I have the DPPH inhibition % of an unknown sample and the % inhibition for a gallic acid standard curve. For the standard I can calculate the IC50 value, but can I use this standard curve to infer the IC50 for the unknown sample? Or should I instead assay multiple concentrations for the unknown sample and deterine the inhibition %, and then create a curve for that sample and use that for the IC50 calculation?
While using a gallic acid standard curve to infer the IC50 for an unknown sample is possible, it’s not ideal because it assumes that the unknown sample behaves similarly to gallic acid in the DPPH assay. For the most accurate IC50 determination, you should assay the unknown sample at multiple concentrations and create its own dose-response curve.
The most accurate way to determine the IC50 for the unknown sample would be to assay it at multiple concentrations and create its own dose-response curve. This approach would give you a direct estimate of the sample's IC50, accounting for any unique antioxidant activities it might have.
I would add to Nicolas Poirier 's answer:
"For the most accurate IC50 determination, you should assay the unknown sample at multiple concentrations "and use different assays.
I agree with what has already been said.
It is possible to use a standard curve of gallic acid to determine the IC50 value of a sample in the DPPH test, even if the sample does not contain gallic acid, but with some considerations. In particular, the precision will depend on how similar the antioxidant activity of the sample is to that of gallic acid.
If the sample contains a compound with antioxidant activity similar to that of gallic acid, meaning it exhibits similar mechanisms of action, then I would expect a similar response in terms of DPPH inhibition.
If the sample contains compounds with very different antioxidant activity compared to gallic acid (for example, other classes of antioxidants that do not reduce DPPH in the same way), the gallic acid curve may not be a perfect representation, and using gallic acid as a reference may not be accurate. In such cases, it would be better to construct a standard curve for the sample itself, if possible.
I think it's possible to compare your extract to gallic acid to evaluate its antioxidant activity. However, to calculate the IC50 of your extract, you need to test multiple concentrations and create your own curve, as each sample is unique. Therefore, using the gallic acid standard curve alone is not appropriate for determining the IC50 of your extract.
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