CONCEPTUAL EXAMPLE 15 1 Classification of Monosaccharides Shown

CONCEPTUAL EXAMPLE 15. 1 Classification of Monosaccharides Shown below are structures of (left to right) erythrulose, gulose, mannose, and ribose. CHO HO CH 2 OH HO C H C O HO C H C H H C OH H C HO C H H C CH 2 OH CHO HO C H OH H C OH CH 2 OH Classify erythrulose and gulose as an aldose or a ketose. 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. Exercise 15. 1 A Classify mannose and ribose as an aldose or a ketose.

CONCEPTUAL EXAMPLE 15. 1 Classification of Monosaccharides continued Exercise 15. 1 B How does the structure of gulose differ from that of glucose? How does the structure of mannose differ from that of glucose?

EXAMPLE 15. 2 Names of Peptides 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. Solution a. b. M-G-F-A-C The endings of the names for all except the last amino acid are changed from ine to yl. The name is therefore methionylglycylphenylalanylcysteine. Exercise 15. 2 A Give the (a) alternate designation using one-letter abbreviations and the (b) full name for the tetrapeptide His. Pro-Val-Ala. Exercise 15. 2 B Use (a) three-letter abbreviations and (b) one-letter abbreviations to indicate the amino acids in the peptide threonylglycylalanylleucine.

EXAMPLE 15. 3 Numbers of Peptides 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. Solution Write out the various possibilities: Met-Val-Phe Val-Met-Phe-Val Val-Phe-Met There are six possible tripeptides. Phe-Val-Met Phe-Met-Val Exercise 15. 3 A How many different tripeptides can be made from two methionine units and one valine unit? Exercise 15. 3 B How many different tetrapeptides can be made from the four amino acids methionine, valine, tyrosine, and phenylalanine?

EXAMPLE 15. 4 Nucleotides Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA or in RNA. 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.

EXAMPLE 15. 4 Nucleotides continued Exercise 15. 4 A Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA or in RNA. Exercise 15. 4 B A nucleotide is composed of a phosphate unit, ribose, and thymine. Does it appear in DNA, RNA, or neither? Explain.