Question

Calculate the overall net charge of the following polypeptide at pH = 12.0, to TWO decimal places. Only add a sign if the answer is NEGATIVE. Gly-Pro-Asp-His-Arg

ide chain pKa values of amino acids: Asp 4.0 Glu 5.0 His 6.5 Cys 8.5 Tyr 10.0 Lys 10.2 Arg 12.5

Average N-terminal amino group has pKa = 9.5

Average C-terminal carboxylic acid group has pKa = 2.5

Answer #1

Ans. The dominant form of acidic and basic groups on different pH group is shown in figure.

A difference in pH and pI by 2 units or more is accepted to provide 1 unit charge on the group as applicable.

At pH = pI, the group has 0.5 units.

Assign charge on the groups accordingly as shown in figure.

Net charge on peptide at pH 12.0 =
**-1.50**

**Note:** The exact
charge on Arginine side chain at pH 12 is 0.760 (calculated using
Henderson-Hasselbalch equation) because pH in not exactly equal to
pI. The difference between pH and pI is not greater than 2.0
units.

So, if you use HH equation, the
exact charge on peptide would be **– 1.24.**

To the nearest hundredth of a unit, what is the net charge of
the peptide GLACIDR at pH 4.5? Use the following set of pKa
values:
Group
pKa
N-ter
8.0
C-ter
3.1
Asp or Glu
4.0
Lys
10.5
Arg
12.4
His
6.0
Tyr
10.5
Cys
8.4

calculate the overall net charge of the following polypeptide
at pH = 6.7
Met-Pro-His-Arg-Asp-Glu-Gln-Lys-Val-Phe
a. 0.39
b. -0.39
c. 0.61
d. -0.61

Calculate the net charge on the following tetrapeptides.
Asp-Arg-Ser-Pro
Trp-Gln-Ile-Lys
Cys-Asn-Glu-Tyr
Which peptide will be retained the longest on a cation-exchange
column at pH 7?

To the nearest hundredth of a unit, what is the net charge of
the peptide GLACIDR at pH 4.5? Use the following set of pKa
values:
Group
pKa
N-ter
8.0
C-ter
3.1
Asp or Glu
4.0
Lys
10.5
Arg
12.4
His
6.0
Tyr
10.5
Cys
8.4
Thoroughly confused about this question. I would like an
explanation. Also, From previous posts I have seen that the answer
is not -1,0,+1, or +4. Thank you!

You wish to determine the sequence of a polypeptide that has the
following amino acid composition.
1 Ala 4 Arg 2 Asn 3 Asp 4 Cys 3 Gly 1 Gln 4 Glu1 His 1 Lys 1 Met
1 Phe 2 Pro 4 Ser 2 Tyr 1 Trp
What is the maximum number of peptides you can expect if you
cleave the polypeptide with cyanogen bromide ?
What is the maximum number of peptides you can expect if you do
a...

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