The amino acids are Zwitterions. In neutral pH, an Amino acid's amino group has a postive charge and Carboxyl group has negative charge. They cancel each others charge thanks to the $Hydrogen$ that's roaming between the two groups.
But, I am not able to understand why Amino acids have positive charges at a low pH and negative at high pH? Given amino acids are Zwitterions, how can both terminals have the same charge (either positive or negative) at low and high pH?
Can someone explain this from general chemistry and pH perspective (acid/base)?
PS: first posted in Biology site but community recommended it to be moved here.
Answer
Firstly a note about terminology. The word "terminus" is reserved for the N- or C-termini of a polypeptide chain. For a free amino acid, you should refer to the carboxyl and amino groups as the $\alpha$-$\ce{COOH}$ and $\alpha$-$\ce{NH2}$ groups respectively.
Anyway, the -$\ce{COOH}$ group is acidic; above a certain pH, typically around 2, it can be deprotonated to form a negatively charged -$\ce{COO-}$ group. On the other hand, the -$\ce{NH2}$ group is basic; below a certain pH, typically around 9, it can be protonated to form a positively charged -$\ce{NH3+}$ group. I made a short table describing the protonation states of the two groups.
$$\begin{array}{c|c|c|c} \textbf{pH} & \textbf{Carboxyl group exists as} & \textbf{Amino group exists as} & \textbf{Net charge} \\ \hline <2 & \ce{-COOH}\text{ (neutral)} & \ce{-NH3+}\text{ (positively charged)} & +1 \\ >2 & \ce{-COO^-}\text{ (negatively charged)} & \ce{-NH3+}\text{ (positively charged)} & 0 \\ >9 & \ce{-COO^-}\text{ (negatively charged)} & \ce{-NH2}\text{ (neutral)} & -1 \\ \end{array} $$
For more information you can read Wikipedia or any biochemistry textbook - the first few chapters will often be devoted to discussing chemistry like this.
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