Cellulose
advantage. This hints to that H-bonds themselves cannot be the main driving force for biomolecular assembly in water, as the total number of H-bonds in a hydrated system will remain more or less constant. In fact, we are probably lucky that H-bonds between biomolecules in water are weak, reversible, and dynamic, since the molecules of life would otherwise be strongly associated in uninteresting lumps. How- ever, their directionality can make them into a significant steering force and thereby proficient orga- nizers of three-dimensional structures (Jeffrey and Saenger 1994). Many processes involving cellulose are non-equilibrium processes. This applies to the biosynthesis of cellulose in the plant cell walls, to the mechanical treatments of fibers by which the elemen- tary fibrils are liberated, and to the application of shear forces (stirring) for efficient dissolution. With that in mind, it makes sense to differentiate between the making and the breaking of H-bonds, and to realize that it may require a large activation energy to both make or break, even if the net effect in the free energy froman equilibrium point of view is zero. A single H-bond is rather unspecific and relatively weak, and thereby can form and break on short (nanosecond) timescales, activated by thermal fluctu- ations alone at biological temperatures. However, consider N H-bonds defining a molecular complex. These bonds are subject to N 1 = 2 k B T thermodynamic fluctuations. If N is large the kinetic stability of this complex is decided by the relation of that term to NCk B T , where Ck B T is the average activation energy to break an intermolecular H-bond. In other words, since the total activation energy scales with N but the random force with N 1 = 2 , massively H-bonded molec- ular complexes are not easy to disassemble since many bonds have to break simultaneously without reform- ing. Once formed they can thermally be prone to remain assembled, even in the case of a favorable free energy for dissolution. Thus, their strength lies in their number, which combined with ordering in specific patterns can lead to considerable specificity. When H-bonds are broken, they can readily re-form provided that the geometry is right. This allows for flexible structures which is utilized in, e.g., spider silk (Nova et al. 2010), and can be exploited in self-healing biomaterials (Brochu et al. 2011). Thus, H-bonds is utilized by nature both to create kinetically stable, highly ordered structures, and to dissipate energy in
interaction mechanisms. Thus, results from modeling studies are central to this text. Our most important message is that H-bonding is just one of several such mechanisms, and the main reason why it stands out is because its importance is frequently exaggerated sometimes to the extent as if it was the only reason behind a variety of complex cellulose-related phenomena. One example for this interpretational havoc is the debate concerning the physical mechanisms behind cellulose dissolution. Owing to the abundance of accessible hydroxyl groups on its surface, cellulose is rightfully considered a hydrophilic molecule with a pronounced hygroscopic character and wetting. At the same time, as distinct molecules it is completely insoluble in water at ambient conditions, which has been attributed to the formation of H-bonds between the cellulose molecules themselves leading to large and readily precipitating aggregates. However, more than a decade ago MD simulations were used to show that the contribution from H-bonding to the insolubil- ity of cellulose in an aqueous environment was an order of magnitude smaller than hydrophobic solva- tion energies (Bergenstra˚hle et al. 2010). About the same time, in a series of papers, Bjo¨rn Lindman and co-workers argued that H-bonding can hardly be the driving force for aggregation in water, and that one should treat cellulose as an amphiphilic molecule dressed with several interactions (Medronho et al. 2012; Lindman et al. 2021) which at the time was coined the ‘‘Lindman hypothesis’’ (Glasser et al. 2012). A few years later Nishiyama (2018) showed that London dispersion interaction is the dominating contribution to the total cohesive energy of cellulose. Today, it seems a large fraction of the cellulose community concurs with the notion that H-bonds play only a minor role for the precipitation of cellulose chains in aqueous environments, but solubility is not the only area where H-bonding effects are being exaggerated. In this context, it is important to consider that all biological processes take place in water, the hydrogen- bonding liquid par excellence . This means that H-bonds within or between biomolecules always have to compete with H-bonds to water, and in this competition the relatively high mobility, both trans- lational and rotational, of the water molecules with the unparalleled capacity of forming four hydrogen bonds for a puny molar mass of 18 gives the latter an
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