Quiz 7 Answer Key April 2, 2005

Question 1. The structure of amino acid isoleucine is shown below. The amino acid carbon (C2) bears an S-configuration.

a. There is only one more chiral (stereogenic) center in this molecule. Which carbon? C 3 carbon. There are four different groups attach to the C3 carbon, see below. b. Draw all possible stereoisomers (with C2 in S-configuration) and clearly show the absolute configuration (R or S) at each chiral center. Note: the solid wedge and dashed wedge convention (see page 7).

c. How are each of these stereoisomers (in part b) related to the others? Are they enantiomers or diastereomers? (2S, 3R) and (2S, 3S) are diastereomers to each other. d. Draw a representative Newman projection for each stereoisomer by looking down from C2 to C3, showing the staggered conformations.

Quiz 7 Answer Key April 2, 2005

Question 2. There are 20 common natural amino acids in proteins, see Table 23.1 on p960-1. Most amino acids have an S-configuration at the amino acid carbon (see p964). Please depict an S-configuration on paper. Please identify amino acids that do not have an S-configuration. Also identify other stereogenic centers in these 20 amino acids, if there is any.

S-configuration of an amino acid:

COOH H

H2N H R COOH

H2N R

Amino acids that do not have an S-configuration: O

H2N ƒ Achiral amino acid GLYCINE OH ƒ Amino acid CYSTEINE with R-configuration at the amino acid carbon S H

N

O

Amino acids with stereogenic centers other than the amino acid carbon:

Quiz 7 Answer Key April 2, 2005

Question 3. Catalytic hydrogenation follows a syn addition pathway, i.e., the two hydrogen atoms are added to a multiple bond on the same side, see the inserted scheme. Please draw all hydrogenation products from the E/Z olefins, and assign absolute configuration to each chiral center. What are relationships among these isomers? Is each stereoisomer optical active? How about the reaction mixtures? How many 13C-NMR signal you expect for each isomer and the mixtures?

+

identical compound (meso, so mirror image relationship among isomers: but can be superimposed on each other) optically active? no no reaction mixture: no (meso compound)

13C-NMR: 2 peaks 2 peaks reaction mixture: 2 peaks

+

relationship among isomers: Enantiomers (1R, 2R) and (1S, 2S)

optically active? yes yes reaction mixture: no (racemic mixture)

13C-NMR: 2 peaks 2 peaks reaction mixture: 2 peaks Quiz 7 Answer Key April 2, 2005

Question 4. Are the melting points of (+)-serine, (-)-serine and (±)-serine the same or different? Why?

The melting points are ƒ (+)- serine 220 °C ƒ (-)- serine 222 °C (difference from the melting point of (+)-serine is insignificant, so it can be assumed that (+)-serine and (-)-serine have the same melting point) ƒ racemic mixture 240 °C

The crystal structures for (+)-serine and (-)-serine are mirror images of each other with exactly same nature of interactions (directions of interactions, of course, are mirror images), so the melting points for (+)-serine and (-)-serine are the same. Interactions in the crystal structure of racemic mixture are different resulting in the higher melting point.