108 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
molecular orbitals after absorption of light energy.
Ultraviolet photons have sufficiently high energies
to excite electrons into higher molecular orbitals.
Combined with vibrations and rotations, these tran-
sitions give rise to very broad bands in the UV spec-
trum. As a result, gaseous, liquid, and solid forms of
water strongly absorb UV light (Berkowitz 1979).
The absorption intensities and regions of water va-
por are different from those of ozone, but both are
responsible for UV absorption in the atmosphere.
Incidentally, both triatomic water and ozone mole-
cules are bent.
HYDROGENBONDING ANDPOLYMERICWATER
INVAPOR
Attraction between the lone pairs and hydrogen
among water molecules is much stronger than any
dipole-dipole interactions. This type of attraction is
known as the hydrogen bond (O–H⎯O), a very
prominent feature of water. Hydrogen bonds are
directional and are more like covalent bonds than
strong dipole-dipole interactions. Each water mole-
cule has the capacity to form four hydrogen bonds,
two by donating its own H atoms and two by accept-
ing H atoms from other molecules. In the structure of
ice, to be described later, all water molecules, except
those on the surface, have four hydrogen bonds.
Attractions and strong hydrogen bonds among
molecules form water dimersand polymeric water
clustersin water vapor. Microwave spectroscopy
has revealed their existence in the atmosphere (Gold-
man et al. 2001, Huisken et al. 1996).
As water dimers collide with other water mole-
cules, trimer and higher polymers form. The direc-
tional nature of the hydrogen bond led to the belief
that water clusters are linear, ring, or cage-like
rather than aggregates of molecules in clusters (see
Fig. 5.4). Water dimers, chains, and rings have one
and two hydrogen-bonded neighbors. There are
three neighbors per molecule in cage-like polymers.
Because molecules are free to move in the gas and
liquid state, the number of nearest neighbors is
between four and six. Thus, water dimers and clus-
ters are entities between water vapor and condensed
water (Bjorneholm et al. 1999).
By analogy, when a few water molecules are inti-
mately associated with biomolecules and food mole-Figure 5.4.Hydrogen bonding in water dimers and cyclic forms of trimer and tetramer. Linear and transitional forms
are also possible for trimers, tetramers, and polymers.