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The mnemonic derives from the mythical tale of Scylla and Charybdis in Homer's Odyssey (chapter 12). It takes the form of an imaginary headline in a newspaper: SCYLLA MEETS CHARYBDIS - EPIC NEWS MUCH ALARMS SICILY. The first two or three letters in each of these eight words remind the user that the nine configurational prefixes are scyllo-, meso-, (or myo-), chiro- [(+) and (-)], epi-, neo-, muco-, allo-, and cis-, respectively. The mnemonic also arranges the prefixes in an order that allows the configurations to be derived in a logical manner by performing a defined sequence of imaginary configurational inversions (epimerizations) around a cyclohexane ring. The all-equatorial, chair conformation of scyllo-inositol is selected as the starting point, and the sequence of inversions is defined by a systematic permutation of possibilities for performing one, two or three inversions in succession (1; 1 and 2; 1 and 3; 1 and 4; 1, 2 and 3; 1, 2 and 4; and finally 1, 3 and 5). In the case of the two chiro-inosit...

Under ordinary analytical conditions, the proportion of periodate-resistant- D -glucose residues in starches and glycogens was consistently about one-third of the proportion of branching points. The resistant D -glucose residues became freely oxidisable after the limit-oxidised glucans had been reduced with sodium borohydride. The results can be explained by assuming that, when a D -glucose residue carrying a branch at position 6 is oxidised, the resulting two aldehyde groups both form 6-membered hemiacetal rings with the closest hydroxyl groups on neighbouring, unoxidised residues in the same, (1→4)-linked chain, whereas when the other D -glucose residues are oxidised, only one of the aldehyde groups shows a strong tendency to form a hemiacetal of this kind. It is suggested that, in the unbranched units, the other aldehyde group preferentially forms a hemiacetal with the primary hydroxyl group in the same unit.

By adding finely powdered sodium nitrite to solutions of amylose in 85% (w/w) orthophosphoric acid, a series of d -glucurono- d -glucans was prepared, having number-average molecular weights of ∼104 and GlcA/Glc ratios varying from 0.5 to 3.0. The products were soluble in water, both as sodium salts and free acids, formed complexes with iodine having λmax in the range of 508–574 nm, and exhibited strongly pH-dependent chiroptical properties.

A novel type of natural, water-soluble copolymer has been obtained from mangrove leaves in ∼20% yield. Evidence is presented that the polymers contain covalently bonded flavolan and high-molecular-weight glycan components, probably in a range of proportions, and are therefore designated flavologlycans. Fractionation of the polymers has been conducted by complexing with lead acetate and by sorption on nylon (polyamide) and on Sepharose.

Oxidation of methyl α- and β- d -galactopyranoside with periodic acid in dimethyl sulphoxide gave, in each case, one major, monomeric product, isolated as a diacetate. These syrupy products were characterised by m.s. and 1 H- and 13 C-n.m.r. spectroscopy as (1 S ,2 S ,4 R ,7 S )-7-acetoxy-4-acetoxymethyl-2-methoxy-3,6,8-trioxabicyclo[3.2.1.]octane and (1 S ,2 R ,4 R ,7 S )-7-acetoxy-4-acetoxymethyl-2-methoxy-3,6,8-trioxabicyclo[3.2.1.]octane, respectively. Chemical shifts and coupling constants for the products were determined partly from the experimental n.m.r. spectra, and partly by an iterative fitting-procedure with the computer program NEMEN.

After depolymerisation to d.p.n ∼20-40, galactomannans from guar, carob (locust bean), lucerne and clover seeds gave good 1H- and 13C-n.m.r. spectra. The resonances of the anomeric protons and the twelve carbon atoms were fully resolved and easily identified. Measurement of the ratios of corresponding peak-areas gave results for monomeric composition (Gal: Man ratio) in good agreement with those of chemical analysis. Resonances from C-4 of d-mannose residues were split, in evident dependence upon the nearest-neighbour probabilities (“diad frequencies”) Of d-galactosyl groups along the mannan chains. The relevant peak-area ratios gave values for these probabilities that were roughly consistent with a random arrangement of d-galactosyl groups in all four galactomannans, but they do not exclude the possibility of more complex kinds of arrangement.

