Problem 6 Atomic and molecular orbitals - PianetaChimica.it

33 rd International Chemistry Olympiad ∗ Preparatory Problemsa. Every amide group in the polypeptide backbone, including its flanking α-carbons, is a planar unit. Explain.b. The α-carbons across each amide unit occur in a trans geometricalarrangement. However, in case of the amino acid proline, both cis and transamide arrangements are almost equally favored. Why?c. The conformational choices of amino acid residues in a polypeptide chain arestereochemically controlled. For nineteen of the genetically coded aminoacids, the conformational choice is largely restricted to the α (folded) and β(extended) regions of the Ramachandran diagram. For the amino acidglycine, however, the conformational choices are much wider. Explain.d. When a linear polypeptide folds forming a globular protein, an amino acidresidue may assume α or β conformation. However it is observed thatconsecutive residues generally assume α or β conformation, rather than arandom combination of α and β. Explain.e. In an aqueous environment polypeptides generally fold into compact globularprotein structures. The reason is (select one)1. The R groups in polypeptides are largely polar.2. The R groups in polypeptides are largely nonpolar.3. Both polar and nonpolar R groups occur in comparable proportion.Justify your answer.f. The pattern of R group polarities has an important role in determining whetherα-helix or β-sheet will form when a polypeptide folds in water at an apolarsurface. Explain the role of R group polarities.Problem 24 Protein sequencingSequencing of a protein (polypeptide) involves the following steps: a) purification, (b)determination of N-terminal amino acid, (c) cleavage of the polypeptide chain bychemical or enzymatic methods, (d) isolation of the peptide fragments and (e)determination of their sequence by an automated sequencing machine (sequenator).It is also possible to sequence the mixture of peptide fragments without resolving it.3434Mumbai, India, July 2001