Worked Example 18.1 Determining Side-Chain Hydrophobicity/ Hydrophilicity Consider the structures of phenylalanine and serine in Table 18.3. Which of these two amino acids has a hydrophobic side chain and which has a hydrophilic side chain? Identify the side chains. The side chain in phenylalanine is an alkane. The side chain in serine contains a hydroxyl group. The hydrocarbon side chain in phenylalanine is an alkane, which is nonpolar and hydrophobic. The hydroxyl group in the side chain of serine is polar and is therefore hydrophilic.

Worked Example 18.2 Drawing Zwitterion Forms Look up the zwitterionic structure of valine in Table 18.3. Draw valine as it would be found (a) at low pH and (b) at high pH. Analysis At low pH, which is acidic, basic groups may gain H+. At high pH, which is basic, acidic groups may lose H+. In the zwitterion form of an amino acid, the — COO– group is basic and the — NH3+ is acidic. Solution Valine has an alkyl-group side chain that is unaffected by pH. At low pH, valine adds a hydrogen ion to its carboxyl group to give the structure on the left. At high pH, valine loses a hydrogen ion from its acidic — NH3+ group to give the structure on the right.

Worked Example 18.3 Determining Chirality Lactic acid can be isolated from sour milk. Is lactic acid chiral? Analysis A molecule is chiral if it contains one C atom bonded to four different groups. Identify any C atoms that meet this condition. Solution To find out if lactic acid is chiral, list the groups attached to each carbon atom: Next, look at the lists to see if any carbon atom is attached to four different groups. Of the three carbons, carbon 2 has four different groups, and lactic acid is therefore chiral.

Worked Example 18.4 Drawing Dipeptides Draw the structure of the dipeptide Ala-Gly. Analysis You need the names and structures of the two amino acids. Since alanine is named first, it is the amino-terminal and glycine is the carboxyl-terminal. Ala-Gly must have a peptide bond between the — COO– on alanine and the — NH3+ on glycine. Solution The structures of alanine and glycine, and the structure of the Ala-Gly dipeptide are

Worked Example 18.5 Drawing Side-Chain Interactions What type of noncovalent interaction occurs between the threonine and glutamine side chains? Draw the structures of these amino acids to show the interaction. Analysis The side chains of threonine and glutamine contain an amide group and a hydroxyl group, respectively. These groups do not form salt bridges because they do not ionize. They are polar and therefore not hydrophobic. They form a hydrogen bond between the oxygen of the amide carbonyl group and the hydrogen of the hydroxyl group. Solution The noncovalent, hydrogen bond interaction between threonine and glutamine is as follows: