Question

A tetrapeptide with the sequence, (lysine-isoleucine-asparatate-seriene), has been isolated from a protein hydrolysate mixture.  A solution of the tetrapeptide (500. ml of 0.80 M) is mistakenly left out of the freezer for several days, and the investigator suspects that trace amounts of proteolytic enzymes may be present.  A.  Explain (using a minimum of words) how the investigator can determine by titration whether the solution contains the intact tetrapeptide or an equimolar mixture of the individual amino acids (lysine-isoleucine-asparatate-seriene) (assume these are the only two possibilities).  

B.  Schematically show the titration curves expected in each case.  Be sure to label the axes.

Answer 1

A. Like amino acids, peptides also have characteristic titration curve. They have only one free -amino group and one free -carboxyl group.

In addition to this, they also contain the ionizable R groups present in some amino acids. The acid-base behaviour of any peptide can thus be predicted based on the constituent amino acids- their free amino and carboxyl group and the number of ionizable R groups.

Based on the above information,
   (i) If the tetrapeptide had not degraged, you should expect a single continuous curve with 5 points of inflection
   (ii) If the tetrapeptide had degraded, 4 different titrations would be observed