Here you go TEC
Silks: Properties and Uses of Natural and Designed Variants
Empowered by evolutionary selective pressures that
yield biopolymers with properties optimized for
survival, Nature often proves to be the most creative
synthetic organic chemist. Scientists across disciplines
have been confronting the challenge of trying to
improve on materials that Nature produces by focusing on
the rational design of natural variants that exhibit predictably
altered properties. These efforts to improve on nature are
empowered by a growing arsenal of technical tools for
manipulating the genomes of organisms, new chemical
reagents for selectively modifying molecular structures, and
controlled processing conditions. Silk represents one of the
most attractive targets of such efforts. This special issue of
Biopolymers explores the range of approaches being used
by leading laboratories for characterizing and rationally
manipulating natural and designer variants of this fascinating
biopolymer.
Silk has been used in textiles for millennia, but in the past
several decades has been subject to a surge of investigation
into its synthesis (both natural and arti?cial), structure,
function, and natural diversity. This research has been driven
by both scienti?c curiosity with the aid of improved
analytical tools as well as the ability to process silk into new
materials that are well-suited for biomedical applications. Of
particular interest are the impressive mechanical properties
of spider silk as well as the ability of silkworms and spiders
to produce ?bers in an aqueous and ‘‘green’’ process with relatively low energy requirements.
1
The origins of spider silk’s high toughness (energy
absorbed to breakage, a combination of strength and elasticity) are still unclear. The variable, metastable, and hierarchical
nature of the ?ber makes it dif?cult to study. Furthermore,
inter-lab (and likely intra-lab) variability in silk sampling
processes can confound results. To address this problem,
Reed et al. discuss the variability in Bombyx mori silk mechanical properties, and determine the variables necessary to
control when handling these ?bers before study. Furthermore,
through the development of new analytical tools with high
spatial and temporal resolution new insight has been
gained into the molecular structure of silk ?bers. The paper by
Holland et al. reports on the combination of birefringence
measurements and mechanical testing to study molecular orientation during the tensile testing of Nephila edulis dragline
?bers. Lefe`vre et al. describe the recent advances in linearly
polarized Raman microspectroscopy,
2,3
and the insight gained
from its use into the structure of single spider silk ?bers and
silk solution. Advances in computational modeling are now
also allowing insight into the origins of strength in silk at
many different structural hierarchies, from crystallites to
?brils to ?bers to webs.
4–6
Herein, Bratzel and Buehler present molecular dynamics simulations of the b-sheet crystals in
Nephila clavipes dragline silk, and their response to various
mechanical loading conditions.
In their review, Lefe`vre et al. describe the use of linearly
polarized Raman microspectroscopy to study the structure of
silk dope in its native, solution state. Within the gland, silk
proteins are secreted from the epithelial lining and are stored
at concentrations over 30% w/w, in an aqueous environment.
1
The stability of silk proteins at such high
concentrations for extended periods of time is unique and
intriguing. As evidenced by several recent publications, the
N- and C- terminal domains are largely responsible for this
stability.
7–10
Eisoldt et al. outline and summarize the recent
progress in understanding how these highly evolutionarily
conserved terminal domains assist in aqueous processability,
storage, and assembly of silk proteins.
These terminal domains are pH responsive and, within
the silk gland, a gradual acidi?cation along the gland length
contributes to the assembly process. The work by Leclerc
et al. uses NMR spectroscopy and dynamic light scattering to
study the contents of the Nephila clavipes major ampullate
gland, solubilized major ampullate ?bers, and recombinant
versions of the MaSp1 and MaSp2 proteins and investigate
the role of pH in secondary structure formation. Yazawa
et al. approach the problem by synthesizing a model dragline
peptide and use solid-state NMR to investigate the role of
acidi?cation in its conformational change.
In addition to changes in pH, as the spinning dope
traverses through the gland and undergoes a phase transition
from liquid to ?ber, it experiences stresses from the force of
the ?ber being drawn, and stresses exerted by the gland
geometry.
