Biology Reference
In-Depth Information
As mentioned earlier, some viruses have a fairly simple geometrical shape,
which renders relatively easy a theoretical calculation of their mechanical
properties. For example, a Young's modulus of about 6 GPa has been calculated
for the Tobacco mosaic virus.
5
If the virus to be studied has a complex shape,
then numerical tools such as inite-element modelling must be implemented
to interpret the FD curves. A detailed handling of the inite-element method
in relation to viral capsids has been published by Gibbons and Klug
6
and
Klug
et al.
8
have applied inite-element modelling to determine
the elastic properties of both wild-type and single-point-mutant strains of
the cowpea chlorotic mottle virus, in an empty as well as in an RNA-illed
state, from AFM measurements. As anticipated, the RNA-illed capsids were
less deformable than the empty ones. Interestingly, a single-point mutation
suficed to modify the stiffness of the capsid: Young's moduli for the wild-type
and mutant forms were 140 MPa and 190 MPa, respectively. Using a similar
set-up, Ivanovska
et al.
7
Michel
et al.
9-11
have measured the elastic properties of empty and
DNA-illed lambda-phages. Also in this study, the DNA-illed viruses were
found to be stiffer than their empty counterparts, and they could withstand
forces twice as high before irreversible damage occurred. Kol
et al.
12,13
have
explored the evolution of mechanical properties as a function of maturation
stage using the human immunodeiciency and murine leukaemia viruses.
The immature particles (930 MPa) were found to be 14-fold stiffer than the
mature ones (115 MPa). These data suggest that the maturation phase of
viruses may play an important role in their capacity to penetrate host tissues
and thus in determining their infectivity.
Finite-element modelling was used in these studies to analyze the FD
curves that were recorded for the investigated viruses. An analysis of this kind
is based upon the assumption that the paradigm for continuum mechanics
still applies on a nanometric scale. Molecular dynamics modelling yields a
more accurate simulation of the experimental situation, but requires huge
computational powers. Zink and Grubmüller
14
have conducted one such
study. For this purpose, the icosahedral southern bean mosaic virus was
used, and the simulation of its interaction with an AFM tip was conducted
in an also simulated liquid medium. The model consisted of 4.5 millions
atoms, 1 million of which stemmed from water molecules. The simulated
time was approximately 100 ns. Indentation of the AFM tip was simulated
at numerous grid points on the capsid and revealed the viral shell to exhibit
a highly elastic behaviour. However, the curves varied greatly according to
the point of indentation. This heterogeneity may relect differences either in
geometry or in the interaction forces operating between the proteins within
the capsid. This type of simulation is currently the most accurate means of
predicting the deformation of molecular structures under loading conditions.
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