Chapter 5 – Detection and Imaging Tools that Use Nonoptical Waves  193

A terahertz spectrometer is very similar in design to an FTIR spectrometer, in measuring

the laser transmission in a cryofixed sample over a typical frequency range of ~0.3–​10 THz.

Samples are mounted in polyethylene holders, which are transparent to THz radiation, and

held on an ultracooled stage at a temperature of ~4 K or less by liquid helium to minimize

vibrational noise in the sample. THz spectroscopy has been applied in particular to investi­

gating different topologies and flexibility of both DNA and RNA molecules in a cryofixed but

physiologically relevant hydrated state, with an ability to detect base pair mutations in short

oligonucleotide sequences from differences to the THz transmission spectra. THz spectros­

copy can also be adapted to slow confocal scanning techniques across a thin sample, thus

allowing terahertz imaging.

T-​rays also have biophysical applications for tissue imaging. Intense T-​rays can be con­

trollably generated from a variety of sources, for example, both a synchrotron and free-​

electron laser (FEL) in addition to generating a continuum of x-​ray radiation can be utilized

to provide a stable source of T-​rays, but also smaller sources that do require a very large

facility such as lower power FEL sources or free-​electron masers (which generate T-​rays

through cyclotron resonance of electrons in a device called a “gyrotron”), in this case due to

high-​frequency T-​rays overlapping with low-​frequency microwaves in the electromagnetic

spectrum. Unlike x-​rays, T-​rays are nonionizing due to a lower photon energy and so do

not result in the often high level of cellular damage of x-​rays, especially due to damage of

cellular DNA.

However, T-​rays can penetrate into millimeters of biological tissues, which have low water

content, such as fat, but have a high reflectivity for high water content tissues. Thus, T-​rays

can be used to measure tissue differences in water content, which has been used for the

detection of various forms of epithelial cancer. Similarly, T-​rays have been applied to gener­

ating more accurate images of teeth compared to x-​rays in dentistry (see Chapter 7).

A recent application of T-​rays has involved investigations of the structural states of the

protein lysozyme, which was used as a model enzyme system (Lundholm et al., 2015). Here,

the time-​resolved structure of lysozyme was monitored at as low as ~1 K temperatures using

x-​ray crystallography, before and after bombarding the crystals with T-​rays. Instead of being

dissipated rapidly in a few nanoseconds as heat in the anticipated process of thermaliza­

tion, the T-​rays were absorbed in a spatially extended state of several coupled lysozyme

molecules, extending the absorption lifetime to time scales three to six orders of magni­

tude longer than expected for single molecules. This coupled system is consistent with a

state of condensed matter theoretically predicted in 1968 by Herbert Fröhlich (1968) as a

possible theoretical mechanism for ordered energy storage in dielectric biomolecules in cell

membranes, but never experimentally confirmed until now, called the Fröhlich condensate,

which is the lowest order vibrational mode of condensed dielectric matter analogous to the

Bose–​Einstein condensate of a gas of bosons in quantum mechanics—​in essence, long-​range

electrostatic Coulomb forces are coupled between molecules in a pool, resulting in coherent

electrical oscillations, thus trapping absorbed energy of the right frequency (T-​rays, in this

case) for much longer than would be expected from individual electric dipole oscillations.

The result is still being hotly debated as it could have enormous relevance to the existence of

nontrivial quantum mechanical effects in many biological processes (see Chapter 9) and cer­

tainly may have implications for the real mode of operation of enzymes on their substrates,

that is, potentially involving more physical-​based processes cooperatively than what were

imagined previously.

Worked Case Example 5.2: NMR Spectroscopy

An NMR spectrometer contains a bespoke superconducting solenoid magnet with a length

of 7 cm and an inner bore diameter of 5 cm, and an outer diameter of 6 cm was composed

of tightly wound, superconducting wire with a diameter of 0.85 mm, with each wire com­

prising ~500 individual conducting filaments. If cooled to ~4 K, a stable coil current of

~100 A was possible in each filament.