logo.gif (3296 bytes)Introduction to Protein Structure

Overall Charge on a Peptide -- Example 3


Cysteinylglutamylprolylthreonylleucinyllysylglutaminylarginine
(Cys-Glu-Pro-Thr-Leu-Lys-Gln-Arg; SEVQK)

wpe22.jpg (16050 bytes)

Note: the ionizable groups are "boxed"

In addition to the amino group (pKa ~9 ) and the carboxyl group (pKa ~ 3) at each end of the molecule, the sidechains of the cystiene (sulfhydryl; pKa ~ 8.5), the glutamic acid (carboxyl group; pKa ~ 4.4), the lysine (amino group; pKa ~ 10) and the arginine (guanidinium group; pKa ~ 12) residues are ionizable.

Thus, the net charge on this peptide at three different  pH's ( 2, 7 and 11) is as follows:

Charge on Functional Group
pH N-terminus Cys (-SH) Glu (-COOH) Lys (-NH2) Arg -C(NH2)NH C-terminus Net Charge
2.0 1+ 0 0 1+ 1+ 0 3+
7.0 1+ 0 1- 1+ 1+ 1-

1+ 

11.0 1- 1- 0 1+ 1- 2- 

Copyright 1998, 1999, 2007 by Frank R. Gorga;   Page maintained by F.R. Gorga;   Last updated: 12-Mar-2007