The Hydration Structure of Guanidinium and Thiocyanate Ions: Implications for Protein Stability in Aqueous Solution

Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2003-04, Vol.100 (8), p.4557-4561
Main Authors: Mason, P. E., Neilson, G. W., Dempsey, C. E., Barnes, A. C., Cruickshank, J. M.
Format: Article
Language:English
Subjects:
Anions
Atoms
Biological Sciences
Biophysical Phenomena
Biophysics
Cations
Coordination numbers
Correlations
Drug Stability
Guanidine - chemistry
Hydrogen
Hydrogen Bonding
Hydrogen bonds
Ions
Molecules
Nitrogen
Protein Denaturation
Proteins
Proteins - chemistry
Solutions
Thiocyanates - chemistry
Water - chemistry
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Summary:Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were determined. Each ion is shown to interact weakly with water: Guanidinium has no recognizable hydration shell and is one of the most weakly hydrated cations yet characterized. Hydration of thiocyanate is characterized by a low coordination number involving around one hydrogen-bonded water molecule and approximately two water molecules weakly interacting through "hydration bonds." The weak hydration of these denaturant ions strongly supports suggestions that a major contribution to the denaturant effect is the preferential interaction of the denaturant with the protein surface. By contrast, solute species such as many sugars and related polyols that stabilize proteins are strongly hydrated and are thus preferentially retained in the bulk solvent and excluded from the protein surface.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0735920100