A formate buffer (pka = 3.75) is prepared by mixing 50 mL of a 4 M formic acid a
ID: 597178 • Letter: A
Question
A formate buffer (pka = 3.75) is prepared by mixing 50 mL of a 4 M formic acid and 2 g of solid sodium hydroxide (MW= 40g/mol) in total volume of 1.0 L. A peptide with the amino acid sequence C-P=T-Y-R-L-D-Q (MW = 995.12 g/mol) is dissolved in this solution at a concentration of 5 micromol.a. What is the pH of this solution?
b. What is the net charge of the peptide in solution?
c. Calculate the approximate pH at which this peptide would have a net charge of zero.
Explanation / Answer
I will not do all of the problems (pH) for you, but I give you an idea of how to solve for the net charge at each pH. pH = 3 Within your peptide chain, you have some acidic residues and some neutral ones like leucine and isoleucine. Anyway, you know at pH =3 you have more H+ ions in solution, which implies that NH2---> NH3+ and COO- ---> COOH and so on. That is all you have to do for this. Figure out which rediues will be protonated (+1), neutral (0), or deprotonated (-). But here is the catch. You need to know the pKa values of each of the side chains so that you can determine which ones will be neutral (or basically, the isoelectric point). Each side chain has its own isoelectric point, for where it will be neutral. The way I would attempt this problem is like so: You should know that the alpha amino group has a pKa = ~9 and the alpha carboxylic group to have a pKa =~2. (this will be for the terminal groups of the peptide chain). Then you should calculate a tritration curve for each of the amino acids. Once you have a tritration cure, you can figure what will happen to each amino acid at a particular pH. For example, for the alpha amino group, at a low pH, the NH2--> NH3, which would be +1. Notice for some residues, they will be chemically neutral, which only contain carbon side chains. I am sure that you can find tritration curves for each of the amino acids in your biochemistry book or on the internet. If you have any further question, let me know. good luck.
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