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Radioactivity in Life Sciences
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Radioactivity can be used in life sciences as a radiolabel to visualise
components or target molecules in a biological system. Some radionuclei are
synthesised in particle accelerators and have short half-lives, giving them high
maximum theoretical specific activities. This lowers the detection time
compared to radionuclei with longer half-lives, such as carbon-14. In some
applications they have been substituted by fluorescent dyes.
Examples of radionuclei
 Tritium (hydrogen-3) is a very low energy emitter that can be used to
label proteins, nucleic acids, drugs and toxins, but requires a tritiumspecific film or a tritium-specific phosphor screen. In a liquid scintillation
assay (LSA), the efficiency is 20–50%, depending on the scintillation
cocktail used. The maximum theoretical specific activity of tritium is 28.8
Ci/mmol (1.066 PBq/mol). However, there is often more than one tritium
atom per molecule: for example, tritiated UTP is sold by most suppliers
with carbons 5 and 6 each bonded to a tritium atom. C-14, S-35 and P-33
have similar emission energies. P-32 and I-125 are higher energy emitters
-> inaccurate, see beta vs gamma radiation.
 Carbon-14 has a long half-life of 5,730±40 years. Its maximum specific
activity is 0.0624 Ci/mmol (2.31 TBq/mol). It is used in applications such
as radiometric dating or drug tests.
 Sodium-22 and chlorine-36 are commonly used to study ion transporters.
However, sodium-22 is hard to screen off and chlorine-36, with a half-life
of 300,000 years, has low activity.
 Sulfur-35 is used to label proteins and nucleic acids. Cysteine is an amino
acid containing a thiol group which can be labeled by S-35. For
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nucleotides that do not contain a sulfur group, the oxygen on one of the
phosphate groups can be substituted with a sulfur. This thiophosphate
acts the same as a normal phosphate group, although there is a slight bias
against it by most polymerases. The maximum theoretical specific
activity is 1,494 Ci/mmol (55.28 PBq/mol).
 Phosphorus-33 is used to label nucleotides. It is less energetic than P-32
and does not require protection with plexi glass. A disadvantage is its
higher cost compared to P-32, as most of the bombarded P-31 will have
acquired only one neutron, while only some will have acquired two or
more. Its maximum specific activity is 5,118 Ci/mmol (189.4 PBq/mol).
 Phosphorus-32 is widely used for labeling nucleic acids and
phosphoproteins. It has the highest emission energy (1.7 MeV) of all
common research radioisotopes. This is a major advantage in experiments
for which sensitivity is a primary consideration, such as titrations of very
strong interactions (i.e., very low dissociation constant), footprinting
experiments, and detection of low-abundance phosphorylated species.
32P is also relatively inexpensive. Because of its high energy, however, a
number of safety and administrative controls are required (e.g., acrylic
glass). The half-life of 32P is 14.2 days, and its maximum specific
activity is 9131 Ci/mmol.
 Iodine-125 is commonly used for labeling proteins, usually at tyrosine
residues. Unbound iodine is volatile and must be handled in a fume hood.
Its maximum specific activity is 2,176 Ci/mmol (80.51 PBq/mol).
 A good example of the difference in energy of the various radionuclei is
the detection window ranges used to detect them, which are generally
proportional to the energy of the emission, but vary from machine to
machine: in a Perkin elmer TriLux Beta scintillation counter , the H-3
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energy range window is between channel 5–360; C-14, S-35 and P-33 are
in the window of 361–660; and P-32 is in the window of 661–1024.
Detection
Quantitative
In a liquid scintillation assay (LSA), or liquid scintillation counting (LSC), a
small aliquot, filter or swab is added to scintillation fluid and the plate or vial
counter in a scintillation counter.
A Geiger counter is a quick and rough approximation of activity. Lower energy
emitters such as tritium can not be detected.
Qualitative
Autoradiography: A membrane such as a Northern blot or a hybridised slot blot
is put against a film that is then developed.
Phosphor storage screen: The membrane is placed against a phosphor storage
screen which is then scanned in a phosphorimager. This is ten times faster and
more precise than film and the result is already in digital form.
Microscopy
Electron microscopy: The sample is not exposed to a beam of electrons but
detectors picks up the expelled electrons from the radionuclei.
Micro-autoradiography imager: A slide is put against scintillation paper and in a
PMT. When two different radiolabels are used, a computer can be used to
discriminate the two.
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Scientific methods
Schild regression is a radioligand binding assay. It is used for DNA labelling (5'
and 3'), leaving the nucleic acids intact.
Radioactivity concentration
A vial of radiolabel has a "total activity". Taking as an example γ32P ATP,
from the catalogues of the two major suppliers, Perkin Elmer NEG502H500UC
or GE AA0068-500UCI, in this case, the total activity is 500 μCi (other typical
numbers are 250 μCi or 1 mCi). This is contained in a certain volume,
depending on the radioactive concentration, such as 5 to 10 mCi/mL (185 to 370
TBq/m3); typical volumes include 50 or 25 μL.
Not all molecules in the solution have a P-32 on the last (i.e., gamma)
phosphate: the "specific activity" gives the radioactivity concentration and
depends on the radionuclei's half-life. If every molecule were labelled, the
maximum theoretical specific activity is obtained that for P-32 is 9131
Ci/mmol. Due to pre-calibration and efficiency issues this number is never seen
on a label; the values often found are 800, 3000 and 6000 Ci/mmol. With this
number it is possible to calculate the total chemical concentration and the hotto-cold ratio.
"Calibration date" is the date in which the vial’s activity is the same as on the
label. "Pre-calibration" is when the activity is calibrated in a future date to
compensate for the decay occurred during shipping.
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