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-Glywhat 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