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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.

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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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  • $\begingroup$ To be frank, when it comes to academic related things (science), Wikipedia hasn't been an easy diegest feed. I found out the video, that I commented below. Also thanks to your explanation, I also checked Morrison & Boyd, plus a lecture from MIT open courseware. $\endgroup$ – bonCodigo Jun 13 '15 at 23:55
  • $\begingroup$ @bonCodigo - RE: Wikipedia - You're right. When Wikipedia first started out the articles were written at a fairly low level. Then the articles got rewritten by experts so that the articles were absolutely technically correct. The result many times is a lot of jargon and complicated formulas which are difficult for a beginning student to understand. $\endgroup$ – MaxW Sep 18 '16 at 18:41
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By increasing the pH you are actually increasing the concentration of hydrogen ions present.

By the simplistic definition,

$pH = -\log\ce{[H+]}$

Thus at a low pH you have lots of $\ce{[H+]}$ present. This can protonate your amino acid resulting in a positive charge overall. The inverse is true for high pH (low concentration of $\ce{[H+]}$)

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  • $\begingroup$ My question : How can both terminals get protonated to result in one charge? Can you explain the chemical process of how each NH2, COOH terminals getting protonated and deprotonated? $\endgroup$ – bonCodigo Jun 9 '15 at 14:17
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    $\begingroup$ Both terminals will not be protonated (under reasonable conditions). To start off with you have a single positive charge and a single negative charge. This is your zwitterion with no net charge. If you have lots of H+ you will protonate, removing a minus. So overall you have a single positive charge. $\endgroup$ – Christopher Jun 9 '15 at 14:32
  • $\begingroup$ At physiological pH (7.4) the amino group (which has a pKa of around 9) is protonated. $\endgroup$ – orthocresol Jun 9 '15 at 14:34
  • $\begingroup$ I watch this video, things became much clearer. Isn't pH 7.4 considered to be in neutral range? $\endgroup$ – bonCodigo Jun 10 '15 at 14:17
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Look for the $pK_a/pK_b$ values for each amino acid. When the pH of the solution is lower than both $pK_a$s, the amino acid will be in its protonated, non-zwitterionic form. When the pH is greater than both $pK_a$s, the amino acid will be in its deprotonated, anionic form. When the pH is between the two $pK_a$s (around neutral pH), the amino acid will be in its zwitterionic form.

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