Cellulose
fiber diffraction several different H-bonding arrange- ments were concluded leading to two, mutually exclusive, H-bonding pattern models existing along- side each other in (local) equilibrium in both cellulose Ib (Nishiyama et al. 2002) and cellulose I a (Nishiyama et al. 2003). Based on MD and DFT, alternate H-bonding patterns for cellulose II and III I were also proposed (Chen et al. 2015). From computational modeling it was shown that for Ib specifically, one of the two suggested patterns (denoted pattern A) was energetically more stable than the other one (pattern B) but that (presumably) static domains exhibiting pattern B still existed within real samples, possibly as disordered regions (Nishiyama et al. 2008). The preference for pattern A was also revealed by comparing DFT calculations with dichro- ism data from polarized IR spectroscopy of aligned cellulose samples (Lee et al. 2015). For cellulose II, however, the energy difference between the different patterns was not large enough to exclude the possibil- ity of co-existence in dynamic equilibrium (Hayakawa et al. 2017). Interestingly, the notion that the cellulose crystal structure is not entirely static and homoge- neous, but prone to structural disorder and possibly fluctuations may be coupled to the dynamical hetero- geneity of the glucan chains that is manifested in broad distributions of 13 C NMR T 1 relaxation times, also within the supposedly crystalline fibril core (Terenzi et al. 2015; Chen et al. 2019). The sensitivity to small variations in the cellulose crystal structure is also seen in numerous MD simulations. Using the experimentally determined atomic coordinates as input structure, including the hydroxyl groups’ hydrogen positions, MD simulations employing common force fields typically yield struc- tures that are stable in periodic crystal models, i.e. where the cellulose chains are covalently bonded to their own periodic image (Mazeau and Heux 2003; Bergenstra˚hle et al. 2007). Such structures can to reproduce crystallographic lattice parameters with reasonable deviations (within 1–8%). On the contrary, significant structural disorder including deviations from the experimental H-bonding patterns is obtained in simulations of finite fibrils, i.e. models including cellulose chain ends and interfaces to water, especially in long (microsecond) simulations (Matthews et al. 2012). Further, at high temperature a structural transition, initiated by a change in the conformation of hydroxymethyl groups from tg to gg has been
cellobiose analogues that lack H-bonding, French (2012) showed that the region of 2-fold conformations was the most stable one, also in vacuum. Thus, in an environment where intermolecular H-bonds are easily exchanged (e.g., in water) the propensity for the (near) 2 1 -fold conformation in b -1,4-linked carbohydrates such as cellulose seems to be caused mainly by steric forces.
Intermolecular H-bonds and the formation of fibrils
During the biosynthesis, glucan chains coalesce into extended structures – the elementary fibrils. These fibrils are often perceived as being constituted by a crystalline core covered by more disordered surface chains and, moreover, occasional regions along the fibril where the crystalline order may be lacking. From X-ray, neutron diffraction, and NMR studies, four major crystalline allomorphs have been reported – cellulose I, II, III and IV. Cellulose I is the native form and also the most widely studied. It is present in all plant cell walls and is further divided into two sub forms, I a (Nishiyama et al. 2003) and Ib (Nishiyama et al. 2002). Native cellulose is most commonly a combination of these two allomorphs (Atalla and VanderHart 1984) in proportions that depend on the source. Cellulose II is irreversibly obtained from cellulose I upon regeneration or alkali treatment (Langan et al. 1999, 2001). Cellulose III (Wada et al. 2004a, b) can be obtained from both cellulose I (in that case called cellulose III I ) and cellulose II (cellulose III II ), by soaking in liquid ammonia or organic amine at low temperatures, whereas cellulose IV can be formed by thermal treatment of cellulose III in glycerol (Wada et al. 2004a, b). All these allomorphs are recognized by conformational differences, various packing arrangements and, importantly, their different intra- and inter-molecular hydrogen bonding patterns (Kovalenko 2010) (Fig. 2). The exact location of hydrogens in the crystal structure is difficult to determine experimentally due to their small X-ray scattering power. Furthermore, even if heavier atoms are ordered, the hydrogens may be disordered in the structure, as for instance seen in hexagonal ice (Peterson and Levy 1957). Conse- quently, the H-bonding in crystalline cellulose is not necessarily fixed to a single pattern but may be prone to fluctuations. Indeed, based on X-ray and neutron
123
Made with FlippingBook Online document maker