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Chapter 49 Suggested solutions for Chapter 49 465 Purpose of the problem Checking on your understandzing of the binding of the two strands in DNA. Suggested solution The true pairs have hydrogen bonds between NHs and carbonyl groups or pyridine-like nitrogen atoms. In each case we must have a purine-pyrimidine pair. Two purines would be pushed too close together inside the DNA double helix to form hydrogen bonds and two pyrimidines would be too far apart. the A-T base pair the G-C base pair H Me N H thymine H H cytosine N N NR N N NR N H N H N 0 0 H N N N N N R R adenine guanine H There are no OHs in the major tautomers but there are in the minor tautomers. So a group, say C=O, that accepts a hydrogen bond in the main tautomer becomes a hydrogen bond donor in the minor tautomer. You might have chosen a number of different possibilities; here are just two. We have chosen these because they show the GT pairs not observed in natural DNA. thymine tautomer Me Me 0 thymine H H 0 NR guanine tautomer N N NR N H H N N N 0 0 H H N N N N N N R R guanine H H Problem 3 Dialkyl phosphates are generally hydrolysed quite slowly at near-neutral pHs but this example hydrolyses much more rapidly. What is the mechanism and what relevance has it to RNA chemistry? 0 OR P P OH fast OH Revision of Chapter 41. This reaction is subject to general base catalysis. Explain. Purpose of the problem Exploring the similarities of phosphate ester hydrolysis to ordinary ester hydrolysis. Suggested solution The completely uncatalysed reaction involves nucleophilic attack by the OH group on the bond to form a five-covalent intermediate much like the tetrahedral intermediate in carboxylate ester hydrolysis. There is a difference. Phosphorus prefers five-membered rings to any other