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The N-terminal amino group will have a charge of +1 because its pKa is around 9 or so. Likewise, the C-terminal carboxyl group will have a charge of -1 because its pKa is around 2.1 or so. Now, the only side chain that can be ionized is the carboxyl group of glutamic acid (E). Its pKa is about 4.25. Since this is slightly above 4, more of the glu carboxyl groups will be un-ionized, and the molecule will still have a slight negative charge. If the pH was exactly equal to the pKa of the glu carboxyl, you could say that the molecule would have a net charge of -1/2. Since you are at a pH a little below the pKa, the net charge will be less than -1/2, but it won't be 0.
You could actually use the Henderson-Hasselbalch equation to calculate the fraction of side chains that are ionized and calculate a specific charge, but that's probably not what they want you to do.
|charge , peptide , ph4|
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