energy is positive. Thus the hydrophobic effect of nonpolar molecules in
water is a process driven by entropy rather than by enthalpy.
Only when the number of nonpolar molecules increases above a critical
point will the tendency to disperse the nonpolar molecules be overcome,
resulting in phase separation, with the lipids forming structures such
as micelles (Chapter 4). Hydrophobic effects also participate in protein
folding. In an unfolded state, the hydrophobic amino acid residues are
exposed to the surrounding water. Since the sequestering of the hydro-
phobic residues is energetically more favorable, the protein conformation
will change into a configuration with a hydrophobic interior in a pro-
cess termed a hydrophobic collapse. Since the hydrophobic effect is not
specific, this conformation is not unique but represents an intermediate
state before other interactions stabilize the final state.
Secondary structure
One of the most common arrangements for proteins is a compact struc-
ture consisting of αhelices. In these proteins, the αhelices are packed
pairwise against each other so that one side of each helix provides a hydro-
phobic surface facing the interior, and the other side is hydrophilic and
faces the aqueous solution. The classic examples of globular proteins with
αhelices are myoglobin and hemoglobin, which were the first proteins
to have their structures solved in the 1950s by John Kendrew and Max
Perutz (Figure 13.10). These proteins also contain a heme cofactor, which
serves as the binding site for oxygen as the proteins transport oxygen
in the muscles and cardiovascular system. Hemoglobin serves to bind
oxygen under high oxygen levels in the lungs, carry the oxygen through
the bloodstream, and release it to myoglobin in the muscles. The oxygen
is bound to hemoglobin until it is needed for aerobic work. The binding
of oxygen to the four hemes in hemoglobin is regulated allosterically by
interactions among the four polypeptide chains.
Another common protein structure is the β structure, formed by
antiparallel βstrands. The simplest arrangement is obtained when each
successive βstrand is added adjacent to the previous strand, with a small
twist yielding a βbarrel if the protein is cylindrical, or a βsandwich, as
found for more elongated proteins such as the Fenna–Matthews–Olsen,
or FMO, protein (Figure 13.11). These proteins can bind a cofactor in the
center, which is well sequestered from the surrounding environment. For
example, the Fenna–Matthews–Olsen protein binds seven bacteriochloro-
phylls that are part of the light-harvesting pathway in photosynthetic green
bacteria. Notice that, in addition to the βsheets, there are several αhelices;
the presence of both types of secondary structure is very common in pro-
teins. The role of the seven bacteriochlorophylls in transferring light energy
is discussed in Chapter 14.
282 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
