1. CONCEPTUAL EXAMPLE 15.1Classification of Monosaccharides Classify mannose and ribose as an aldose or a ketose. Exercise 15.1A Shown below are structures of (left to right) erythrulose, gulose, mannose, and ribose. CH 2 OH CO CHHO CH 2 OH CHO HOCH CH HCOH HO CH CH 2 OH Classify erythrulose and gulose as an aldose or a ketose. CHO HOCH CH HCOH H C CH 2 OH CHO HOCH HCOH H C CH 2 OH Solution Erythrulose is a ketose. The carbonyl (C O) group is on the second carbon atom from the top; it is between two other carbon atoms, a situation that defines a ketone. Gulose is an aldose. The carbonyl group is on an “end” carbon atom, as it is in all aldehydes.

2. How does the structure of gulose differ from that of glucose? How does the structure of mannose differ from that of glucose? Exercise 15.1B CONCEPTUAL EXAMPLE 15.1Classification of Monosaccharides continued

3. Give the (a) alternate designation using one-letter abbreviations and the (b) full name for the pentapeptide Met- Gly-Phe-Ala-Cys. You may use Table 15.3. Give the (a) alternate designation using one-letter abbreviations and the (b) full name for the tetrapeptide His- Pro-Val-Ala. Exercise 15.2A Use (a) three-letter abbreviations and (b) one-letter abbreviations to indicate the amino acids in the peptide threonylglycylalanylalanylleucine. Exercise 15.2B EXAMPLE 15.2 Names of Peptides Solution a. M-G-F-A-C b. The endings of the names for all except the last amino acid are changed from ine to yl. The name is therefore methionylglycylphenylalanylalanylcysteine.

4. How many different tripeptides can be made from the three amino acids methionine, valine, and phenylalanine? Write them, using the three-letter designations in Table 15.3. How many different tripeptides can be made from two methionine units and one valine unit? Exercise 15.3A How many different tetrapeptides can be made from the four amino acids methionine, valine, tyrosine, and phenylalanine? Exercise 15.3B EXAMPLE 15.3 Numbers of Peptides Solution Write out the various possibilities: Met-Val-Phe Val-Met-Phe Phe-Val-Met Met-Phe-Val Val-Phe-Met Phe-Met-Val There are six possible tripeptides.

5. EXAMPLE 15.4 Nucleotides Solution The sugar has two H atoms on the second C atom and therefore is deoxyribose. The base is a pyrimidine (one ring). The amino (NH 2 ) group helps identify it as cytosine. Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA or in RNA.

6. EXAMPLE 15.4 Nucleotides continued A nucleotide is composed of a phosphate unit, ribose, and thymine. Does it appear in DNA, RNA, or neither? Explain. Exercise 15.4B Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA or in RNA. Exercise 15.4A