Raven H. answered • 10/08/21
Patient, Encouraging Tutor Specializing in Science Courses
Hi Lauren,
-Start by writing out the tripeptide sequence including the N terminus and C terminus:
NH2--Gly--Arg--Lys--COO-
-Notice that you only include the N and C termini for the Glycine and Lysine residues because the N and C groups for the Arg residue are used up in the polypeptide bond that connects them to the other 2 residues.
-In this case, the pH is 11.5. This means the NH2 group would carry a neutral charged due to its pka of around 9. That means that once the pH is higher than 9 (as it is in this case) the NH3+ group loses its proton and becomes an NH2 group.
-The pka of the COO- group is around 2. This means that at any pH higher than 2, the COOH group would lose its proton and become a COO- group carrying a -1 charge.
-Now you look at the side chains:
-Glycine as a neutral side chain.
-Arg has a side chain with a pka of 12.5. This means that at our pH (11.5) the Arg would still have its proton giving it a charge of +1.
-Lys has a side group with pka of 10.5 so at a pH of 11.5, the Lys would have lost its proton giving it a -1 charge.
-Now we add up all the charges
0 from the NH2 group at the N terminus
0 from the neutral Gly side chain
+1 from the protonated Arg side chain
-1 from the unprotonated Lys side chain
-1 from the COO- group at the C terminus
0+0+1-1-1=-1
I hope this helps!
