Possible sequence combinations of a linear polypeptide chain

PaDC

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Nov 18, 2016
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anyone can help with this?


Assuming I have 25 amminoacid positions constituting the whole length of a polypeptide and knowing that there are 20 different types of naturally occurring amminoacids, what is the equation to calculate how many possible combinations of 5 amminoacid positions (out of those 25) are possible?


If it helps to explain my question better, the 5 amminoacid positions can be any subset of those 25 positions (e.g.: the 5 positions could be amminoacids 1-2-3-4-5 as well as 3-14-15-19-21 or any other combination of 5 positions) and each of them can be any of the 20 natural amminoacids. Any of the 20 types of amminoacids can appear more than once.


Cheers
 
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