# Reason for protonation of amino group in amino acids at near neutral pH

What happens to the amino group when amino acid is added to a near neutral solution? How does it come about gaining an extra proton?

I understand what happen to the carboxyl group, since it is a strong acid it will dissociates quickly to donate a proton.

## Prerequisite knowledge

A proton is deprotonated when the $$\mathrm{pH}$$ exceeds the $$\mathrm pK_\mathrm a$$ of the proton.

# Analysis

There are 3 different forms that an amino acid can exist in. For the sake of simplicity of analysis, I am taking the simplest amino acid - glycine.

$$\underset{\mathrm{glycine}}{\ce{HOOC-CH2-NH2}}$$

Continuing to state the conditions of analysis, let's take a highly acidic medium to begin with and observe the changes that take place on increasing $$\ce{pH}$$.

### Case 1: pH of solution < 2.34

In such a scenario, the $$\mathrm{pH}$$ is lower than the $$\mathrm p K_\mathrm a$$ of the acid group and lower than the $$\mathrm p K_\mathrm a$$ of the amino group. This means that both the amino group and acid groups are protonated. The amino acid is positively charged at this $$\mathrm {pH}$$.

$$\underset{\mathrm{glycine\,(pH \,<\,2.34)}}{\ce{HOOC-CH2-NH3+}}$$

Now, we increase the $$\ce{pH}$$ to a value greater than $$2.34$$ but less than 9.6 Specialty of these numbers? We'll see at the end.

### Case 2: pH of solution greater than 2.34 but less than 9.6

In such a scenario, the $$\mathrm{pH}$$ is higher than the $$\mathrm p K_\mathrm a$$ of the acid group but lower than the $$\mathrm p K_\mathrm a$$ of the amino group. This means that the amino group remains protonated and the acid group is now deprotonated.

$$\underset{\mathrm{glycine\,(2.34\,<\,pH \,<\,9.6)}}{\ce{^-OOC-CH2-NH3+}}$$

### Case 3: pH of solution greater than 9.6

In this last case, the $$\mathrm{pH}$$ is higher than both the $$\mathrm pK_\mathrm a$$s of the amino group and the acid group. This means that both groups are now deprotonated. The amino acid is now negatively charged.

$$\underset{\mathrm{glycine\,(pH \,>\,9.6)}}{\ce{^-OOC-CH2-NH2}}$$

## Observations and Conclusion

As you noticed, there are some specific numbers that are so important they are mentioned. They are the $$\mathrm p K_\mathrm a$$ values of the acid group ($$\mathrm pK_{\mathrm a_1}$$) and the amino group ($$\mathrm pK_\mathrm {a_2}$$). There is one more important number and that is the exact point of net average neutrality. It is known as the isoelectric point.

$$\mathrm {pI} = \frac{\mathrm pK_\mathrm {a_1} + \mathrm pK_\mathrm {a_2}}{2}$$