Isolichenan, also known as isolichenin, is a cold-water solubleα-glucan occurring in certain species of lichens. It was first isolated as a component of an extract of Iceland moss in 1813, along with lichenin. After further analysis and characterization of the individual components of the extract, isolichenin was named in 1881. It is the first α-glucan to be described from lichens. The presence of isolichenin in the cell walls is a defining characteristic in several genera of the lichen family Parmeliaceae. The biological function of isolichenan in the lichen thallus is unknown.
Early studies
Isolichenan was first isolated from Cetraria islandica in 1813 by Swedish chemist Jöns Jacob Berzelius, who also concomitantly isolated the cellulose-like hot-water soluble glucan lichenan. Because in these experiments the isolichenan component of the lichen extract had a positive reaction with iodine staining, Berzelius thought it to be similar in nature to starch, and he called it "lichen starch". It was thought to function as a reservefood source for the organism. Later studies showed it to be a mixture of polysaccharides. In 1838, Gerardus Johannes Mulder isolated the blue-staining component of the C. islandica extract, believing it to be starch. Friedrich Konrad Beilstein gave the name "isolichenan" to this substance in 1881. Isolichenan was the first α-glucan described from lichens.
Structure
Isolichenan is made of glucose units joined by a mixture of α- and α- linkages. Using the technique of partial acid hyrolysis, Stanley Peat and colleagues determined that the linkages are of the α-configuration. The ratio of these linkages has been reported differently by various authors in the literature: 11:9, 3:2, 2:1, 3:1, and 4:1. Fleming and Manners found the ratio to be 56.5:43.5 and 57:43 in two separate procedures. The distribution of linkages was found to be somewhat irregular, with both types occurring in groups of two or more in at least some areas; another study suggests that isolichenan has mostly groups of one or two α- bonds surrounded by α- bonds. The relatively weak intensity of the iodine staining reaction of isolichenan, compared with for example amylose, is thought to be a result of its preponderance of linkages. This reduces the formation of the polyiodide-complex that gives the positive reaction its blue colour. The chain length of isolichenan was estimated at 42–44 glucose residues. The reported molecular weight of isolichenan also varies from 26 kD to 2000 kD. The relatively short chain length of isolichenan may explain why it is soluble in cold water after it has been extracted from the lichen thallus. Purified isolichenan has a high positive specific rotation in water. It has been reported as high as +272, although different sources give differing values. The term "isolichenan-type" has been used as a general term for α-D-glucans having - linkages in their main chain. Similar to isolichenan, the α-D-glucan known as Ci-3 consists of 1→3 and 1→4 linked α-D-glucose residues in ratio of 2:1, but with a much higher degree of polymerization and a molecular weight of about 2000 kD. It is also found in Cetraria icelandica. As the discrepancies in reported values demonstrate, lichens produce isolichenan-type polysaccharides with considerable variation in linkage ratios as well as molecular weight, even within the same species. The carbon-13 nuclear magnetic resonance spectrum of isolichenan was reported by Yokota and colleagues in 1979 and also by Gorin and Iacomini in 1984.
Uses
Although isolichenan is not nearly as constant at the genus level as lichenan, the presence of isolichenan in the cell walls is a defining character in several genera of the lichen family Parmeliaceae, including Asahinea, Cetrelia, Flavoparmelia, and Psiloparmelia. In contrast, the absence of isolichenan is a character of genus Xanthoparmelia. Isolichenan is used as an active ingredient in cough lozenges as a component of Cetraria islandica extract.
Research
Isolichenan was shown to enhance hippocampalplasticity and behavioural performance in rats. When administered orally, isolichenan was also shown to improve memory acquisition in mice impaired by ethanol, as well as in rats in which memory impairment had been induced by beta-amyloid peptide. The main α-glucan synthesized by lichens of the genus Ramalina in the symbiotic state is isolichenan. It is not, however, produced by either individual symbiont when cultivated apart from each other. Its absence in this circumstance suggests that it may not have an importance as a structural part of the fungal cell wall; this contrasts with lichenan, where the -β-glucan has been shown to be involved in cell wall structure. Isolichenan is synthesized by the mycobiont only in the presence of its symbiotic partner in a special microenvironment – the lichen thallus. The triggering of this phenomenon and the biological function of isolichenan in the symbiotic relationship between fungi and algae is still unknown.
