What is the structural formula for serine with a pH of 4?
1 Answer
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 (
#"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
- 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