Question

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

Answer 1

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

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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 `"pH"` to `"pK"_a`:

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

Here we see that

• if `["HA"] < ["A"^(-)]`, then `(["A"^-])/(["HA"]) > 1` and `log((["A"^-])/(["HA"])) > 0`. Thus, `"pH" > "pK"_a` and the solution is basic with respect to the proton whose `"pK"_a` is given.

• if `["HA"] > ["A"^(-)]`, then `(["A"^-])/(["HA"]) < 1` and `log((["A"^-])/(["HA"])) < 0`. Thus, `"pH" < "pK"_a` and the solution is acidic with respect to the proton whose `"pK"_a` is given.

Clearly, since `"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 `(-)` and `(+)`, respectively.