Consider a peptide with the sequence KMQPRDK. a) Draw the full structure of the peptide at pH 5 with natural stereochemistry.

b) Draw structures for the two possible conformations of the peptide bond between Gln-Pro. Why is one of these two conformations typically favoured over the other?

c) For the proline residue in the KMQPRDK peptide, would a phi angle of -120 and psi angle of +120 be favoured? Explain why or why not. If not, suggest an angle that would be favoured. (Hint: You may wish to use the Ramachandran plots)

d) Where would cyanogen bromide (NC-Br) cleave the peptide KMQPRDK? e) Would you need to use cyanogen bromide to cleave this peptide before you could carry out Edman degradation and why/why not? f) Write the arrow pushing mechanism of the peptide bond cleavage from part a using cyanogen bromide. Make sure to show all steps and arrows. You can abbreviate the amino acids towards the N and C termini but the amino acids on either side of the cleavage site must be drawn out fully in your mechanism (include correct stereochemistry).