Positive charge on a zwitter ion at pH=7? I faced this question. But I am having some problems understanding the solution to this:


How can the negative charge on the peptide vanish on $\pu{pH = 7}$? Suppose, we have some $\ce{H+}$ donor on either $\ce{R1}$ or $\ce{R2}$. Then the $\ce{H+}$ donor can make the carboxyl radical neutral. But by donating a $\ce{H+}$ ion, the donor becomes negatively charged. Maybe, the donor can take the $\ce{H+}$ from the $\ce{NH3+}$ radical. But the result, a completely neutral tetrapeptide, doesn't satisfy the condition of the question.


Need a hint.
To sum up: of the side chains listed, only the aminoalkyl ones will be positively charged at pH 7, so four peptides -- 4, 6, 8 and 9 -- will have a net positive charge at pH 7.