Question

Predict the charge on the major species of the peptide Ile–Lys–Glu–Ala–Arg–Ser–Asn at pH 7.0

Answer 1

At pH = pK_a, half of the acid is dissociated. For the given polypeptide, the pK_as of carboxylic acid groups are below 5.0. Hence, the the carboxylic acid groups will stay deprotonated at pH 7.0. The pK_a of protonated amine group is above 8.0. Hence, the protonated amine groups will remain protonated.

The charges due to deprotonated carboxylic acid (glutamic side chain + C-terminal COO^-) = -2

The charges due to protonated amine (N-terminal NH₃⁺ + lysine and arginine side chain) = +3

Hence, the net charge on the polypeptide at pH 7.0 = +3 - 2 = +1