228 6.4 Magnetic Force Methods
of different biomolecule types including proteins/peptides, lipids, and several other small
molecules such as synthetic drugs.
SILAC is a popular technique used in quantitative proteomics, in detecting differences
in protein amounts from cell samples using nonradioactive isotope labeling. Typically, two
populations of cells are grown in culture medium, one contains normal amino acids, while
the other contains amino acids labeled with stable nonradioactive heavy isotopes, usually
replacing the normal carbon 12C isotope with the heavier 13C with one more labeled amino
acid type in the growth medium. For example, if arginine (which contains six carbon atoms
per molecule) was used, then all peptides and proteins containing arginine in the cells would
be ~6 Da heavier per molecule of arginine present, compared to the “normal” cell population
grown in parallel. Another approach involves more uniform labeling with 13C or the heavier
15N isotope of nitrogen. Both cell populations are then analyzed using mass spectrometry
and then compared in a pairwise fashion for chemically identical peptide ion fragments.
The measured ratio of signal intensity of such paired fragments in the mass spectrum is an
estimate for the relative abundance of a cellular protein that contains those specific heavier
amino acids. It can thus be used as a tool to measure the different expression levels of different
proteins from a live-cell population.
Biophysical applications of mass spectrometry are significant and include sensitive bio
logical particle detection. The detection sensitivity is around one particle per liter, which
compares favorably relative to other bulk ensemble average techniques. A particle can be
detected with a sampling time resolution of a few minutes. The technique has been applied
for investigations of sample purity quality control, detection of relatively subtle mutations in
nucleic acids, protein conformation and folding studies, and proteomics experiments investi
gating protein–protein interactions. The Simple Analysis at Mars (SAM) instrument suite of
NASA’s rover that landed on Mars on August 5, 2012, included a portable mass spectrometer
device for detection of putative biological material. Also, the spatial resolution of state-of-
the-art mass spec devices now permit precision down to the level of just a few cells and, in
some exceptional cases of relatively large cells, just a single cell.
6.4.3 MAGNETIC TWEEZERS
Magnetic particles that have a length scale range of hundreds to thousands of nanometers
can be controlled directly and efficiently via the manipulation of the local external B-field
using field strengths in the milli-Tesla (mT) range. This force transduction device is com
monly referred to as magnetic tweezers. Both paramagnetic and ferromagnetic beads of
around a micron diameter are typical probes used. This has been used to great success for
investigating the mechanical properties of several types of biopolymer molecules, especially
DNA (Manosas et al., 2010).
The external B-field in the magnetic tweezers setup is usually built as a module to an
inverted optical microscope, with either two permanent bar magnets mounted to have
juxtaposed poles or a combination of multiple electromagnetic coils to generate a suitable
mT B-field placed around the magnetic probe (Figure 6.5a). By moving the microscope stage,
a candidate bead can be captured in the locally generated B-field. Ferromagnetic beads con
tain a permanent magnetic dipole moment m, and the interaction between this and the local
B-field, as indicated by the gradient of their dot product, results in a force F on the bead and a
torque τ, which results from their cross product, rotating the bead in a direction so as to align
the magnetic moment with the B-field:
(6.13)
F
m B
= ∇(
)
.
(6.14)
τ =
×
m
B
It is more common to use paramagnetic beads in magnetic tweezers. In this instance, a mag
netic dipole moment is induced in the bead by the external B-field, and in typical milli-Tesla