First published online May 30, 2008
Journal of Experimental Biology 211, 1948-1957 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014225
The role of proline in the elastic mechanism of hydrated spider silks
Ken N. Savage and
John M. Gosline*
Department of Zoology, 6270 University Boulevard, University of British
Columbia, Vancouver, British Columbia, Canada, V6K 1Z4
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Fig. 1. The micro-beam tensile test apparatus used to measure the thermoelastic
properties of spider silks. A supercontracted silk thread (green line) is
mounted between a glass beam and a moveable micrometer mount. A video
dimension analyzer (VDA) system mounted on a microscope is used to track the
deflection of a glass micro-beam relative to a reference glass beam. This
provides nano-Newton resolution of changes in the elastic force. A thermistor
placed near the silk sample measures temperature.
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Fig. 2. The raw data from a typical thermoelastic experiment on Araneus MA
silk. The top curve represents the raw force–temperature data and the
bottom curve represents the control data for the expansion of the apparatus. A
linear regression fitted to the control data was subtracted from the
regression of the raw data to give the corrected voltage-temperature profile
of the silk (middle curve).
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Fig. 3. (A) The micro-beam test apparatus used to measure the thermal swelling
coefficients for swollen MA silks. The silk sample is mounted between a glass
beam and a moveable micrometer mount in the sample chamber of the aluminium,
temperature-controlled stage. The stage was fixed at one end to a lead block
located to one side of the microscope, and this assembly was position so that
the glass beam was centred in the field of view of the microscope. This setup
ensures that the temperature-controlled stage is thermally isolated from the
microscope stage, so that movement of the glass beam could be used to track
the thermal expansion of the temperature-controlled stage. (B) An expanded
view of the temperature-controlled stage with the silk sample chamber, which
shows the process for measuring the thermal swelling coefficient of MA silk.
In step 1, a silk fibre is mounted in the sample chamber between the glass rod
and a moveable micrometer mount, and the initial length, SL, is measured as
described in the text. In step 2, the temperature is increased, and a new
initial length is measured. The silk sample is then removed and the thermal
expansion of the stage was measured by tracking the movement of the unloaded
beam over the same temperature range (steps 3 and 4). The movement of the
glass beam was subtracted from the calculated length change of the silk with
temperature to give the thermal swelling coefficient of the silk.
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Fig. 4. (A) Typical force–temperature curves, uncorrected for swelling, for
Araneus and Nephila MA silk and Araneus FL silk.
Each curve consists of two points taken from the linear regression of the raw
force data curves. Each curve is labeled for the type of silk and the
extension at which the test was administered. (B) The force–temperature
curves from Fig. 2 re-plotted
as normalized force vs temperature. The regressions plotted in
Fig. 2 from experimental
force–temperature profiles are normalized to the force at 303 K. Each
curve is labeled for the type of silk and the extension at which the test was
administered.
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Fig. 5. Values of Fh/F are plotted against
extension for Araneus and Nephila MA silk. Nephila
MA values are represented by black squares and are fitted to the linear
regression, y=0.13x+0.51; N=7;
R2=0.77; P<0.01. Measurements on
Araneus MA silk are represented by black circles; open circles are
Fh/F values taken from Gosline et al.
(Gosline et al., 1984 ). The
Araneus MA Fh/F data are fitted to the
linear regression, y=0.02x–0.53; N=10;
R2=0.68; P<0.01.
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Fig. 6. Force–length isotherms are plotted to demonstrate the force
correction for a typical thermoelastic test of Araneus MA silk at an
extension of 27%. (A) The force on a fibre at a relative length of 1.27 is
plotted for temperatures of 283 K (reference temperature) and at 298 K. The
dashed line shows the regression between the force measured at 298 K and a
relative length of 0.99, which was calculated based on the thermal swelling
coefficient of 1x10–4°C–1 for
Araneus MA silk. Owing to the small value of the correction, the
uncorrected and corrected isotherms are difficult to distinguish. (B) The
force at a relative length of 1.27 is measured at 283 K (reference
temperature) and at 298 K. At the reference temperature of 283 K, the relative
length at zero force is 1.0 and a relative length of 1.27 is equivalent to an
extension of 27%. However, with a thermal swelling coefficient of
–1x10–2°C–1, the zero force
length at 298 K is reduced to 0.85. This occurs because, at 298 K, the initial
length has decreased because of a thermal de-swelling, and thus the extension
of the fibre has increased. At 298 K, an extension of 27% occurs at a relative
length of 1.08. The linear regression between the measured force and the zero
force length (dashed line) is solved at 1.08 to determine the force at a
constant extension of 27%, and is represented by the black triangle. An
exaggerated thermal swelling coefficient of
–1x10–2°C–1 was chosen to
better demonstrate the correction process.
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Fig. 7. A plot of normalized force against temperature for a sample of
Araneus MA silk held at an extension of 36%. The intercept of the
uncorrected plot is 0.2, indicating that for this sample approximately 20% of
the force is due to enthalpy. When corrected for the effects of thermal
swelling, the slope of the force–temperature plot decreases, and the
intercept rises to 0.30 for this silk sample, indicating that approximately
30% of the force is due to bond energy.
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Fig. 8. Values of Fe/F are plotted against
extension for Araneus and Nephila MA silk. No swelling
correction was applied to Nephila MA silk and so the
Fh/F values
(Fig. 5) are equivalent to
Fe/F. Nephila MA silk values are
represented by black squares and are fitted to the linear regression,
y=0.13x+0.51; N=7; R2=0.77;
P<0.01. Araneus MA silk measurements are represented by
black circles; open circles are Fe/F values
taken from Gosline et al. (Gosline et al.,
1984). The Araneus MA
Fe/F data are fitted to the linear
regression, y=0.02x–0.15; N=10;
R2=0.68; P<0.01.
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Fig. 9. Values of Fh/F (black triangles) and
Fe/F (open triangles) for Araneus
FL silk are plotted against extension. Note that the
Fh/F values are negative at all extensions
tested.
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Fig. 10. The Araneus FL silk values for
Fh/F (black triangles) and
Fe/F (open triangles) from
Fig. 8 are plotted against
extension. The most negative outliers from
Fig. 8 have been omitted in
order to better view the trend about zero on the force axis. The values of
Fh/F and
Fe/F have each been fitted to a linear
regression; neither regression has a slope significantly different from
zero.
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© The Company of Biologists Ltd 2008
