1 research outputs found
Regulation of Surface Charge by Biological Osmolytes
Osmolytes, small
molecules synthesized by all organisms, play a
crucial role in tuning protein stability and function under variable
external conditions. Despite their electrical neutrality, osmolyte
action is entwined with that of cellular salts and protons in a mechanism
only partially understood. To elucidate this mechanism, we utilize
an ultrahigh-resolution frequency modulation-AFM for measuring the
effect of two biological osmolytes, urea and glycerol, on the surface
charge of silica, an archetype protic surface with a p<i>K</i> value similar to that of acidic amino acids. We find that addition
of urea, a known protein destabilizer, enhances silica’s surface
charge by more than 50%, an effect equivalent to a 4-unit increase
of pH. Conversely, addition of glycerol, a protein stabilizer, practically
neutralizes the silica surface, an effect equivalent to 2-units’
reduction of pH. Simultaneous measurements of the interfacial liquid
viscosity indicate that urea accumulates extensively near the silica
surface, while glycerol depletes there. Comparison between the measured
surface charge and Gouy–Chapman–Stern model for the
silica surface shows that the modification of surface charge is 4
times too large to be explained by the change in dielectric constant
upon addition of urea or glycerol. The model hence leads to the conclusion
that surface charge is chiefly governed by the effect of osmolytes
on the surface reaction constants, namely, on silanol deprotonation
and on cation binding. These findings highlight the unexpectedly large
effect that neutral osmolytes may have on surface charging and Coulomb
interactions