Question

Predict the charge on the major species of the peptide Gly-Arg-Asp-Ala-Lys-Ser-Asn at PH 6.0. Please, can you explain? Thanks.

Answer 1

Looking at the pKa of the amino acids we can do this calculation.

pKa for NH2 of glycine is 9.78 so at pH 6.0 it will remain in protonated NH3+ form

pKa of COOH for Asparagine is 2.14 so at pH 6 it will remain as COO- form

pKa of side chain of arginine is 12.48 so it will remain as NH3+ at pH 6

pKa of Lysine side chain is 10.54 so it will remain as NH3+ at pH 6

pKa of asparatic acid is 3.9 so it will remain as COO- at pH 6

Total charge on peptide at pH 6 would be thus +1