Question

The following dipeptide is Asp-Tyr. Indicate the charge state for each structure at the given pH by adding the or – charges to the appropriate functional groups and adding or removing hydrogen atoms to the structure where appropriate.

Answer 1

Assume the pKa of the residues are the same as for the free amino acids in solution then you would have 3 titratable sites:
1) The N-terminal
2) The C-terminal
3) Side chain on Asp1

pKa:
Asp N-terminal: 9.7
Tyr C-terminal: 2.2
Asp side chain: 3.7

Use your knowledge from organic chemistry to remember what the different protonation states of an amide group and carboxyl acid looks like together with the pKa to find the protonation state and structure of your dipeptide at different pHs.

Our assumption is not perfect, as it does not account for induction effects, and generally the structure made by the protein, but it will suffice as it is only a dipeptide and if you have no more data than what you presented us here.