The natural silk spinning process

Abstract

The spinning mechanism of natural silk has been an open issue. In this study, both the conformation transition from random coil to b sheet and the b sheet aggregation growth of silk fibroin are identified in the B. mori regenerated silk fibroin aqueous solution by circular dichroism (CD) spectroscopy. A nucleation-dependent aggregation mechanism, similar to that found in prion protein, amyloid b (Ab) protein, and a-synuclein protein with the conformation transition from a soluble protein to a neurotoxic, insoluble b sheet containing aggregate, is a novel suggestion for the silk spinning process. We present evidence that two steps are involved in this mechanism: (a) nucleation, a ratelimiting step involving the conversion of the soluble random coil to insoluble b sheet and subsequently a series of thermodynamically unfavorable association of b sheet unit, i.e. the formation of a nucleus or seed; (b) once the nucleus forms, further growth of the b sheet unit becomes thermodynamically favorable, resulting a rapid extension of b sheet aggregation. The aggregation growth follows a first order kinetic process with respect to the random coil fibroin concentration. The increase of temperature accelerates the b sheet aggregation growth if the b sheet seed is introduced into the random coil fibroin solution. This work enhances our understanding of the natural silk spinning process in vivo.Keywords: silk fibroin; spinning mechanism; conformation transition; nucleation-dependent; CD spectroscopy.A number of studies have been reported silk processing techniques [1 -11], including those of spider and silkworm. However, the controlling factors that determine the efficiency of silk spinning at ambient temperature and normal pressure remains unclear [6 -8]. The silk fibroin in the silkworm gland possesses relatively low viscosity in a concentrated solution as a result of its storage in a liquid crystalline state before spinning [7 -9]. This is coupled with a low critical shear rate for inducing crystallization in aqueous solution [10], as well as the low draw ratios for the production of uniaxially aligned fibrous structures [11]. Theoretical elucidation of the mechanism for the natural silk spinning process has implications for material science, particularly in the design of engineering polymers.The primary sequence of silk fibroin of the B. mori silkworm predominantly consists of the -(Gly-Ala-Gly-AlaGly-Ser) 8 -motif [12]. It has been demonstrated that two types of conformations exist in the silk fibroin: silk I, a mainly coiled chain conformation in the present silk gland, and silk II which is formed by regularly aligned crystalline b sheet in the silk fibers [7]. Previous studies have suggested that the b sheet conformation can be induced in silkworm fibroin by a stretching force [13], and the formation of spider dragline silk also involves a stress-induced b sheet formation by extensional flow [14]. But the mechanism involved in the conversion of a hydrogel of silk fibroin in the silk I state into the silk II state remains...

Effect of Temperature Treatment during Development ofOsmia rufaL., on Mortality, Emergence and Longevity of Adults

“…In our study, we also distinguished the spinning larval stage, but its duration was not affected by temperature. This stage is mainly related to secretion from silk glands, which are responsible for synthesis of proteins forming the silk fibroin structure (Li et al, 2001). The cocoon is created by intensive movements of the larva, which are not affected by temperature.…”

A microfluidic cell for studying the formation of regenerated silk by synchrotron radiation small- and wide-angle X-ray scattering

“…Therefore, these results do not support a direct conversion of random coil silk protein into ␤-sheet material via a nucleation-dependent aggregation mechanism. 4 …”

“…While the structure and function of spinning ducts in spiders 1 and silk worms 2,3 have been studied extensively, microscopic models of silk protein ͑fibroin͒ aggregation and silk fiber formation have still to be experimentally verified. 1,4,5 In-situ synchrotron radiation ͑SR͒ micro-small-angle X-ray and micro-wide-angle X-ray scattering ͑ -SAXS and -WAXS͒ experiments during forced silking of Nephila spiders 6 provide microstructural information on the nascent silk thread at the exit of the spigots. Information on the fiber-formation process taking place within the spinning duct however, cannot be obtained in this way and the influence of pH, 7,8 ions, [9][10][11][12] and stress 13 on fibroin aggregation have only been addressed by in vitro studies.…”

A novel growth process of calcium carbonate crystals in silk fibroin hydrogel system

“…The formed precursor tended to absorb on the highly oriented crystalline region of silk fibroin because of the nucleation-dependent aggregation of β-sheets [31]. Finally, recrystallization took place on the region of β-sheets due to the high localized concentration of sequestered Ca…”