Biochemistry-charge on a Polypeptide

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Biochemistry-charge on a Polypeptide

Post by greenpak07 on Fri Jan 30, 2009 2:20 am

Hi,

Thanks a bunch, I was wondering if you had any familarity with the calculating the charge on an amino acid?
I'm a bit confused on how to calculate the net charge on a peptide. I get that when the pH is greater than the ionizable groups (lysine, arginine, glutamate, asparagine), the base form predominates and that there will be either a neutral charge or a - charge. But how do you know when you will have a neutral plus or negative charge (i.e.) when do you know that there will be a no charge vs. negative?

I am really confused on this one and it is really bugging me.

Thanks a bunch!

Gina

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Re: Biochemistry-charge on a Polypeptide

Post by Alla on Mon Feb 02, 2009 10:07 am

pI is pH at which a molecule has no net charge. You can calculate it as an average of pKa values of ionizable groups. You can find that there are a few formulas and a few sources for pKa values that do not agree with each other, but you can use any of those to calculate approximate theoretical pI (I would suggest to use those from your textbook).

At pH greater than pI the peptide will have -ve charge, at the pH lower than pI it will have a +ve charge overall. You can calculate charge by looking it each side chain pKa. For example, at pH 7.0 most His residues will be neutral, while all Lys will be +ve, etc.

I hope this helps.

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