Page 11 - PERIODIC Magazine Issue 6

Li
Making ght work

of weighing

molecules

Professor Philipp Kukura, Professor Justin Benesch and colleagues have
developed a new method of weighing single molecules, through using
light. This microscopy technique can also be used to follow the molecular
dynamics of molecules in solution in real time, such as monitoring protein

complexes as they form.

Mass spectrometry is a powerful method that has been glycosylated proteins and lipoproteins. In addition,
used by chemists for decades to deduce the mass (or iSCAMS allows molecular dynamics to be observed,
more accurately, the mass to charge ratio) of molecules such as how two protein units join together.
present in a system. Translating gas phase mass
spectrometry to the solution phase, and in particular One of the initial problems with the technology was that
reaching single molecule sensitivity, however, has been the quality of the technique was insufficient to compete
extremely challenging. with currently available methods that observe molecules
through fluorescence. The researchers at Oxford
Essentially all optical single molecule techniques rely University therefore developed a method to selectively
on fluorescence, which involves the emission of light suppress the background signal (i.e. the light observed
from a molecule after being excited by electromagnetic from sources that aren’t protein molecules) without
radiation. Not all molecules are fluorescent, however, so interfering with the molecular signal. This change led to
recent efforts have concentrated on the development of the development of an imaging technology with a high
more universally applicable methods. enough mass resolution, accuracy and precision to be
applicable to life sciences and eventually diagnostics.
The newly developed technique, interferometric
scattering mass spectrometry (iSCAMS), is an optimised
form of light microscopy whereby the light scattered
by a single protein can be measured. It works on the
basis that proteins are all made up of the same subunits
(amino acids), which are all roughly of the same density
and have similar masses. A protein with twice as many
amino acids will therefore produce twice the scattering
signal when detected interferometrically; through this
technique it is possible to measure the mass of single Arago Biosciences, a spinout company born from the
protein molecules to within 2% of their true masses. research carried out by the Kukura group, was formed
this year to give other scientists and businesses access to
“The scattering signal observed is proportional to the this new technology. Arago offers the detection, imaging
polarisability of the molecule and therefore the number and quantification of single molecules, without using any
of amino acids present,” Professor Kukura explains. labels or matrices, using small instruments that are close
“The greater the number of amino acids present in the to shoeboxes in size - yet another example of Oxford’s
protein, the greater the scattering of light observed.” research being made available to all that require it.
This technique has thus far been used to weigh and
image a range of biomolecules, including polypeptides,

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Periodic
The Magazine of the Department of Chemistry