Why so many restriction enzymes have a very different activity (from very low to high) in NEB Buffer 2 vs NEB CutSmart buffer (https://www.neb.com/tools-and-resources/usage-guidelines/nebuffer-performance-chart-with-restriction-enzymes)?
In principle, these buffers are very similar - they have the same pH 7.9, [Mg2+] and ionic strength. The only difference is that 50 mM Na+ is swapped for 50 mM K+ and ~80 mM Cl- is swapped for ~ 90 mM acetate-.
It would be no surprise that there is some difference for many enzymes (e.g. some inhibition by chloride or sodium ions which are less abundant in the cytosol of many organisms). But why then so many enzymes are that much (e.g. 10 times) sensitive to the exchange Na/K and chloride/acetate. Still, for most restriction enzymes which show a difference between buf2/CutSmart, activity is indeed higher in Cutsmart. For a minority, though, the difference is the opposite (but typically less extreme).
NEBuffer 2.1 1X Buffer Components 50mM NaCl 10mM Tris-HCl 10mM MgCl2 100μg/ml BSA pH 7.9@25°C
CutSmart Buffer 1X Buffer Components 50mM Potassium Acetate 20mM Tris-acetate 10mM Magnesium Acetate 100μg/ml BSA pH 7.9@25°C
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