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Terahertz Radiation
roughly 10 3 to 10 6 times stronger than in the millimeter-wave region (Albert et al. 2001).
This property makes THz spectroscopy a very valuable research tool, and many experts
contend that this property may be the single most important feature of the THz spectral
region (Mittleman 2003).
Numerous gases, liquids, and biological materials undergo intermolecular vibrations,
intramolecular vibrations, collective motions, and molecular rotations at THz wave
numbers (3–334 cm −1 ) or periods (0.1–10 ps) (see Figure 7.1). As a consequence, this
region is intrinsically rich with unique spectroscopic signatures that are intrinsically
unavailable in other spectral bands. Here, we briefly discuss the properties of water and
several biological macromolecules in this spectral range. For a more detailed discussion,
we refer the reader to several excellent books and articles (Albert et al. 2001; Beard et al.
2002; Globus 2003; Mittleman 2003; Svanberg 2004; Fischer 2005; Pivonka et al. 2007;
Dexheimer 2008).
Covering ∼70% of the Earth’s surface and accounting for ∼60%–70% of the total
weight of an average human being, water is the most ubiquitous chemical substance
on planet Earth. Water exhibits a plethora of unique chemical and physical properties
that make it essential to human life: (1) Water is “the universal solvent” in which all
biomolecules (i.e., carbohydrates, proteins, and nucleic acids) are dissolved in; (2) Water
is required for the proper folding and function of all biomolecules (Pal et al. 2002a,b;
Pal et al. 2003a,b; Pal and Zewail 2004); (3) Water consists of oxygen atoms that have
a higher electronegativity (i.e., δ − ) than hydrogen atoms; (4) Water is a polar molecule
with a large dipole moment; and (5) Water molecules form a tetrahedral arrangement of
intermolecular hydrogen bonds to its neighboring molecules (Figure 7.2a).
THz radiation interacts strongly with water due to the slow relaxation time of water
molecules. This property is closely associated with rotational dynamics and intermolecular hydrogen-bond rearrangements (i.e., intermolecular bends), which occur on the picosecond timescale (Ladanyi and Skaf 1993; Russo et al. 2003). Therefore, in order to probe
and capture these dynamics, experimental methods must also function on the picosecond
timescale (Figure 7.2a). Millimeter-wave and IR spectroscopic methods permit measurements on the ∼100 ps and subpicosecond timescale, respectively. However, THz spectroscopic techniques can capture dynamics on the 0.1–10.0 ps timescale. This fundamental
property makes THz tools ideal for probing water dynamics in biological structures.
In addition to water, many biological macromolecules (e.g., carbohydrates, lipids,
proteins, and DNA) undergo vibrational motion on the picosecond timescale. These
collective modes include intramolecular vibrations (i.e., stretch, bond, and torsion),
intermolecular vibrations (i.e., van der Waals forces and hydrogen bonding), and
molecular rotations (i.e., slow relaxation of water). For example, carbohydrates, which
are essential molecules for energy storage and structural maintenance, exhibit several
distinct resonances in the THz region (typically, below 200 cm –1 ) (Walther et al. 2000;
Globus et al. 2002; Xie 2002; Globus 2003). In addition, monosaccharide isomers (i.e.,
glucose, galactose, mannose, and fructose) exhibit vastly different spectra at THz wave
numbers (70–100 cm –1 ), despite the fact that they have very similar molecular structures (Fischer 2005). Studies have also shown that the photoisomerization of retinal, an
integral step in the vision process, exhibits a vibrational mode at THz wave numbers
(∼60 cm –1 ; Wang et al. 1994).