This problem is a good example of isoelectric points and zwitterions. Serine has an NH2 group, a COOH group, and an OH group on its side chain. The side chain OH is very ionizable (it would take a lot of energy to pull off that hydrogen). So don\'t factor the side group into this problem. Instead, focus on the amino and carboxyl groups. There\'s always tables floating around about the different pKa\'s of amino acid groups. In this case, the pKa of the serine carboxyl group is about 2.21, while the pKa of the serine amino group is about 9.15. Remember, pKa is a measure of acid dissociation, and it is expressed on the logarithmic scale. Essentially, the lower the pKa value, the more readily the acid dissociates. In other words, if you have two acids, the one with the lower pKa will lose its proton first before the other acid (if you\'re gradually increasing the pH of the system......and if I just confused you more, the pH of a solution is a measure of the acidity [H30+ content] of the solution, whereas pKa is a property of each molecule itself....although the two are intimately related by the Henderson-Hasselbach equation). In the above titration, you started off at a very low pH and reached a pH of ~2. This pH is not yet at the pKa of the carboxyl COOH group, so the H does NOT dissociate. Additionally, it\'s nowhere near the pKa of the amino group (which acts as a base). ----- >Therefore, at the first arrow from the left, serine will look like it normally does, EXCEPT that instead of the neutral NH2 goup, it\'ll be NH3+. The 2nd arrow corresponds to the zwitterion - the term coined for amino acids that have a positive and negative charge at the same time and are thus neutral. The pH corresponding to this is known as the isoelectric point. Recall that at a pH of ~2.19, the carboxyl hydrogen dissociates. However, the amino group still has not yet reached its pKa. ------>Therefore, at the second arrow, serine will look like it normally does, EXCEPT that the COOH group will instead be COO-, and the amino group will STILL BE NH3+. The 3rd arrow from the left represents the point at which the pH has finally reached the pKa of the amino group (i.e. the hydrogen dissociates from the amino group). ------>Therefore, at the third arrow from the left, serine will have the COO- group on one end and the NH2 group on the other end. Note: when I say \"looks like it normally does,\" I am referring to the normal serine structure of H | NH2------COOH | R where the R is that CH2OH group (the R group doesn\'t change in the above problem). Hope this helps. Solution This problem is a good example of isoelectric points and zwitterions. Serine has an NH2 group, a COOH group, and an OH group on its side chain. The side chain OH is very ionizable (it would take a lot of energy to pull off that hydrogen). So don\'t factor the side group into this problem. Instead, focus on the amino and carboxyl groups. There\'s always tables floating around about the different pKa\'s of.

1. This problem is a good example of isoelectric points and zwitterions. Serine has an
NH2 group, a COOH group, and an OH group on its side chain. The side chain OH is very
ionizable (it would take a lot of energy to pull off that hydrogen). So don't factor the side group
into this problem. Instead, focus on the amino and carboxyl groups. There's always tables
floating around about the different pKa's of amino acid groups. In this case, the pKa of the
serine carboxyl group is about 2.21, while the pKa of the serine amino group is about 9.15.
Remember, pKa is a measure of acid dissociation, and it is expressed on the logarithmic scale.
Essentially, the lower the pKa value, the more readily the acid dissociates. In other words, if you
have two acids, the one with the lower pKa will lose its proton first before the other acid (if
you're gradually increasing the pH of the system......and if I just confused you more, the pH of a
solution is a measure of the acidity [H30+ content] of the solution, whereas pKa is a property of
each molecule itself....although the two are intimately related by the Henderson-Hasselbach
equation). In the above titration, you started off at a very low pH and reached a pH of ~2. This
pH is not yet at the pKa of the carboxyl COOH group, so the H does NOT dissociate.
Additionally, it's nowhere near the pKa of the amino group (which acts as a base). -----
>Therefore, at the first arrow from the left, serine will look like it normally does, EXCEPT that
instead of the neutral NH2 goup, it'll be NH3+. The 2nd arrow corresponds to the zwitterion -
the term coined for amino acids that have a positive and negative charge at the same time and are
thus neutral. The pH corresponding to this is known as the isoelectric point. Recall that at a pH
of ~2.19, the carboxyl hydrogen dissociates. However, the amino group still has not yet reached
its pKa. ------>Therefore, at the second arrow, serine will look like it normally does, EXCEPT
that the COOH group will instead be COO-, and the amino group will STILL BE NH3+. The
3rd arrow from the left represents the point at which the pH has finally reached the pKa of the
amino group (i.e. the hydrogen dissociates from the amino group). ------>Therefore, at the third
arrow from the left, serine will have the COO- group on one end and the NH2 group on the other
end. Note: when I say "looks like it normally does," I am referring to the normal serine
structure of H | NH2------COOH | R where the R is that CH2OH group (the R group doesn't
change in the above problem). Hope this helps.
Solution
This problem is a good example of isoelectric points and zwitterions. Serine has an
NH2 group, a COOH group, and an OH group on its side chain. The side chain OH is very
ionizable (it would take a lot of energy to pull off that hydrogen). So don't factor the side group
into this problem. Instead, focus on the amino and carboxyl groups. There's always tables
floating around about the different pKa's of amino acid groups. In this case, the pKa of the
serine carboxyl group is about 2.21, while the pKa of the serine amino group is about 9.15.

2. Remember, pKa is a measure of acid dissociation, and it is expressed on the logarithmic scale.
Essentially, the lower the pKa value, the more readily the acid dissociates. In other words, if you
have two acids, the one with the lower pKa will lose its proton first before the other acid (if
you're gradually increasing the pH of the system......and if I just confused you more, the pH of a
solution is a measure of the acidity [H30+ content] of the solution, whereas pKa is a property of
each molecule itself....although the two are intimately related by the Henderson-Hasselbach
equation). In the above titration, you started off at a very low pH and reached a pH of ~2. This
pH is not yet at the pKa of the carboxyl COOH group, so the H does NOT dissociate.
Additionally, it's nowhere near the pKa of the amino group (which acts as a base). -----
>Therefore, at the first arrow from the left, serine will look like it normally does, EXCEPT that
instead of the neutral NH2 goup, it'll be NH3+. The 2nd arrow corresponds to the zwitterion -
the term coined for amino acids that have a positive and negative charge at the same time and are
thus neutral. The pH corresponding to this is known as the isoelectric point. Recall that at a pH
of ~2.19, the carboxyl hydrogen dissociates. However, the amino group still has not yet reached
its pKa. ------>Therefore, at the second arrow, serine will look like it normally does, EXCEPT
that the COOH group will instead be COO-, and the amino group will STILL BE NH3+. The
3rd arrow from the left represents the point at which the pH has finally reached the pKa of the
amino group (i.e. the hydrogen dissociates from the amino group). ------>Therefore, at the third
arrow from the left, serine will have the COO- group on one end and the NH2 group on the other
end. Note: when I say "looks like it normally does," I am referring to the normal serine
structure of H | NH2------COOH | R where the R is that CH2OH group (the R group doesn't
change in the above problem). Hope this helps.