The Foundations of Chemistry

RH. Figure 28-6 shows this mirror-image relationship for optical isomers of phenyl-
alanine, in which RXCH 2 C 6 H 5. All naturally occurring phenylalanine in living systems
is the L-form. In fact, only one isomer of each of the various optically active amino acids
is found in proteins.

CONFORMATIONS

The conformationsof a compound differ from one another in the extent of rotation about
one or more single bonds.The CXC single bond length, 1.54 Å, is relatively independent
of the structure of the rest of the molecule.
Rotation about a single CXC bond is possible; in fact, at room temperature it occurs
very rapidly. In drawings and models, like those in Figure 28-7, it might at first appear
that rotation about a single bond would be unrestricted. The electrons of the groups

28-3

As we saw in Section 27-3, rotation

does not occur around carbon–carbon

double bonds at room temperature.

28-3 Conformations 1113

H

C

HH

H

C

HH

H

C

HH

C

HH

H

Staggered

conformation

Eclipsed

conformation

(a) (b)

Figure 28-7 Two possible conformations of ethane. (a) Staggered. (b) Eclipsed. Rotation of
one CH 3 group about the CXC single bond, as shown by the curved arrows, converts one
conformation to the other.

Figure 28-6 Models of the two optical isomers of phenylalanine. The naturally occurring
phenylalanine in all living systems is the form shown on the left.