Functionalization of Colored/Fluorescent Silkworm Silk Fibrous Materials - Soft Fibrillar Materials: Fabrication and Applications - page 206

staves in 550 AD. Subsequently, the West began cultivating their own silk. It was
only in the thirteenth century, the time of the Second Crusades, that Italy began silk
production with the introduction of 2000 skilled silk weavers from Constantinople.
Eventually, silk production became widespread in Europe. Since 1978, China has
quickly recovered its silk production capacity, and now enjoys market supremacy.
During the last 30 years, world silk production has approximately doubled in spite
of man-made fibers replacing silk for some uses [17]. Currently, the world annual
production of raw silk is about

∼

150 000 tons, of which China accounts for about
85%, followed by India 12%, and so on [20].

7.3
The Structure of Silkworm Silk

In order to functionalize silk fibers without disturbing their performance, the
structure of silk fibers needs to be understood. Although the Bombyx mori silkworm
silk structure has been investigated for some time, the hierarchical structure did
not become clear until recently.
The density of raw silk fibers is about 1.33-1.45 g cm -3 . Raw cocoon silk fibers
have good extensibility (25-35%) and reasonably good strength (300-600MPa).
For different silkworms, silk properties also are very different. Some silk has strong
mechanical properties, and other silk shows good biodegradability. The structure
of the different types of silk has become a research focus. Recently, the diversity of
test instruments is also facilitating the study of silk structure [21, 22].
A Bombyx mori silkworm silk thread consists of two triangular water-insoluble
silk fibroin fibers glued by water-soluble sericin (see Figure 7.1a). Many views
concerning the detailed structure of silk fibers have been published so far. Based
on solid-state NMR, Asakura et al . [23-26] reported the structural analysis of
natural protein fibers with mixed parallel and antiparallel β -sheet structures.

Alanine tripeptide samples with 100% parallel β -sheet structure and with 100%

antiparallel -sheet structures

following 13 C solid-state NMR experiments. All 13 C resonances of the tripeptides
were assigned by a comparison of the methyl 13 C resonances of Ala3 with different
[3-13C]Ala labeling schemes and also by a series of RFDR (radio frequency driven
recoupling) spectra observed by changing mixing times. The differences between
two tripeptides were studied and compared. The results found in the NMR
parameters for the tripeptides were applied to assign the parallel and antiparallel
β -sheet 13 C resonances in the asymmetric and broad methyl spectra of [3-13C]-
Ala silk protein fiber of a wild silkworm, Samia cynthia ricini . Teramoto et al .
investigated the characteristics of sericin [27, 28]. They concluded that sericin
forms a largely random coil structure in the native state in the middle silk
gland and undergoes no remarkable structural changes during the spinning by
the silkworm. It was also found that Asp residues periodically contained in the
repeated region of sericin might constitute points vulnerable toward hydrolysis by
heat treatment.

β -sheet structure were chosen to characterize the β