Many cells synthesize significant quantities of zwitterionic osmolytes to cope with the osmotic stress induced by excess salt. In addition to their primary role in balancing osmotic pressure, these osmolytes also help stabilize protein structure and restore enzymatic activity compromised by high ionic strength. This osmoprotective effect has been studied extensively, but its electrostatic aspects have somehow escaped the mainstream. Here, we report that, despite their overall neutrality, zwitterions may dramatically affect electrostatic interactions in saline solutions of biological relevance. Using atomic force microscopy, we study the combined effect of osmolytes and salts on electrostatic interactions between two negatively charged silica surfaces in mixtures of five salts (NaCl, KCl, CsCl, MgCl2, and CaCl2) and five zwitterionic osmolytes (betaine, proline, trimethylamine N-oxide, glycine, and sarcosine) as a function of solutes concentration and pH. All osmolytes are found to counteract the screening effect of salt on electrostatic repulsion, albeit to a different extent. They do so by both increasing the screening length shortened by added salts and by desorbing bound protons and cations, hence enhancing the negative surface charge. Both effects are traced to the osmolytes’ higher molecular polarizability compared with water. In addition to this direct effect on the solution’s dielectric constant, we identify an osmolytic Hofmeister effect with the more hydrophobic osmolytes more efficiently desorbing weakly hydrated cations from weakly hydrated silica and the less hydrophobic osmolytes better desorbing strongly hydrated cations from strongly hydrated silica. The combined results shed light on Coulomb interactions in a more realistic model of the cytosol, a relatively unexplored territory.

While recent studies clarify the effect of osmolytes on Coulomb interaction at elevated concentrations of salt, little is known about the way osmolytes affect the same interaction in cryoprotection. In this Communication we explore the effect of cold on the interaction between two charged surfaces immersed in ternary solution containing salt and osmolyte and find that the effect of cold parallels that of excess salt, i.e., low temperatures increase adsorption of salt counterions to the surface, thus neutralizing it. Two osmolytes, proline and glycine-betaine, are then shown to recharge the surface by releasing the adsorbed counterions. The ability to counteract effects of both cold and excess salt on Coulomb interactions renders these known osmolytes cryoprotectants as well as osmoprotectants, explaining why plants, fish, insects and bacteria accumulate them in response to either drought or cold stress.

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Non-contact, frequency modulated atomic force microscopy is often operated in the constant-frequency mode to obtain a height map of the sample’s surface. Once linearized, the dynamics of the constant-frequency closed-loop system are reduced to a single transfer function. By modifying the bandwidth of this transfer function, a tradeoff is achieved between image noise and imaging speed. In this article, a new constant-frequency feedback loop is developed, utilizing the self-excitation technique for resonating the cantilever. Along with the proposed controller, it will be shown with the root locus that one needs to vary a single parameter, the loop gain, to modify the closed-loop bandwidth. The result is a robust, low-order, real-poled, feedback loop that is very easy to tune. The methodology is validated experimentally on a single-board field-programmable gate array device.

The interaction between two adjacent charged surfaces immersed in aqueous solution is known to be affected by charge regulationthe modulation of surface charge as two charged surfaces approach each other. This phenomenon is particularly important near surface neutrality where the stability of objects such as colloids or biomolecules is jeopardized. Focusing on this ubiquitous case, we elucidate the underlying thermodynamics and show that charge regulation is governed in this case by surface entropy. We derive explicit expressions for charge regulation and formulate a new universal limiting law for the free energy of ion adsorption to the surfaces. The latter turns out to be proportional to k B T, and independent of the association energy of ions to surface groups. These new results are applied to the analysis of unipolar as well as amphoteric surfaces such as oxides near their point of zero charge or proteins near their isoelectric point.