plus simultaneous conversion of the aldehyde (C-1) to a hydroxyl group. The result is a ring that consists of five carbon atoms bridged by one oxygen atom, a structure that is termed pyranose and if derived from glucose glucopyranose (Fig. 1.15b). It assumes a chair-like shape (Fig. 1.15c) to accommodate the angled C–C bonds that result from the hybridisation of the carbon’s valence electron orbitals (see p. 17). The polar hydroxyl groups extending from the entirely nonpolar cyclic core are placed the ring’s plane in such a way that tensions and mutual interference are minimised.

Glucose always carries one aldehyde group. In its cyclic form it is hidden in the hemiacetal group. However, in aqueous solution a small amount of glucose molecules (0.26%) occur in the open-chain form (Fig. 1.15d), where the aldehyde is accessible to reactants and thus is free to act as reducing agent. Therefore glucose is classified as reducing sugar, as it is capable of scaling-down reducible substances, e. g. it reduces copper-(II) ions in Fehling’s reagent and simultaneously becomes oxidised (see Appendix A9.11, p. 560–562). When the open-chain aldehyde reacts and thus is removed from the system, it is replaced by the continuous shifting of the equilibrium from the cyclic to the open form. Cellulose – the linear polymer of β-glucose linked together by C–O–C bonds between glucopyranose units as shown in Fig. 4.2 (p. 83) therefore features one reducing end-group. When a cellulose chain is broken down into smaller units, one additional reducing end group is created per any newly formed fragment. Hence, degraded cellulose or aged paper is more vulnerable due to an increased sensitivity towards oxidation (see Chapters 8 and 9), a fact that would need alternative chemical interventions beyond traditional conservation treatments.

As mentioned above, three forms of glucose – the open-chain form and two cyclic isomers α- and β-glucose exist in equilibrium in aqueous solution. The equilibrium is due to the limited stability of hemiacetals. The spontaneous disintegration and reforming of the hemiacetal group cause the ring to open and close, thus creating two different molecular structures of identical composition. They only differ in the orientation of the hydroxyl group created at the 1-carbon. The designation α means that the hydroxyl group attached to the 1-carbon and the –CH₂OH group (C-6) joined to the 5-carbon lie on opposite sides of the ring’s plane, a structure that is also called a “trans” arrangement (Fig. 1.16a). The prefix β denotes that these groups are on the same side of the ring’s plane, i.e. they are in “cis” position (Figs 1.15b and 1.16b).

Glucose molecules can link to each other by forming an oxygen bridge between the 1-carbon of one molecule and the 4-carbon of a