is a strong stabilizer of folded proteins (because of the dominant effect of unfavorable interactions of KGlu with hydrocarbon groups and amide oxygens exposed in unfolding).
