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I want to convert the absorption of a protein into concentration. Hence ı need to calculate the extinction coefficient of the protein (GFP). ı found the formula (nTrp)5500+(nTyr)1490 on the internet but not able to use it since ı dont understand what n in front of the amino acids stand for and how ı reach to the n value

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Only the number ("n") of the aromatic amino acids Tryptophan (Trp) and Tyrosin (Tyr) in your protein contribute significantly to its absorption at 280 nm (and Cystein (Cys), albeit to a lesser extend), see also here.

You could retrieve the protein's amino acid sequence from uniprot (see GFP here). Looking through the GFP sequence, one can identify n=11 (4.6%) Tyr and n=1 (0.4%) Trp. Now you can insert those in your formula and end up with an extinction coeffient of 21'890 M^-1 * cm^-1. This formula is only valid if all Cys are reduced (GFP has n=2 Cys that could form a disulfide bridge).

If you want to avoid all the formulas I suggest you copy the sequence of GFP and paste it into ExPASy ProtParam. This way you get a nice overview of the amino acid distribution of your protein and also retrieve the extinction coefficient under oxidizing conditions, which BTW is 22'015 M^-1 * cm^-1.

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