I tried to do it but what I got was not matching up with the aswer. So far I hav
ID: 1071800 • Letter: I
Question
I tried to do it but what I got was not matching up with the aswer. So far I have figured out that we only need to regard the first C-terminus cysteine, the Glutamate, and the Histidine bc they are the only ones in the sequence with side R groups. After that I am a bit confused because I thought that if pH was less than pKa then the protonated (+1) would predominate. For example at pH 2 Cysteine would have a +1 bc 2<8.4, glutamic acid would have a +1 bc 2<4.1 and Histidine would have a +1 bc 2<6.0 but these were wrong and the answer for overall charge at ph 2 was +2. Am I missing something? DO I need to know the structure of the amino acid to get this problem? Any help is appreciated. Thanks9. oxytocin is a nonapeptide (a nine-residue peptide) hormone in- volved in the milk-releasing response in lactating mammals. The sequence of a synthetic version of oxytocin is shown below. What is the net charge of this peptide at (a) pH 2.0, (b) pH 8.5, and (c) pH 10.7? Assume that the ionizable groups have the pKa val- ues listed in Table 3.2. The disulfide bond is stable at pH 2.0, pH 8.5, and pH 10.7. Note that the C-terminus is amidated. Cys --Phe--lle Glu-- Asn Cys Pro His Gly- NH
Explanation / Answer
To answer the question, we need to identify the amino acids with charged side chains and uncharged side chains.
List the amino acids with no side chains first:
Cys, Phe, Ile, Asn, Cys, Pro, Gly
List the amino acids with side chains next:
Glu (-COOH), His (-NH2)
The SH groups in Cys doesn’t interfere. The N-terminus is Cys and and the C-terminus of Gly is amidated and hence doesn’t count.
(a) At pH 2.0, the –COOH group of the Glu side chain is protonated while the N-terminus amino group is protonated. The –NH2 side chain of His is protonated.
N-termins Cys
Glu –COOH side chain
His –NH2 side chain
Total
+1
0
+1
+2
(b) At pH 8.5, the Glu –COOH side chain is deprotonated while the –NH2 side chain of His is deprotonated.
N-terminus Cys
Glu –COOH side chain
Terminal –NH2 on Gly
Total
+1
-1
0
0
(c)At pH 10.7, the –COOH group of Glu side chain is deprotonated.The N-terminus –NH2 as well as the His side chain –NH2 group are both protonated.
N-terminus Cys
Glu –COOH side chain
Terminal –NH2 on Gly
Total
0
-1
0
-1
N-termins Cys
Glu –COOH side chain
His –NH2 side chain
Total
+1
0
+1
+2