[elyse07]

I've had a go at the question but I'm unsure if its correct, I'm a little confused as to when the charge is to be 0 or 1/2.

Here is the question:
Oxytocin is a nonapeptide (a nine-residue peptide) hormone involved in the milk-releasing response in lactating mammals. the sequence of a synthetic version of oxytocin is shown bellow:
Cys-Phe-Ile-Glu-Asn-Cys-Pro-His-Gly
what is the net charge of this peptide at (a) pH 2.3, (b) pH 6.0, (c) pH 8.5 and (d) 11.5 ?
Assume that the ionisable groups have the following pKa values: Glu 4.2, His 6.0, N-term-NH₃⁺, C-term -COOH 2.3. The disulfide bond is stable between pH 2.3 and 12.5.

This is how i worked it out, but as i said I'm a little confused about the 1/2 and 0 values for the charge. I worked off pH>pKa proton tends to be off, pH<pKa the proton tends to be on and pH=pKa half of the molecules are protonated and half are deprotonated.

                    NH3+          Glu         His       COOH
     pKa          10.3           4.2         6.0        2.3

(a) pH 2.3
net charge       +1            +1          +1       +1/2
Net charge = +3.5

(b) pH 6.0
net charge       +1            -1        +1/2        -1
Net charge = -0.5

(c) pH 8.5
net charge      +1             -1          -1         -1
Net charge = -2

(d) pH 11.5
net charge       0              -1          -1         -1
Net charge = -3

It would be greatly appreciated if someone could let me know if i did it right and if not explain where i went wrong. Thanks 
   

[Yggdrasil]

When Glu, His, and the C-terminal carboxyl group are protonated, they have a charge of 0, not +1.  Otherwise, you have the right idea.  The +1/2 charge makes sense, not because all the individual molecules will each have a +1/2 charge (impossible), but because half of the molecules will have a charge of +1 and half will have a charge of 0 (giving an average charge of +1/2 over the entire population of molecules).

[elyse07]

Thanks for your help 

[ashleigh]

HEY RGGDRASIL,

just wondering i'm a little stuck as to where you get the charges from, and what other working out do you think would be expected for this type of question?
thanks!

[Yggdrasil]

Charges on proteins result from simple acid/base chemistry.  Acidic and basic groups on the proteins lose/gain protons depending on the pH of the solution.  The loss or gain of the proton will alter the charge of the protein.  Therefore, to be able to answer a question such as this, you need to know which amino acids are acidic/basic and their pKa values (the pH at which they become protonated/deprotonated.  Aspartate, glutamate, and cysteine are acidic and have a negative charge at high pH and a neutral charge at low pH (serine and threonine can also be considered in this group although they have a fairly high pKas).  Lysine, arginine, and histinde are basic and have a neutral at high pH and have positive charge at low pH.  You also need to take into account the effects of the amino group on the N-terminus of the peptide and the carboxyl group at the C-terminus of the peptide.