11,12
Under these conditions, silk dope exhibits
liquid crystalline textures, thought to assist in the low-energy
spinning process. Rey et al. provide a comprehensive review
of liquid crystal models, how they apply to biological
materials, and how they relate to silk processing and help us
understand the spinning process. Such models are often
complemented by rheological measurements, which have
recently become a powerful tool for gaining insight into the
spinning processes in both spiders and silkworms.
13,14
Holland et al. have provided a unique perspective for utilizing rheology as a tool to compare the phylogenetic relationships of four species of silkworm (domesticated and wild
silkworms) in regards to their energetic input toward silk
production. Further exploring less conventional silkworm
silks, Kundu et al. reviews non-Mulberry silks and their
utility for biomedical applications.
In addition to silkworms and spiders, there are many
more silks that exist in nature, presumably the vast majority
of which are still undescribed. These likely have unique and
interesting material properties, each having evolved for a
speci?c ecological function while minimizing production
energy. As evidenced by the recent discovery of the Darwin
Bark Spider through ‘‘bioprospecting,’’ which has dragline
?bers with two times the toughness of other spiders, it is
likely that we are only just beginning to characterize the
diversity of natural silks.
15
To this end, this issue contains
two papers on ‘‘alternative’’ silks. The paper by Sutherland
et al. reviews the coiled coil silk produced by bees, ants, and
hornets that are used in ?ber or sheet form for mechanical
structures, thermal regulation, and humidi?cation.
Ashton et al. report on an underwater silk adhesive made by
Caddis?ies for protective shelters and food harvesting nets.
Just as one ?nds many different uses of silk in nature,
reprocessing and reengineering of silk into new materials and
structures has broadly expanded the uses of silk materials.
Both reconstituted silkworm silk and genetically engineered
spider silk can be formed into a multitude of materials ranging from ?bers to ?lms to sponges to microspheres to ?brous
mats.
16
These materials have been used extensively for
biomedical applications because of silk’s exceptional biocompatibility and biodegradability. Pritchard et al. discuss the
particular bene?ts of aqueous processability, and the emerging unique features of silk in stabilizing a range of different
labile molecules in reconstituted silk materials. Due to its
widespread availability, the majority of the work into silk
materials has been performed with Bombyx mori silkworm
silk. However, other silks could have unique medical roles,
and Widhe and Herrera-Valencia discuss the more recent
use of natural and recombinant spider silk proteins for
biomedical applications.
As Widhe and Herrera-Valencia conclude, the scale of
recombinant silk production systems is still too limited for
all but the most low-volume applications. However more
advanced recombinant hosts are being engineered to tackle
these problems (metabolically engineered E. coli to produce
full-length silk-like proteins
17
and engineered salmonella to
export silk-like monomers
18
). In addition, in 2007, we saw
the publication of the ?rst full length silk sequences,
19
which
provided tremendous insight into the structure and evolution of dragline proteins and will hopefully help lead to the
faithful synthetic replication of natural silk. Despite the dif?-
culty in sequencing silks due to their long lengths and repetitiveness, with improvements in sequencing technology, we
should expect to see the sequences of many more silks
shortly. This will provide additional insight into the composition and structure of silk, as well as the translational
machinery necessary to produce such large amounts of
highly glycine and alanine-biased protein.
As genetic engineering and gene sequencing become more
sophisticated, enabling the rapid creation of custom structural and functional silks at high yield, and the processing
mechanisms of native silks are further elucidated, one can
only speculate what the future applications of silk will be and
how silk itself will be rede?ned. For example, Teule´ et al.
have created a chimeric ‘‘silk’’ composed of Nephila clavipes
?agelliform and MaSp2 domains, expressed in E. coli. In
addition, we have already seen several recombinant, functionalized silk block copolymers.
20–22
Perhaps soon the
vision of designer protein polymers, from plug-and-play
sequence modules, will be realized.
16,23
Combined with
the discovery of new silks through bioprospecting, the
future application space of the ?eld seems almost impossibly
broad.
