Question

What would be the charge of this peptide, Asn-Arg-Met-Lys-His-Pro-Trp-His-Thr-Tyr-Ser-Gly-Glu-Leu-Val-Trp, at a pH of 1 and a pH of 11?

Answer 1

The charge of peptide will depend upon the pKa of the amino acids present in the peptide

Remember : That below the given pH all the groups will be protonated and above it deprotonated

The amino acids with pKa of side chains are

Asn : pKa of NH2 = 8.72

Arg: 12.48

Met:

Lys:10.54

His: 6.04

Pro:

Trp:

His: 6.04

Thr:

Tyr: 10.46

Ser:

Gly:

Glu: 4.047

Leu:

Val:

Trp:carboxylic 2.46

a) At pH =1 ,

Glu : It will be neutral at this pH

Lys : It will have +1 charge

His : (two molecules) +1 charge each

Arg : +1 charge

NH2 group of Asn : +1 Charge

Total charge : +5

2) at pH = 11

Negative charge will be due to

Terminal COOH = -1

-1 Charge due to glu :

Total charge = -2