Page 308 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GRB Final Pages
Encyclopedia of Physical Science and Technology EN016B-738 July 31, 2001 14:0

Stereochemistry 89

up conformation called a β or pleated sheet stabilized by
hydrogen bonding between rather more distant members
of the chain folded onto each other. “Tertiary structure” of
proteins comprises the combination of α-helices, pleated
sheets, and some random-coil areas, which gives rise to
their three-dimensional shape.
The distinction between configuration and conforma-
FIGURE 11 Stable conformers of butane.
tion is usually based on whether the interconversion of
the pertinent stereoisomers is slow or fast. Since a bar-
(French for “skew”). In Fig. 11B the torsion angle is rier of 84 kJ/mole between two species corresponds to
−1
◦
180 ; this conformation is called “anti.” Torsion angles an interconversion rate at 25 Cof1.3 × 10 −2 sec , i.e.,
◦
often deviate from the ideal values (60 or 180 ) for stag- a half-life of 1 min, making isolation of the individ-
◦
◦
gered conformations; thus it may be desirable to specify ual species quite difficult, one might say that the divi-
the exact torsion angle ω when known (e.g., from X-ray sion between configuration and conformation comes at
structure determination). When C(1)–C(2)–C(3)–C(4) barriers of about 84 kJ/mole. However, such a precise
describe a right-handed helical turn, ω is positive; for distinction is problematic. At lower temperatures, inter-
a left-handed turn, it is negative. As an alternative, a conversion rates decrease and isomers that differ only
system of semiquantitative conformational descriptors in conformation (cf. Fig. 9) may become isolable. Also,
more detailed than gauche and anti has been developed by the technique of observation matters. Infrared and Raman
Klyne and Prelog (1960) and is described in the original spectroscopy are “very fast” and thus the vibrational spec-
reference and in Eliel and Wilen (1994). tra of the conformational isomers of 1,2-dibromoethane
In straight-chain hydrocarbons larger than butane, ro- are distinct. Nuclear magnetic resonance (NMR) is in-
tation about each single bond is possible, giving rise to termediate, and there are numerous instances where an
a large number of conformations; this situation exists es- averaged NMR spectrum is seen at one temperature but
pecially in linear polymers, where it was studied early by spectra for the individual conformers emerge at lower
P.Flory.Evenwhenconformationsinwhichthechaincoils temperatures.
upon itself in such a way as to generate excessive van der An interesting example of the fluidity of the delin-
Waals (steric) repulsion are excluded, the number of low- eation between configuration and conformation is seen
energy conformers will be quite large and a Monte Carlo in the biphenyls (Fig. 12). In biphenyl itself, rotation is

approach may have to be used to find the family of pop- fast; thus a 3,3 ,5,5 -tetrasubstituted biphenyl (Fig. 12A)

ulated (low-energy) conformers. (Conformers which lie cannot be resolved into enantiomers, even though con-
12 kJ/mole or more above the lowest energy conformer are formations in which the two rings are not coplanar are
populated to the extent less than 1% of the total and may chiral. However, as soon as sizable substituents are intro-

be neglected for most purposes.) By way of an example, in duced at positions 2, 2 , 6, and 6 (Fig. 12B, X
= Y) the

linear polyethylene, only a minor fraction of the molecules compounds become resolvable; they display axial chiral-
will be in the most stable zigzag (all-anti) conformation. ity. When X and Y (in Fig. 12B) are different and other
These considerations are important in the conforma- than F or CH 3 O, the enantiomers are stable. When one
tion of proteins (natural polymers). When polypeptides are of the four substituents is H, however, the compounds
synthesized—in the laboratory and perhaps also in vivo— are resolvable but usually racemize readily either at room
they are first formed as linear strings (so-called “random temperature or above by rotation about the Ar–Ar bond.
coil” conformations, similar to those of a polyethylene). Biphenyls with only two ortho substituents are generally
But in polypeptides and proteins additional considerations not resolvable unless the substituents are very bulky, as

come into play, notably hydrogen bonding between amino in 1,1 -dinaphthyl (Fig. 12C, Z = H). (The enantiomeric

acid residues and hydrophobic forces generated by the re- 2,2 -dihydroxybinaphthyls, Fig. 12C, Z = OH, have found
luctance of hydrocarbon side chains to be in contact with manifold uses, e.g., as parts of chiral reagents and chiral
the common water solvent. Additional interactions be- catalysts.)
tween nonadjacent amino acids may come about because
of oxidation of cysteine to cystine residues (2 –SH →
–S–S–). These various interactions lead to a folding of the XII. CYCLOALKANES AND THEIR
chain into so-called secondary structures, which include a CONFORMATIONS
helical (α-helix) conformation (Pauling et al., 1951) sta-
bilized mainly by hydrogen bonding between nonadjacent Before considering conformation in cyclic molecules
but close amino acids in the polymer chain, and a doubled- (which is more complex since rotations about individual