As the ?rst published compilation since the 1994 ACS
Symposium Series ‘‘Silk Polymers: Materials Science and Biotechnology,’’ we hope this special issue of Biopolymers will invigorate a more general scienti?c interest in silk polymers
and serve as a guide to those looking to learn about the latest
knowledge in the ?eld. We also hope the enduring interest in
and continued insight into this unique family of protein
polymers will attract further students and investigators to
explore the many unanswered questions in the ?eld.
DAVID N. BRESLAUER
Refactored Materials, Inc
DAVID L. KAPLAN
Tufts University
REFERENCES
1. Vollrath, F.; Knight, D. P. Nature 2001, 410, 541–548.
2. Lefe`vre, T.; Boudreault, S.; Cloutier, C.; Pe´zolet, M. J Mol Biol
2011, 405, 238–253.
3. Rousseau, M.-E.; Lefe`vre, T.; Beaulieu, L.; Asakura, T.; Pe´zolet,
M. Biomacromolecules 2004, 5, 2247–2257.
320 Editorial
Biopolymers. Nova, A.; Keten, S.; Pugno, N. M.; Redaelli, A.; Buehler, M.
J Nano Lett 2010, 10, 2626–2634.
5. Keten, S.; Xu, Z.; Ihle, B.; Buehler, M. J Nat Mater 2010, 9, 359–
367. Epub 2010 Mar 14.
6. Keten, S.; Buehler, M. J. J R Soc Interface 2010, 7, 1709–1721.
7. Gaines, W. A.; Sehorn, M. G.; Marcotte, W. R. J Biol Chem
2010, 285, 40745–40753.
8. Hagn, F.; Thamm, C.; Scheibel, T.; Kessler, H. Angew Chem Int
Ed 2010, 50, 310–313.
9. Hagn, F.; Eisoldt, L.; Hardy, J. G.; Vendrely, C.; Coles, M.;
Scheibel, T.; Kessler, H. Nature 2010, 465, 239–242.
10. Askarieh, G.; Hedhammar, M.; Nordling, K.; Saenz, A.; Casals,
C.; Rising, A.; Johansson, J.; Knight, S. D. Nature 2010, 465,
236–238.
11. Knight, D.; Vollrath, F. Proc. R. Soc. London, Ser. B 1999, 266,
519–523.
12. Breslauer, D. N.; Lee, L. P.; Muller, S. J Biomacromolecules
2009, 10, 49–57.
13. Holland, C.; Terry, A. E.; Porter, D.; Vollrath, F. Nat Mater 2006,
5, 870–874.
14. Holland, C.; Terry, A. E.; Porter, D.; Vollrath, F. Polymer 2007,
48, 3388–3392.
15. Agnarsson, I.; Kuntner, M.; Blackledge, T. A. PLoS One 2010, 5,
e11234.
16. Omenetto, F. G.; Kaplan, D. L. Science 2010, 329, 528–531.
17. Xia, X.-X.; Qian, Z.-G.; Ki, C. S.; Park, Y. H.; Kaplan, D. L.; Lee,
S. Y. Proc Natl Acad Sci USA. 2010, 107, 14059–14063.
18. Widmaier, D. M.; Tullman-Ercek, D.; Mirsky, E. A.; Hill, R.; Govindarajan, S.; Minshull, J.; Voigt, C. A. Mol Syst Biol 2009, 5, 309.
19. Ayoub, N. A.; Garb, J. E.; Tinghitella, R. M.; Collin, M. A.;
Hayashi, C. Y. PLoS one 2007, 2, e514.
20. Bini, E.; Foo, C. W. P.; Huang, J.; Karageorgiou, V.; Kitchel, B.;
Kaplan, D. L. Biomacromolecules 2006, 7, 3139–3145.
21. Gomes, S. C.; Leonor, I. B.; Mano, J. F.; Reis, R. L.; Kaplan, D. L.
Biomaterials 2011, 32, 4255–4266.
22. Cappello, J.; Crissman, J.; Dorman, M.; Mikolajczak, M.; Textor,
G.; Marquet, M.; Ferrari, F. Biotechnol Prog 1990, 6, 198–202.
23. O’Brien, J.; Fahnestock, S.; Termonia, Y.; Gardner, K. Adv Mater
1998, 10, 1185–1195.
AI
