Silk Polymers - American Chemical Society

Chapter 25 Mechanism

of 1

Fiber Formation of

Silkworm

2

3

Jun Magoshi , Yoshiko Magoshi , and Shigeo Nakamura

Downloaded by CHINESE UNIV OF HONG KONG on March 7, 2016 | http://pubs.acs.org Publication Date: December 3, 1993 | doi: 10.1021/bk-1994-0544.ch025

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National Institute of Agrobiological Resources, Tsukuba, Ibaraki 305, Japan National Institute of Sericultural and Entomological Science, Tsukuba, Ibaraki 305, Japan Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-ku, Yokohama 221, Japan

2

3

Larvae of the silkworm, Bombyx mori, control the molecular orientation of their cocoon fiber with several sophisticated spinning techniques. This chapter reviews the interactions of behavior, morphology, and biochemistry in the production of silkworm silk. Numerous physical and chemical variables can induce transitions among the four known phases of this silk: random-coil, α, β, and well-oriented β form conformation. Fibroin solution becomes fibrous as it moves anteriorly through the silk gland. Cations mediate a gel-to-sol transition. Viscosity falls and strength increases as pH drops to about 4.9 and the incipient fibers assume a liquid crystal character. Strain-rate effects on yield stress are profound in lab-drawn fibers, the larva pulls fibers by segments in a figure-eight pattern and cannot spin if its movements are restrained. The resulting fiber is superdrawn. New textures of synthetic fiber under development include si Ik-I ike ( 1) / spun-I ike(2)/ and Ieather-I ike (3) materials. These are intended to serve as imitations of natural fibers. Much of the development of synthetic-fiber technology prog ressed by imitation of the spinning of silkworms. Attractive features of silk fiber include its pearl-like gloss and its light velvety touch. Silk cloth can be easily dyed. It is comfortable to wear. Silk is fashionable fiber.

0097-6156/94/0544-0292$06.00/0 © 1994 American Chemical Society Kaplan et al.; Silk Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

25. MAGOSHI ET AL.

tion

Mechanism of Fiber Formation of Silkworm

293

In t h i s chapter we examine the d e t a i l s of c r y s t a l l i z a and f i b e r f o r m a t i o n during cocoon s p i n n i n g .


EXPERIMENTAL Mater i a I s For experiments on the mechanism of f i b e r f o r m a t i o n / liquid s i l k was o b t a i n e d from the p o s t e r i o r part of the middle d i v i s i o n of the s i l k gland in f u l l - g r o w n larvae (one day before spinning or cocooning) of the silk worm Bombyx mori(4) . The second s i l k p r o t e i n / s e r i c i n in l i q u i d s i l k was removed by washing the silk gland thoroughly with de i o n i z e d water. The s e r i c i n surrounds the bulk fiber f i b r o i n as a s e p a r a t e layer and s e r v e s as a l u b r i c a n t in the cocoon s p i n n i n g . For s t r e s s - s t r a i n measurements/ the s i l k gland was cut about 3cm in l e n g t h ( 5 ) . To examine the l i q u i d crystal of fibroin/ the specimen was prepared from the a n t e r i o r d i v i s i o n of the s i l k g l a n d . A l l experimental m a t e r i a l s of s p i n n i n g of silkworm used were f u l l - g r o w n larvae in s p i n n i n g . Method S t r e s s - s t r a i n curves were measured on a T e n s i l o n t e n s i l e t e s t e r ( U M - 1 - 2 ) at 2 0 ° C . S t r a i n r a t e s were v a r i e d in a range of 10-1000 mm/min. The cross s e c t i o n of specimen was c a l c u l a t e d from the length and weight of the specimen. X - r a y d i f f r a c t i o n p a t t e r n s were recorded with a Rigaku D-3F x - r a y a p p a r a t u s . Raman s p e c t r a were taken on a JASCO Mod Ie R-80 Raman s p e c t r o m e t e r . B i r e f r i n g e n c e measurements were made by the optical method using a compensater/ and optical diffraction patterns were obtained by laser light scattering. V i s c o s i t y of liquid silk was determined by penetration measurement using a p e n e t r a t e r made by Fudo Kogyo L t d . The l i q u i d c r y s t a l was observed under p o l a r i z e d l i g h t by a Nikon AMF m i c r o s c o p e . RESULTS C r y s t a l l i z a t i o n of S i l k F i b r o i n F i b r o i n has a r e l a t i v e l y simple s t r u c t u r e and is e a s i l y available from natural products. Three types of c o n f o r m a t i o n / the random c o i l / the & f o r m C f i b r o i n 1/ crank shaft pleated-sheet structure)(7/ 8)/ and the β formCfibroin 11/ antipara I I e I - c h a i η Ρ Ieated-sheet structure)(7/ 8) have been observed by x - r a y d i f f r a c t i o n and infrared spectroscopy. Three forms of f i b r o i n f i l m can be

