Can ions be countered

You could not be signed in, please check and try again. Sign in with your library card Please enter your library card number. Related Content 'counter ion' can also refer to Show Summary Details Overview counter ion. Reference entries counter ion in A Dictionary of Chemistry 6 Length: 54 words.

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You have access to this article. Please wait while we load your content E-mail: m. E-mail: j. E-mail: p. The extent of protein and counter-ion interactions in solution is still far from being fully described and understood. In low dielectric media there is documented evidence that counter-ions do bind and affect enzymatic activity. However, published crystal structures of macromolecules of biological interest in aqueous solution often do not report the presence of any counter-ions on the surface.

The extent of counter-ion interactions within subtilisin in an aqueous medium has been investigated crystallographically using CsCl soak and X-ray wavelength optimised anomalous diffraction at the Cs K-edge. The presence of counter-ions substantially changes the protein surface electrical charge. The surface charge distribution on a protein is commonly discussed in relation to enzyme function.

The correct identification of counter-ions associated with a protein surface is necessary for a proper understanding of an enzyme's function. Some interactions take place at relatively strong and specific binding sites, and have been well studied.

These cations show a strong preference for aspartic and glutamic acid side chains, with minor contributions from other side chains or from carbonyl oxygens. Protein X-ray crystallography represents a source of information on these interactions with ions. If their identification is specifically targeted, confidence in ion assignments can be improved. Replacing these common counter-ions with ones based on heavier elements, but as similar as possible in their size and their chemistry, combined with anomalous diffraction measurements with softer X-rays 12,13 can be used to unambiguously identify their binding sites.

We report here that the same enzyme, shown to retain activity in 3. This finding is rather unexpected, because the improved solvation of ions in water might be expected to reduce their binding to the protein in comparison with their behaviour in non-aqueous media. The electrical potential surface of the protein will be substantially affected by the presence of these counter-ions. In protein crystallography ion's partial occupancies and ion's B factor are a measure of ion mobility likelihood.

It is reasonable to think that the protein molecule has only some of the sites fully occupied, and perhaps some are mutually exclusive. The distribution of cesium ions on the subtilisin surface in an aqueous medium was predicted by molecular dynamics simulations in a preliminary study and showed good agreement with the cation sites observed here crystallographically.