A sample of soluble humic acid from peat-bog water was a glycoconjugate containing 46% of a glycuronoglycan moiety and 54% of a dark-brown chromophore. These accounted for 37% and 63%, respectively, of the titratable acidity of the polymer. Cation-exchange capacities, and cationic selectivity coefficients relative to magnesium ions ( K Mg Me ), were measured on the humic acid for Pb 2+ , Cu 2+ , Zn 2+ , Ba 2+ , Ca 2+ , and Sr 2+ , and compared with those of extractive-free Sphagnum and other mosses, their chlorite holocelluloses, and two soluble fragments of Sphagnum holocellulose, prepared by acidic and alkaline degradation, respectively. The humic acid showed considerably higher K Mg Me values than most of the control materials, the enhancement being especially marked for Pb 2+ , Cu 2+ , and Ca 2+ . Scatchard plots showed that both parts of the glycoconjugate contributed to its selectivity, and that the selectivity of the carbohydrate part was greater in the humic acid than in the holocellulose or its soluble fragments. The results are explained by assuming that there are enhanced possibilities for cross-linking in the colloidal humic-acid complexes.

After correction for overoxidation, the periodate-oxidation limit of the soluble, sodium salt of C-6-oxycellulose was 0.63 mol per non-terminal, hexuronic acid residue, compared to 0.44 mol for a (1→4)-linked β- d -mannuronan isolated from alginate. This suggested that, whereas both the aldehyde groups of oxidised hexuronic acid residues formed stable inter-residue hemiacetals in the mannuronan, only the one derived from C-2 did this in the glucuronan. Formation of the other hemiacetal in the oxidised glucuronan would require C-5 of the oxidised residue to become axial with respect to the resultant 1,4-dioxane ring, and the p orbitals of O-1 and O-5 in the adjacent unoxidised residue to become eclipsed. The extent of the destabilisation suggests that both conditions have to be recognised as important instability factors in this kind of hemiacetal.

A detailed theoretical analysis of the kinetics of the periodate oxidation of guaran and locust-bean gum, together with the results of methylation analyses carried out by Lindberg's group on partially oxidised and barohydride-reduced samples of the galactomannans, has provided new evidence for the arrangement of the α- d -galactopyranosyl side-groups along the mannan chains. The results indicate that, in guaran, the d -galactosyl groups are arranged in small groups of mostly two to four units, attached to contiguous d -mannosyl residues in the chains. These are separated mostly by small groups of two, or occasionally three, contiguous, unsubstituted d -mannosyl residues. In locust-bean gum, there are long blocks of contiguous, unsubstituted d -mannosyl residues, long blocks in which every second d -mannosyl residue is substituted with d -galactose, and shorter blocks in which there is a high density of d -galactosyl groups.

A kinetic analysis is described of the periodate oxidation of a dextran in which all the 93% of oxidisable D -glucose residues contained a 2,3,4-triol system. Measurements were made of the periodate consumed and the formic acid liberated by the dextran, the periodate consumed and the formaldehyde liberated by samples that had been partially oxidised and then reduced with sodium borohydride, and the glycerol and erythritol released from these samples by acid hydrolysis. Initially, the oxidisable D -glucose residues decayed according to second-order kinetics. After the first oxidative attack, ~ 40% of the singly oxidised residues very rapidly consumed a second mole of periodate, while the remainder consumed further periodate at about one-seventh of the rate of an intact D -glucose residue. Residues cleaved between positions 3 and 4 were generated 7.5 times faster than residues cleaved between positions 2 and 3, but the two kinds of singly oxidised residue subsequently decayed at similar rates. Towards the end of their reaction, the rate of decay of intact, oxidisable D -glucose residues declined in a way that was simply correlated with the proportion of doubly oxidised residues in the chains. A simple scheme is presented that explains these facts in terms of intra-residual hemiacetal formation by singly oxidised residues, and inter-residual hemiacetal formation between doubly oxidised residues and intact D -glucose residues adjacent to them in the chains.

For oxidation in aqueous sodium metaperiodate at 20°, rate coefficients describing the decay of intact glycoside ( k G ) and the consumption of periodate ( k P ) were measured for the methyl α- and β-glycopyranosides of d -glucose, d -galactose. d -mannose, d -xylose, and d -(or l -)arabinose, and for methyl α- l -rhamnopyranoside. For glycosides containing a cis -1,2-diol group, k G was four to sixteen times larger than for those containing only trans -1,2-diol groups. For anomeric pairs, ( k G ) α /( k G ) β varied from 0.6 to 1.7. The kinetics showed that the singly oxidised intermediates existed partly in an unreactive form, which was inferred to be a cyclic hemiacetal. For hexopyranosides, but not pentopyranosides, the stabilities of the hemiacetals, as indicated by the limiting values of k P , were much greater with α than with β anomers. From a consideration of possible structures and conformations of the hemiacetals, it was inferred that this result was a manifestation of the anomeric effect.

