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First published online May 30, 2008
Journal of Experimental Biology 211, 1937-1947 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014217
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The effect of proline on the network structure of major ampullate silks as inferred from their mechanical and optical properties

Ken N. Savage and John M. Gosline*

Department of Zoology, 6270 University Boulevard, University of British Columbia, Vancouver, British Columbia, Canada, V6K 1Z4


Figure 1
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  		Fig. 1. The consensus sequences for the fibroins expressed in the MA glands of Nephila clavipes and Araneus diadematus, as presented in Gatsey et al. (Gatsey et al., 2001). Nc-MA-1 is spidroin-1 type and Nc-MA-2 is spidroin-2 type. Ad-MA-1 and Ad-MA-2 are both spidroin-2 type and were previously called ADF-3 and ADF-4 by Guerette et al. (Guerette et al., 1996Go). The percentage values at the end of each sequence give the relative size of the poly-alanine block and the percentage proline in each consensus repeat.

 

Figure 2
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  		Fig. 2. A conceptual model representing the distribution of hydrogen bond energies contained within proline-rich (+Pro) Araneus and proline-deficient (–Pro) Nephila MA silk networks. Green peaks represent bond energies in poly-alanine β-sheet crystals; black peaks represent bond energies in the network chains in Araneus silk, and red peaks represent bond energies in the network chains in Nephila MA silk. (A,B) Two possible scenarios for the bond energy distributions of the silks in the dry state. In A the distribution of hydrogen bond energies in +Pro and –Pro networks are not sufficiently different to be detected in the dry mechanics. In B the difference in hydrogen bond energies between +Pro and –Pro networks is sufficiently large that there are significant differences in the dry mechanics. According to both dry models, the peaks associated with hydrogen-bonded structures are well above the energy associated with Brownian motion, kT, and thus represent stable structures. (C) Hydration weakens the hydrogen bond strengths of the +Pro networks, and consequently the +Pro peak shifts to levels that are near or below kT. It is unclear where on this scale the –Pro peak might shift; however, if it remains above kT, we hypothesize that there would be a difference in the properties of hydrated MA silks. The poly-alanine β-sheet crystals are stable both in the dry and hydrated state, and it is unclear if the crystals `soften' in the presence of water. We arbitrarily shifted the poly-alanine peak to the left slightly, but the position well to the right of kT indicates that the poly-alanine crystals are quite stable in water.

 

Figure 3
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  		Fig. 3. Typical tensile test data for dry MA silk collected from Araneus and Nephila. Traces were chosen to demonstrate the range of data collected. Dashed traces are data from Nephila, unbroken traces are data from A. diadematus. The black circle represents the mean failure stress and strain for Araneus with error bars representing one standard deviation. The black square represents the same values for Nephila. Each sample was from a different spider.

 

Figure 4
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  		Fig. 4. Supercontracted properties of Araneus MA silk. Note that the individual samples were not tested to failure, but to the highest strain possible with each experimental setup. (A) The complete data set; (B) the average behaviour of the data set. Predicted values were averaged from all the curves for which a value was available. Filled circles represent values averaged from all 12 tests and open circles represent values averaged from all tests for which a value at that extension was available. The numbers above each data point are the number of curves included in the average. The calculated average behaviour is fitted to the fourth order polynomial, 450.05x4–310.64x3+109.49x2–4.29x+0.22, r2=1. Error bars are one standard deviation. Each sample was from a different spider.

 

Figure 5
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  		Fig. 5. Supercontracted properties of Nephila MA silk. Note that the individual samples were not tested to failure, but to the highest strain possible with each experimental setup. (A) The completed data set; (B) the average behaviour of the data set. Predicted values were averaged from all the curves for which a value was available. Filled squares represent values averaged from all 24 tests and open squares represent values averaged from all tests for which a value at that extension was available. The numbers above each data point are the number of curves included in the average. The calculated average curve is fitted to a third order polynomial, 266.23x3+81.42x2+89.11x–2.21, r2=0.97. Error bars are one standard deviation. Each sample was from a different spider.

 

Figure 6
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  		Fig. 6. The average stress–strain behaviour presented in Fig. 4B (Araneus) and Fig. 5B (Nephila) are presented on the same axis for the purposes of direct comparison. Squares represent Nephila MA silk and circles represent Araneus MA silk. As for Figs 4 and 5, filled symbols represent values from all tests for which a value was available, and open symbols represent values from all tests that were available at that extension. The average Nephila MA silk behaviour is stiffer and the stress rises more quickly with increasing extension than is the case with Araneus MA silk.

 

Figure 7
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  		Fig. 7. (A) The instantaneous modulus of MA silk is plotted against strain. The Araneus curve is difficult to distinguish from the axis because of the relatively high modulus of Nephila. (B) To better distinguish the Nephila and Araneus curves, the data from A is plotted as log modulus versus strain. The solid squares indicate the initial stiffness of Nephila silk, and the solid circles indicate the initial stiffness of Araneus silk.

 

Figure 8
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  		Fig. 8. Birefringence measurements versus strain for supercontracted MA silk from Araneus. (A) The complete data set and (B) the average behaviour of the data set. Predicted values were averaged from all the curves for which a value was available. Filled circles represent values averaged from all nine tests and open circles represent values averaged from all tests for which a value at that extension was available. The numbers above each data point represent the number of curves included in the average. The calculated curve is fitted to the second order polynomial, y=0.0122x+0.0070, r2=0.99. Error bars are one standard deviation. Each sample was from a different spider.

 

Figure 9
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  		Fig. 9. Birefringence measurements versus strain for supercontracted MA silk from Nephila (A)The complete data set and (B) the average behaviour of the data set. Predicted values were averaged from all the curves for which a value was available. Filled squares represent values averaged from all seven tests and open squares represent values averaged from all tests for which a value at that extension was available. The numbers above each data point represent the number of curves included in the average. The calculated average behaviour is fitted to the linear regression, y=0.0456x2+0.0197, r2=1. Error bars are one standard deviation. Each sample was from a different spider.

 

Figure 10
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  		Fig. 10. The average birefringence curves in Fig. 7B (Araneus) and Fig. 4B (Nephila) are presented on the same axis for direct comparison.

 

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© The Company of Biologists Ltd 2008