# Question #c0a10

Aug 21, 2016

When the protein shifts from the R-form to the T-form at pH 7.0, the histidine goes from its neutral to its ionic form.

#### Explanation:

The neutral and the ionic forms of the imidazole side-chain are shown below.

(From chem.libretexts.org)

There are three points to remember:

• If $\text{pH = p} {K}_{a}$, the neutral and ionic forms are present in equal amounts.
• If $\text{pH < p} {K}_{a}$ (i.e. more acidic), the ionic (protonated) form will predominate.
• If $\text{pH > p} {K}_{a}$ (i.e. more basic), the neutral (non-protonated) form will predominate.

For the R-form, $\text{p} {K}_{a} = 6.0$.
∴ The R-form is ionic when pH <6.0 and neutral when pH > 6.0.

For the T-form, $\text{p} {K}_{a} = 8.1$.
∴ The T-form is ionic when pH < 8.1 and neutral when pH > 8.1.

At pH = 7.0, the R-form is neutral, but the T-form is ionic.

This corresponds to Statement 1.

Statement 2 is wrong, because the R-form is neutral at pH 7.0.

Statement 3 is wrong for the same reason.

Statement 4 is wrong, because the T-form is ionic at pH 7.0.

Statement 5 is wrong, for the same reason.