To the nearest hundredth of a unit, what is the net charge of the peptide GLACID

Question

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!

Explanation / Answer

The amino acids present are GLACIDR

G : Glycine

L : Leucine

A : alanine

C: Cysteine

I: Isoleucine

D: Aspartic acid

R:Arginine

We have to consider that all the group with given pKa will be protonated at pH =4.5 if pKa > 4.5

So there will be positive charge due to

N-ter = +1

Arg = +1

Cysteine (disulphide bond is stable)

Now the pKa with > 4.5 will contribute -1 charge

C-terminus = -1

Aspartic acid = -1

So the total charge = 0 [No confustion]

Generally we dont cosider the charge on cysteine between 2.3- 12pH

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