Base-free preparation of low molecular weight chitin from crab shell
Introduction
Chitin, the second most abundant biomass on the earth, is a polysaccharide polymerized from β-1, 4-N-acetyl-d-glucosamines. Chitin mainly exists in the arthropod shells such as shrimp and crab, and other sources including insects, jellyfish, algae, fungi, squid pens and so on (Abdou, Nagy, & Elsabee, 2008; Kurita, 2001; Lavall, Assis, & Campana-Filho, 2007). Owning to its antimicrobial activity (Benhabiles et al., 2012; Park & Kim, 2010), chitin demonstrates its potentially promising application in the fields of food, cosmetics, and biomedicines, etc (Anitha et al., 2014; Rinaudo, 2006).
Industrially, chitin is produced by treating the exoskeletons of crustaceans with acid and alkali alternately. On a small-scale trial, after being dissolved in certain solvents, chitin could be regenerated from anti-solvents such as water, alcohol and acetone (Barber et al., 2013; Hu et al., 2007; Poirier & Charlet, 2002). Generally, the above obtained chitin has a high molecular weight (HMW), which is as large as 1300 kDa or above (Domard, 2011; Kumar, 2000; Percot, Viton, & Domard, 2003). The HMW-chitin exhibits adequate mechanical properties which could meet the requirements for preparing high-strength chitin-derivative materials (Qin, Lu, Sun, & Rogers, 2010; Shamshina et al., 2014). However, HMW-chitin is insoluble in water, dilute acid or alkali (Chen, Chew, Kerton, & Yan, 2014; Qin et al., 2010). It must be converted into chitin derivatives and then could be soluble (Younes & Rinaudo, 2015). On the conversion of HMW-chitin to its derivatives, the physical and chemical conditions should be very demanding to break down the chains, which implies that it is not a cost and energy effective process.
By contrast, the molecular chains of low molecular weight (LMW)-chitin are much less entangled and easier to break down. Therefore, the conversion of LMW-chitin to its derivatives would be supposed to be less consumptive, which benefits the expected cleaning process, green energy, and environmental protection. Also, the LMW-chitin shows great antimicrobial activity against some specific tumor cell or bacterial strain (Salah et al., 2013), thus it could be used as a potential target for antifungal therapy. However, as of now, the raw materials employed for preparing LMW-chitin in the previous reports are all HMW-chitins (Vasileva, Lopatin, & Varlamov, 2016), which should be first prepared from crab or shrimp shells, resulting in a complicated process and extra consumption of reagents. In some cases, the condition to obtain LMW-chitin is unconventional, and very difficult to achieve, i.e. electron-beam plasma (Vasilieva et al., 2017). Moreover, the optimal conditions for preparing LMW-chitins with acid have been not specifically researched yet.
Here, we present a one-step method to produce LMW-chitin from crab shell directly in a mild condition. The demineralization and deproteinization of crab shell were both investigated. The molecular weight of LMW-chitin and its distribution was controlled in a narrow range by simply modifying the treatment conditions such as acid concentration, temperature, and time. Also, the structure of the LMW-chitin was explored, and compared with the commercial LMW-chitin product. Finally, it should be emphasized that in this study the demonstrated feasible base-free protocol to produce the LMW-chitin directly from crab shell under mild conditions, might be extended to various chitin sources such as shrimp shell, krill, crayfish, and fungi.
Section snippets
Materials
The shell of Snow Crab (Alaska State, USA) was supplied by Zhejiang Golden-Shell Pharmaceutical Co. Ltd, China. The shell was dipped into tap water for one day to wash away the dust and soluble substances in the crab shell. After drying at 60 °C, the shell was smashed to about 60 mesh. Then, by using plenty of tap water to wash the powder until its conductivity gets balanced, the powder was lyophilized with a FD-1D-80 Lyophilizer (Beijing Boyikang Scientific Instruments Co., Ltd).
Commercial
Demineralization and deproteinization of crab shell versus acid concentration
As shown in Table 1, the analysis of chemical composition demonstrates that the raw crab shell contains 16% protein, 9.7% water, 37.8% ash, and 0.7% lipid, which implies that almost half of crab shell needs to be removed to obtain a pure chitin product.
To evaluate the effect of acid concentration on the crab shell, we carried out the demineralization and deproteinization experiments at different acid concentrations. As the acid concentration was increased from 0.5 wt% to 1 wt%, the chitin yield
Conclusions
We propose a new one-step base-free strategy to prepare LMW-chitin from crab shell by hydrochloric acid. The demineralization of crab shell is accomplished in minutes, while the deproteinization requires at least 180 min to obtain a high purity product. Moreover, the deproteinization of crab shell is proved as a minerals-independent process.
The obtained LMW-chitin owns a molecular weight of 53–80 kDa and a purity as high as 92%. It holds raw α-allomorph crystal structure, which is identical to
Acknowledgments
This work was supported financially by National Natural Scientific Fund of China (Nos. 21476234, 21506231, 21336002, 21210006).
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