Cellulose
and Lennard-Jones energies for atoms in the H-bond- ing geometries. Although it may seem so, this fact is in no way lending support for the non-covalent side in the controversy discussed above but is rather indicative of successful parameterization of the models. It has been noted that quantum mechanical contributions are required to capture the fine details of the structure of liquid water in simulations (Chen et al. 2003), and that inclusion of explicit polarizability affects both H-bond strength and kinetics (Xu et al. 2002), but in general empirical force fields are accurate enough to repro- duce structure and dynamics of massively H-bonded systems, such as hydrated polysaccharides (Chen et al. 2018). Since H-bonds in MD do not possess a distinct and fundamental identity, they are usually identified in the post-processing of the molecular trajectories based on a set of geometrical criteria. A common choice is a donor-acceptor distance less than 0.35 nm, and a hydrogen-donor-acceptor angle below 30 . The actual H-bond definition will affect the number of detected bonds at a certain instant, but the H-bond dynam- ics(Luzar 2000) can be analyzed in a way such that it becomes independent of the cutoffs.
form if two consecutive glucose units attain a 2 1 -fold (or close to) conformation, meaning that the two glucose units are twisted 180 with respect to each other around the chain axis (Fig. 1). This conforma- tion is indeed the most dominant one in water soluble cellooligomers both in solution (Kroon-Batenburg et al. 1993) and in the solid state (Chu and Jeffrey 1968; Ham and Williams 1970). One question that arises is whether these hydrogen bonds play an active role in driving the molecular conformation, or if they are merely a consequence of favorable geometry. The influence of trans-glycosidic H-bonding was investigated by several computer modeling studies where the H-bonding capability was modulated either by chemical substitution of the hydroxyls (French et al. 2005, 2021), by using glucose epimers having the hydroxyl groups in axial position as opposed to equatorial (e.g., mannose or allose) (Wang et al. 2013), or by looking at hemicelluloses (Berglund et al. 2016, 2019, 2020), which, due to its variation in chemical structure, can be viewed as a combination of both approaches. Indeed, both Density Functional Theory (DFT) calculations with solvent effects included and MD simulations in explicit water show that the conformation of the b -1,4 linkage is very consistent regardless of the number of H-bonds. The exceptions to this are chemical substitutions that have a relatively large effect on the steric environment (Yu et al. 2019), which can both shift the lowest energy conformation and influence the rigidity of the struc- ture. Another example is xylan, where the hydrox- ymethyl group is removed altogether, which favors a twisted conformation in solution but has a relatively low free energy barrier for conversion to 2 1 -fold (Berglund et al. 2016; Ling et al. 2020). The conclu- sion is that since the intra-molecular H-bonds can be effectively replaced by H-bonds to water molecules they do not contribute to the energy of the different conformations. In the gas phase on the other hand, trans-glycosidic H-bonding may indeed have pro- found effects on the conformation of celloligomers, as revealed by both modeling(Strati et al. 2002; French 2012) and experiments (Anggara et al. 2021). In this context, using a combination of vibrational spec- troscopy and QM calculations (Cocinero et al. 2009), it was shown that the cis (untwisted) conformation produced the lowest energy conformers of phenyl b - cellobioside in vacuum, which were stabilized by H-bonding between O2’ and O3. Furthermore, using
Cellulose from the inside
The cellulose polymer
Cellulose is a linear polymer composed of D -glucopy- ranose units linked by b -1,4-glycosidic bonds. The native polymer has high molecular weight with a degree of polymerization sometimes exceeding 10,000 (Grale´n and Svedberg 1943). It is also rather inflexible with a persistence length that has been estimated to * 15 nm (or * 30 D -glucopyranose residues) from molecular modeling (Kroon-Batenburg et al. 1997). Since the sugar rings are relatively stiff, conformational freedom around the glycosidic bonds, commonly described by the u and w torsional angles (Fig. 1), lends flexibility to cellulose, and polysaccha- rides in general. As a consequence of having its hydroxyl groups equatorially positioned a cellulose polymer can form intra-molecular hydrogen bonds between sequential glucose units: between the hydro- xyl group on C3 and the ring oxygen (O3H3 O5) and between the hydroxyl groups on C2 and C6 respec- tively (O2H2 O6 or O6H6 O2). Due to the geom- etry of the molecule, these hydrogen bonds can only
123
Made with FlippingBook Online document maker