This paper reviews recent findings on wood–water interaction and puts them into context of established knowledge in the field. Several new findings challenge prevalent theories and are critically discussed in an attempt to advance current knowledge and highlight gaps. The focus of this review is put on water in the broadest concept of wood products, that is, the living tree is not considered. Moreover, the review covers the basic wood–water relation, states and transitions. Secondary effects such as the ability of water to alter physical properties of wood are only discussed in cases where there is an influence on state and/or transition.

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A critical discussion of the physics of wood–water interactions

The dynamics of water surrounding a solute is of fundamental importance in chemistry and biology. The properties of water molecules near the surface of a bio-molecule have been the subject of numerous, sometimes controversial experimental and theoretical studies, with some suggesting the existence of rather rigid water structures around carbohydrates and proteins [Pal, S. K., Peon, J., Bagchi, B. & Zewail A. H. (2002) J. Phys. Chem. B 106, 12376–12395]. Hydrogen bond rearrangement in water occurs on the picosecond time scale, so relevant experiments must access these times. Here, we show that terahertz spectroscopy can directly investigate hydration layers. By a precise measurement of absorption coefficients between 2.3 THz and 2.9 THz we could determine the size and the characteristics of the hydration shell. The hydration layer around a carbohydrate (lactose) is determined to extend to 5.13 ± 0.24 A from the surface corresponding to ≈123 water molecules beyond the first solvation shell. Accompanying molecular modeling calculations support this result and provide a microscopic visualization. Terahertz spectroscopy is shown to probe the collective modes in the water network. The observed increase of the terahertz absorption of the water in the hydration layer is explained in terms of coherent oscillations of the hydration water and solute. Simulations also reveal a slowing down of the hydrogen bond rearrangement dynamics for water molecules near lactose, which occur on the picosecond time scale. The present study demonstrates that terahertz spectroscopy is a sensitive tool to detect solute-induced changes in the water network.

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Solute-induced retardation of water dynamics probed directly by terahertz spectroscopy

Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

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Gas-phase spectroscopy of biomolecular building blocks.

We present new terahertz (THz) spectroscopic measurements of solvated sugars and compare the effect of two disaccharides (trehalose and lactose) and one monosaccharide (glucose) with respect to the solute-induced changes in the sub-picosecond network dynamics of the hydration water. We found that the solute affects the fast collective network motions of the solvent, even beyond the first solvation layer. For all three carbohydrates, we find an increase of 2-4% in the THz absorption coefficient of the hydration water in comparison to bulk water. Concentration-dependent changes in the THz absorption between 2.1 and 2.8 THz of the solute-water mixture were measured with a precision better than 1% and were used to deduce a dynamical hydration shell, which extends from the surface up to 5.7 +/- 0.4 and 6.5 +/- 0.9 A for the disaccharides lactose and trehalose, respectively, and 3.7 +/- 0.9 A for the glucose. This exceeds the values for the static hydration shell as determined, for example, by scattering, where the long-range structure was found to be not significantly affected by the solute beyond the first hydration shell. When comparing all three carbohydrates, we found that the solute-induced change in the THz absorption depends on the product of molar concentration of the solute and the number of hydrogen bonds between the carbohydrate and water molecules. We can conclude that the long-range influence on the sub-picosecond collective water network motions of the hydration water is directly correlated with the average number of hydrogen bonds between the molecule and adjacent water molecules for carbohydrates. This implies that monosaccharides have a smaller influence on the surrounding water molecules than disaccharides. This could explain the bioprotection mechanism of sugar-water mixtures, which has been found to be more effective for disaccharides than for monosaccharides.

Long-range influence of carbohydrates on the solvation dynamics of water--answers from terahertz absorption measurements and molecular modeling simulations.

Combining laser desorption with a supersonic expansion together with the selectivity of IR/UV double resonance spectroscopy makes it possible to isolate and characterise the gas phase of remarkable backbone conformations of short peptide chains mimicking protein segments. A systematic bottom-up approach involving a conformer-specific IR study of peptide sequences of increasing sizes has enabled us to map the spectral signatures of the intramolecular interactions, which shape the peptide backbone, in particular H-bonds. The precise data collected are directly comparable to the most sophisticated quantum chemistry calculations of these species and therefore constitute a stringent test for the theoretical methods used. One-residue chains reveal the local conformational preference of the backbone and its dependence upon the nature of the residue. The investigation of longer chains provides evidence for a competition between simple successions of local conformational preferences along the chain and more folded structures, in which a new H-bonding network, involving distant H-bonding sites along the backbone, takes place. From three residues, the issue of helical folding can also be addressed. The present review of the gas phase literature data emphasizes the observation of remarkable secondary structures of biology, including short segments of β-strands, γ- and β-turns, combinations of turns, including a 310 helix. It also provides evidence for the flexibility of the peptide chains, i.e., a critical influence of rather minor interactions (like side-chain/backbone interactions) on the conformational stability. Finally, the paper will discuss future promising directions of the present approach.

Probing the competition between secondary structures and local preferences in gas phase isolated peptide backbones

The conformation of phenyl-substituted monosaccharides (mannose, galactose, and glucose) and their singly hydrated complexes has been investigated in the gas phase by means of a combination of mass selected, conformer specific ultraviolet and infrared double resonance hole burning spectroscopy experiments, and ab initio quantum chemistry calculations. In each case, the water molecule inserts into the carbohydrate at a position where it can replace a weak intramolecular interaction by two stronger intermolecular hydrogen bonds. The insertion can produce significant changes in the conformational preferences of the carbohydrates, and there is a clear preference for structures where cooperative effects enhance the stability of the monosaccharide conformers to which the water molecule chooses to bind. The conclusions drawn from the study of monosaccharide−water complexes are extended to the disaccharide lactose and discussed in the light of the underlying mechanisms that may be involved in the binding of carbo...

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Hydrogen bonding and cooperativity in isolated and hydrated sugars: mannose, galactose, glucose, and lactose

The ‚(1f4) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of H (H1-C1-O-C4') 180° and aH (C1-O- C4'-H4') 0° which correspond to a rotation of 150° about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it.

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Probing the glycosidic linkage: UV and IR ion-dip spectroscopy of a lactoside

The intrinsic conformer specific vibrational spectra of two important subunits of the core pentasaccharide of N-linked glycans, the alpha(1,3) and alpha(1,6) dimannosides, have been recorded in the gas phase. Coupling these measurements with a computational exploration of their conformational landscapes has enabled their conformational assignment and has identified characteristic vibrational signatures associated with particular conformational families-including those that do or do not display inter-ring hydrogen bonding across the glycosidic linkage. In addition, the IR spectra of the monosaccharide moieties provide benchmarks, through which the conformational assignments can be refined. This introduces a general concept of modularity and secondary structure in oligosaccharides--essential for the success of similar studies on larger oligosaccharides in the future.

Rebecca A Jockusch

Papers

Hydrogen bonding and cooperativity in isolated and hydrated sugars: mannose, galactose, glucose, and lactose

Probing the glycosidic linkage: UV and IR ion-dip spectroscopy of a lactoside

Building Up Key Segments of N-Glycans in the Gas Phase: Intrinsic Structural Preferences of the α(1,3) and α(1,6) Dimannosides