Question

Question 10 (10 pts) Peptides are biological oligomers formed by condensing multiple amino acids into an amide backbone. Although this amide backbone is common to all peptides, the so- called 'side-chan' groups attached to this backbone vary depending the amino acids used in the condensation reaction. Shown below are four different tetrapeptides (so-named because they are the products of the condensation of four amino acids). Based on their structures, predict the order of elution that one might expect if separating them using reversed-phase HPLC with a C- 18 stationary phase and a mixture of acetonitrile and water as the mobile phase General structure of a tetrapeptide "N-terminus HN OH C-terminus 0 R OR First to elute amide backbone residue side-chains Second to elute Peptide A Peptide B Third to elute HEN OH Fourth to elute OH OH alanine-threonine-alanine-tyrosine Peptide alanine-valine-alanine phenylalanine Peptide D HN OH OH O он o COOH valino-Isoleucine-Isoleucine-phenylalanine NH HN NH serine-arginine-serine-aspartic acid (Hint - recall that C18 stationary phases have long hydrocarbon chains that interact with analyte principally through dispersion forces).