Net charge of a peptide Why is it +2 not +3 (Asp-Gly-Arg-His) at pH1?
 

I had this question on a quiz:
The overall charge of a peptide of the following peptide sequence at pH 1 would be (Asp-Gly-Arg-His)??

I though the answer was +3 but I was wrong, it is +2. Why?

This is what I thought, Where am I going wrong???
At PH1 the amino group with pka of about 9 would be +1
At PH1 the Arginine group with pka of about 12.5 would be +1
At PH1 the Histidine Side chain with pka of about 6 would be +1
At PH1 the carboxyl group with pka of about 2 would be 0

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Re: Net charge of a peptide Why is it +2 not +3 (Asp-Gly-Arg-His) at pH1?
 

I hope the peptide asked in your quiz is cyclic (i.e. amino of aspartic is covalently bound with carboxyl of Histidine) and if so then you loss +1 of amino group of aspartic so you remain with his and arg +1 so total charge on peptide is +2.

In general nomenclature peptide sequence written as Asp-Gly-Arg-His but in your question it written as (Asp-Gly-Arg-His) so may be it indicate that it have to cyclised but in that case questioner must have to write word cyclo also,but this is only explanation to explain charge +2.

Regards
Kunal pandya:hyper: