I have the following problem:
A polypeptide has the following sequence (amino end on the left) $$\mathbf{lysine - aspartic~acid-histidine-arginine-serine}$$
Using the $pK_a$ information that follows, what would be the approximate charge on this molecule at pH = 3.3?
$pK_a$ values for the various groups in the molecule are as follows:
\begin{array}{|cc|}\hline \rm Alpha~carboxyl & 2.2 \\ \rm Alpha~amino & 9.5 \\ \rm R-group~amino & 11.7 \\ \rm R-group~carboxyl & 4.5 \\\hline\end{array}
After my calculations I got +2 and my answer was wrong. How to solve it correctly?
My effort:
$\ce{NH3}$ has $+1$ when on and $0$ when off. $\ce{NH3}$ are Lysine, Histadine, and arginine. $\ce{COOH}$ has $0$ when on and $-1$ when off. $\ce{COOH}$ are Serine and aspartic acid. Based on the pH and $pK_a$ values the answer should be 2. Lysine=+1 Aps=0 His=+1 Arg=+1 Serine=-1