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Abstract
Using static and dynamic light scattering we have investigated the effects of either
strongly chaotropic, nearly neutral or strongly kosmotropic salt ions on the hydration
shell and the mutual hydrodynamic interactions of the protein lysozyme under conditions
supportive of protein crystallization. After accounting for the effects of protein
interaction and for changes in solution viscosity on protein diffusivity, protein
hydrodynamic radii were determined with +/-0.25 A resolution. No changes to the extent
of lysozyme hydration were discernible for all salt-types, at any salt concentration
and for temperatures between 15-40 degrees C. Combining static with dynamic light
scattering, we also investigated salt-induced changes to the hydrodynamic protein
interactions. With increased salt concentration, hydrodynamic interactions changed
from attractive to repulsive, i.e., in exact opposition to salt-induced changes in
direct protein interactions. This anti-correlation was independent of solution temperature
or salt identity. Although salt-specific effects on direct protein interactions were
prominent, neither protein hydration nor solvent-mediated hydrodynamic interactions
displayed any obvious salt-specific effects. We infer that the protein hydration shell
is more resistant than bulk water to changes in its local structure by either chaotropic
or kosmotropic ions.