Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: elyse07 on September 13, 2009, 09:19:53 PM
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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 :)
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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).
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Thanks for your help :)
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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!
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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.