Final answer:
The net charge of the peptide at pH 7, considering typical pKa values and assuming histidine is neutral, should be +1, as lysine and arginine side chains are positively charged, whereas aspartic acid and glutamic acid side chains are negatively charged. However, this net charge does not match any of the provided options – there may be a typo or error in the question or answer choices.
Step-by-step explanation:
To find the net charge of the peptide lys-his-gly-his-asp-glu-tyr-cys-ala-asp-ala-arg at pH 7, we need to consider the ionizable side chains of the amino acids as well as the N- and C-termini. At pH 7, lysine (lys) and arginine (arg) have positively charged side chains, while histidine (his) is close to its pKa and could be partially positively charged. Aspartic acid (asp) and glutamic acid (Glu) have negatively charged side chains. Since the question specifies that we are at pH 7, the N-terminus (lysine) will be positively charged and the C-terminus (cysteine) will be neutral.
By counting up the charges, we find that lysine (+1), arginine (+1), and possibly each histidine (his) contribute positively at pH 7. On the other hand, aspartic acid (asp) and glutamic acid (Glu) contribute negatively with a charge of -1 each. If we assume histidine (his) is neutral at pH 7 (which is a common assumption since its pKa is around 6), the peptide will have a net charge of +1 (from the N-terminal lysine) +1 (from the lysine side chain) +1 (from the arginine side chain) -1 (from the first aspartic acid side chain) -1 (from the glutamic acid side chain) -1 (from the second aspartic acid side chain), summing up to a net charge of +1. However, this net charge does not match any of the provided answer options. It appears that there might be a typo or an assumption error in the question or answer choices, as with the given peptide sequence and typical pKa values, the net charge should be +1 at pH 7