Kaplan et al.; Silk Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


294

SILK POLYMERS: MATERIALS SCIENCE AND BIOTECHNOLOGY

obtained by v a r y i n g the preparation conditions such as concentration of fibroin and the casting and quenching temperatures of the aqueous s o l u t i o n . The CL and β form c o n f o r m a t i o n s are very s t a b l e to heating and treatment with organic solvents. However/ conformational changes occur e a s i l y from the r a n d o m - c o i l to the Λ , β and we I I - o r i e n ted β forms by mechanical shearing/ treatment with organic s o l v e n t s / heat t r e a t m e n t / or by v a r y i n g t r e a t i n g c o n d i t i o n s and under s e v e r a l other conditions(4). C r y s t a l l i z a t i o n of s i l k f i b r o i n was s t u d i e d in d e t a i l from d i l u t e aqueous s o l u t i o n and from amorphous random-coil film. The effects of crystallization conditions on the c o n f o r m a t i o n of f i b r o i n are summarized in F i g u r e 1. C r y s t a l l i n e f i l m s in the a and β form c o n f o r m a t i o n s can be obtained by c a s t i n g an aqueous s o l u t i o n at 0 to 40 °C and above 50 *C / respectively/ whereas by c a s t i n g of a d i l u t e s o l u t i o n at 0 to 45 *C r e s u l t s in amorphous f i l m s with a random-coil c o n f o r m a t i o n (Figure 2) (6) . When an aqueous s o l u t i o n of fibroin is frozen by quenching p r i o r to d r y i n g / β form c r y s t a l s are obtained irrespective of the c o n c e n t r a t i o n of f i b r o i n at quenching temperature from -2 to -20 "C. On the other hand/ when the s o l u t i o n is quenched to below -20 *C and then d r i e d at 20 *C, both d , β form c r y s t a l s and an amorphous f i l m with random-coil conformation are obtained depending on the c o n c e n t r a t i o n of f i b r o i n in s o l u t i o n . F i b r o i n molecules in dilute aqueous solutions are in the random-coil conformation. T h e r e f o r e / extremely r a p i d f r e e z i n g of a very d i l u t e s o l u t i o n does not permit a c o n f o r m a t i o n a l change to occur to the β form/ and the random-coil conformation in a d i l u t e s o l u t i o n is maintained in f i b r o i n f i l m s . Silk

g I and

The s i l k glands are a p a i r of tubes l y i n g one on each s i d e of the larvae as shown in F i g u r e 3. The schematic diagram and photograph show the s i l k gland from the silkworm Bombyx mori. Each gland is divided into three divisions: posterior/ middle and a n t e r i o r d i v i s i o n s . The epithelial wall of the gland c o n s i s t s of large hexagonal c e l l s where the s i l k f i b r o i n and s i l k s e r i c i n are s y n t h e s i z e d . The s i l k p r o t e i n s are s t o r e d i n s i d e the g l a n d . The p o s t e r i o r d i v i s i o n / which is c l o s e d at one end/ is narrow and very c o n v o l u t e d . T h i s is where the main s i l k p r o t e i n / f i b r o i n / is s y n t h e s i z e d . The f i b r o i n moves forward into the wider/ middle division/ which serves as a


Mechanism of Fiber Formation of Silkworm 295

MAGOSHI ET AL.

β - Fori ( Fibroin - Π )


Aiorphous (Randoi-coi1)

fell-oriented β -fori

a — Fori (Fibroin - I )

1) Casting from aqueous solution C>5%) at 0 to 45 C. Quenching aqueous solution (>5%) below -20*C prior to drying. Crystallization from aqueous solution at pH above 6.0. Epitaxial growth on Nylon 66. Treatment with water at 0 to 50*C e

2) Application of high pressure. 3) Casting from aqueous solution 5X) above 45*C. Quenching aqueous solution (>5%) at 0 to -20*C prior to drying. Mechanical shearing. Application of an electric field to aqueous solution. Treatment with polar organic solvents.

Figure

1.

Crystallization from aqueous solution at PH below 4.5. Heat treatment at 190 to 200 C. Treatment of film with hot water at above 60*C. Treatment of aqueous solution with enzyme (Chymotrypsiη) . e

ισ>

4) Application of high pressure. 5) Drawing of liquid fibroin or film. 6) Application of high pressure. Treatment with salt. 7) Heating to 270 C . Mechanical shearing. 8) Treatment with salt solution. 9) Mechanical shearing. Epitaxial growth. 10) Application of high pressure. 11) Mechanical shearing. 12) Heat treatment by wet process.

C r y s t a l l i z a t i o n of s i l k various c o n d i t i o n s .

e

fibroin

113

induced under



296

SILK POLYMERS: MATERIALS SCIENCE AND BIOTECHNOLOGY

~ ο οα Ε Φ

Ο) C +*

CO ce ϋ

100(

β—form

80 40 60 20 10

ϋ

-20h α Ε -4θί Φ Ι α -60f

20 30 40 β—form Concn(%)

α—form +

(0 ~σ c os Φ _c _c - M —

CL

(0

Silk Polymers - American Chemical Society

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