# What is the structural formula for serine with a pH of 4?

Jul 22, 2018

Read over this, and then see if you can draw the structure. You should have the structure of serine memorized.

Well, serine has two pKas to consider; the carboxyl and the amine pKa's ($2.21 , 9.15$). As you have seen, the Henderson-Hasselbalch equation relates $\text{pH}$ to ${\text{pK}}_{a}$:

"pH" = "pK"_a + log((["A"^-])/(["HA"]))

Here we see that

• if $\left[{\text{HA"] < ["A}}^{-}\right]$, then $\left(\left[\text{A"^-])/(["HA}\right]\right) > 1$ and $\log \left(\left(\left[\text{A"^-])/(["HA}\right]\right)\right) > 0$. Thus, ${\text{pH" > "pK}}_{a}$ and the solution is basic with respect to the proton whose ${\text{pK}}_{a}$ is given.

• if $\left[{\text{HA"] > ["A}}^{-}\right]$, then $\left(\left[\text{A"^-])/(["HA}\right]\right) < 1$ and $\log \left(\left(\left[\text{A"^-])/(["HA}\right]\right)\right) < 0$. Thus, ${\text{pH" < "pK}}_{a}$ and the solution is acidic with respect to the proton whose ${\text{pK}}_{a}$ is given.

Clearly, since $\text{pH} = 4$, and $2.21 < 4 < 9.15$,

• The solution is basic with respect to the carboxyl group.
• The solution is acidic with respect to the amine group.

Hence, the carboxyl group is DEprotonated, and the amine group is protonated. Their charges are thus $\left(-\right)$ and $\left(+\right)$, respectively.