Understanding protein thermodynamics as it occurs inside cells is a fundamental goal of biophysics, and, from a practical point of view, will facilitate the design and improvement of protein-based drugs and catalysts. By measuring the temperature dependence of protein stability inside Escherichia coli cells, we show, contrary to predictions, that proteins are not necessarily stabilized inside cells compared with buffer alone. We also show that crowding-induced charge–charge interactions slow folding because of preferential interactions with the unfolded ensemble, and reducing these interactions increases protein stability
