Natural Protein Fibers

Keywords

Mussel • Anchoring thread • Rigid fiber • Flexible fiber • Collagen distribution

Marine animals such as mussels produce fibrous attachments generally called byssus as shown in Fig. 41.1. Each thread in a byssus is about 2-3 cm long and about 100-200 qm in diameter [13Lin]. Byssal threads were reportedly woven into fabric in Greece to produce fine clothing [07Ald]. These byssal threads have extraordinary structural arrangement and properties not seen in other protein fibers. The thread consists of two regions, the distal portion (threads) which is rigid and stiff and the proximal region (threads) that is approximately 50-fold less stiff than the distal threads due to the unique composition and structure of the proteins in the threads [01Vac].

Tensile properties of the mussel threads have been examined under various conditions. Table 41.1 provides a comparison of the tensile properties of the distal mussel threads treated in different conditions. As seen in the table, dehydrated threads had higher stiffness, whereas the hydrated threads were more flexible and had high elongation. It was suggested that in addition to the preCol domains in the fibers, the adjacent domains assist in load dissipation and are crucial for the load­bearing ability of the threads [11Hag]. Recent studies have suggested that the high strength of the byssal threads is mainly due to the combination of the stiff and soft regions (distal/proximal) in 80/20 ratio [13Qin]. Other researchers have also reported tensile stress of 0.5 g/den for fresh hydrated distal mussel threads and 0.6 g/den for aged and hydrated threads [07Ald]. Corresponding values for the dehydrated threads were 0.7 g/den, respectively. The ability of the threads to recover from stretching in water with various chemicals was studied by Vaccaro and Waite [01Vac]. Table 41.2 shows the Young’s modulus and energy dissipated by distal byssal threads after three stress-strain cycles. Chemical treatments decreased the modulus and energy dissipation of all the fibers. EDTA-treated fibers

Table 41.1 Comparison of the properties of the mussel distal threads treated in artificial seawater and distilled water

showed different behavior than the other proteins due to the chelating action and stabilization of the proteins.

In terms of composition, proximal threads consist of oriented collagen — containing fibrils dispersed in a soft proteinaceous matrix, whereas the distal threads are composed of packed fibrous bundles (Fig. 41.2) held together by covalent and non-covalent bonds. The matrix consists of glycine-, tyrosine-, and

Fig. 41.2 Schematics of the distribution and properties of byssal threads. From Harrington and Waite [09Har]

asparagine-rich proteins that have a unique repeated sequence motif and are distributed throughout the thread [09Sag]. Such an arrangement provides the rigidity and strength required for the threads to adhere to surfaces and resist the external forces and at the same time provides flexibility required to move and capture prey [01Vac, 11Hag]. Threads are about 95 % protein and consist of fibrils composed of collagen embedded in a protein matrix surrounded by a granular cuticle as shown in Fig. 41.3 [11Hag]. Further investigations have shown that the threads are composed of collagen and are termed PreCol (preCol-D, — NG, and — P) that forms 96 % of the distal region and 66 % of the proximal part [08Har, 09Har, 13Arn]. Protein chains (motifs) in preCol-D resemble silk, in preCol-NG resemble glycine-rich plant cell wall proteins, and that of preCol-P are similar to that of elastin. The composition and arrangement of these three PreCol components vary across the length of the fibers, and this distribution is crucial in determining the properties of the thread. Some studies have suggested that the preCol-NG serves as a mediator between preCol-D and preCol-P and responsible for the gradient properties seen in the fibers [98Qin]. A schematic of the distribution of the PreCol components is shown in Fig. 41.3b.

Hagenau et al. have shown that the distal region is composed of high levels of P-sheets that are well oriented, whereas the proximal part is less oriented and mainly consists of a-helices [09Hag]. X-ray diffraction studies (Fig. 41.4) have clearly shown that the distal portion is well oriented, the proximal portion is less oriented, and the middle portion has an orientation in between that of the distal and proximal regions. Further analysis of these portions using FTIR revealed that the proximal portion is composed of proteins with about 47 % a-helices, 15 % p-sheets, 25 % of aggregate sheets, and 13 % of triple helix structures. Comparatively, the distal portion was composed of 70 % p-sheets, 22 % triple helices, and 8 % of aggregate sheets [09Hag]. However, it was suggested that uncertainties existed in the prediction of the structure of the Byssal threads using X-ray diffraction and FTIR studies due to the presence of mixture of proteins. SEM image of a distal byssal thread in Fig. 41.5 shows a dense fibrillar network inside a proteinaceous matrix.

References

[98Qin] Qin, X., Waite, J. H.: Proc. Natl. Acad. Sci. USA 95, 10517 (1998)

[01Vac] Vaccaro, E., Waite, J. H.: Biomacromolecules 2, 906 (2001)

[07Ald] Aldred, N., Wills, T., Williams, D. N., Clare, A. S.: J. R. Soc. Interface 4, 1159 (2007) [08Har] Harrington, M. J., Waite, J. H.: Biomacromolecules 9, 1480 (2008)

[09Hag] Hagenau, A., Scheidt, H. A., Serpell, L., Huster, D., Scheibel, T.: Macromol. Biosci. 9, 162 (2009)

[09Har] Harrington, M. J., Waite, J. H.: Adv. Mater. 21, 440 (2009)

[09Sag] Sagert, J., Waite, J. H.: J. Exp. Biol. 212, 2224 (2009)

[11Hag] Hagenau, A., Papadopoulos, P., Kremer, F., Scheibel, T.: J. Struct. Biol. 175, 339 (2011)

[13Arn] Arnold, A. A., Byette, F., Seguin-Heine, M., LeBlanc, A., Sleno, L., Tremblay, R., Pellerin, C., Marcotte, I.: Biomacromolecules 14, 132 (2013)

[13Lin] Lintz, E. S., Scheibel, T. R.: Adv. Funct. Mater. 23, 4467 (2013)

[13Qin] Qin, Z., Buehler, M. J.: Nat. Commun. 4, 2187 (1998)