Abstract
Cellulose is the most familiar and most abundant strong biopolymer, but the reasons for its outstanding mechanical performance are not well understood. Each glucose unit in a cellulose chain is joined to the next by a covalent C–O–C linkage flanked by two hydrogen bonds. This geometry suggests some form of cooperativity between covalent and hydrogen bonding. Using infrared spectroscopy and X-ray diffraction, we show that mechanical tension straightens out the zigzag conformation of the cellulose chain, with each glucose unit pivoting around a fulcrum at either end. Straightening the chain leads to a small increase in its length and is resisted by one of the flanking hydrogen bonds. This constitutes a simple form of molecular leverage with the covalent structure providing the fulcrum and gives the hydrogen bond an unexpectedly amplified effect on the tensile stiffness of the chain. The principle of molecular leverage can be directly applied to certain other carbohydrate polymers, including the animal polysaccharide chitin. Related but more complex effects are possible in some proteins and nucleic acids. The stiffening of cellulose by this mechanism is, however, in complete contrast to the way in which hydrogen bonding provides toughness combined with extensibility in protein materials like spider silk.
Highlights
IntroductionCellulose has a simple primary structure, a linear chain of βglucose units joined covalently by 1,4′ glycosidic (C−O−C)
Cellulose has a simple primary structure, a linear chain of βglucose units joined covalently by 1,4′ glycosidic (C−O−C)links (Figure 1)
The Fourier transform infrared (FTIR) and diffraction data support a mechanism for elastic extension of the cellulose chain in which much of the additional chain length is obtained by straightening the kink in the chain at each glycosidic linkage.[9,12]
Summary
Cellulose has a simple primary structure, a linear chain of βglucose units joined covalently by 1,4′ glycosidic (C−O−C). Cellulose chains are packed into partially crystalline fibres called microfibrils, typically ∼3 nm in diameter.[1] Within a microfibril, the chains are arranged in sheets, with hydrogen bonding between chains and between monomers in each chain[2,3] (Figure 1). The two crystalline allomorphs cellulose Iα and Iβ are exceptionally stiff and strong, outperforming steel weight for weight[4,5] and inviting comparison with carbon nanotubes.[6]. Cellulosic materials like wood can stretch in two ways. Irreversible, time-dependent slippage can occur between the cellulose microfibrils, which reorient into line with the applied force.[7] When the force aligns with the cellulose orientation, the microfibrils themselves stretch reversibly.[8] We explored this second, elastic, stretching mechanism
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