Samples of sodium alginate from Laminaria digitata were oxidised in aqueous solution to different extents by the addition of calculated amounts of sodium metaperiodate. An empirical measure of the molecular stiffness of the products, both before and after reduction with sodium borohydride, was obtained by studying the sensitivity of their intrinsic viscosities to changes in the ionic strength of the solution. The results showed that, whereas the stiffness decreased very sharply with increasing degree of oxidation, it changed very little upon subsequent reduction of the products. From this, and the known structures of the inter-residue hemiacetals in the unreduced products, it was inferred that the increase in flexibility brought about by oxidation is specifically associated with rotation about the three bonds adjoining C-4, C-5, the original ring-oxygen atom, and C-1 of the oxidised hexuronic-acid residues. This rotation was considerably less-hindered than the rotation around the glycosidic linkage in both the intact alginate and in the partially oxidised and reduced products. The conversion of only a small percentage of the stiff pyranoid rings into these centres of high, local flexibility had a marked effect on the overall flexibility of the chains, showing that the possible presence of such centres should be taken into account in any discussion of the solution properties of heteropolysaccharides or of polysaccharides that may have suffered slight chemical modification.

Publisher Summary This chapter reviews algal polysaccharides. Fossil evidence indicates that many of the algae have changed very little in the hundreds of millions of years and it may be somewhat reckless to assume that the evolution of algal glycan structures has proceeded exactly in parallel with morphology. As new fossil evidence comes continually to light, ideas about the phylogenesis of algae are constantly being revised and there are inevitably areas of disagreement among authorities. There is unanimous agreement that the blue-green algae (Cyanophyta) are by far the oldest and there is reasonable evidence that the red algae (Rhodophyta) appeared next and the brown algae (Phaeophyta) last. In view of the very primitive morphology of algae, one might expect that the structures of the glycans would also be simple. In fact, the reverse seems to be true. Many algal glycans surpass even the gum exudates of terrestrial plants in apparent complexity. Some disorder might perhaps be expected in the glycans of organisms that are low on the evolutionary scale, because it implies that the biosynthetic enzymes are low in specificity rather than large in number. The apparent structural complexity of some algal glycans is increased by a special feature of the reproductive cycle of many algae, whereby the organism passes successively through gametophytic (haploid, male and female) and sporophytic (diploid) forms. There is growing evidence that many algal glycans exist in the native state, at least partly, as proteoglycans and that accepted procedures for isolation usually entail some degradation of these more complex macromolecules, with the liberation of the glycan moiety.

For solutions of starch, diethylaminoethylamylose, amylose sulphate, and carboxymethylamylose in aqueous inorganic acids or salts at 20°, plots of the optical density at 680 nm against the concentration of added potassium tri-iodide (KI3) showed maxima whose values were highly dependent upon the concentration and identity of the ions present. The absorption spectra in the different media were also different, and similar changes in the absorption spectra could not be brought about by varying the concentration of I3– in the system.With polyvinyl alcohol, there was no tendency to develop different maxima in different media, and the absorption spectrum was independent of the medium. It thus appeared that the acids and salts were bringing about a conformational change in the glucans, leading to an increase or decrease in the number of sites that were able to bind I3– strongly.By working with a very large excess of I3–, it was possible to avoid complications arising from the competitive binding of species other than I3–, or salt effects upon the activity coefficient of I3–, and to study the effect of different media upon the conformational change alone. These media included aqueous sulphuric, phosphoric, hydrochloric, and hydrobromic acids, alkali-metal halides, and various other salts of lithium, sodium, and potassium.The effect of the acids and salts upon the maximal optical density at 680 nm could not be correlated with their effect upon the volume of gels prepared by cross-linking starch with epichlorohydrin, or with their salting-out parameters for non-polar compounds. This was interpreted as evidence that the conformational changes were not primarily associated with changes in the stability of intramolecular hydrogen bonding, or of hydrophobic interactions.On the other hand, there was a close correlation between the maximal optical density in the four acids named above, and their previously demonstrated capacity to change the magnitude of the anomeric effect in the methyl glucopyranosides and in cellobiose and maltose. With an apparent exception that was noted for fluorides, the effect of all the other salts upon the optical density correlated well with their expected capacity to modify the magnitude of the anomeric effect, based upon the idea that the ring and glycoside-oxygen atoms are selectively desolvated by salts whose anion is the stronger electrostrictor, and solvated by salts whose cation is the stronger electrostrictor.It was therefore concluded that the conformational changes were mainly due to changes in the magnitude of the exo-anomeric effect, which would be expected to modify the relative stabilities of the rotamers generated about the C(1)–O(1) bonds, and, hence, the ease with which the linear segments of the glucan chains are able to take up the helical, V-conformation in solution. It is pointed out that similar changes can be expected in any flexible molecule, whose net dipole moment in different conformational states can be expected to vary.