The Identification of Imbricaric Acid and a New Imbricaric Acid-Containing Lichen Species

Abstract

Cetraria alaskana, a new, endemic, tundra-inhabiting species from western Alaska, is described. It differs from its nearest relatives morphologically in lacking soredia and chemically in producing the depsides imbricaric acid and atranorin. Pure imbricaric acid was isolated from the new species and its identity proven chemically. The pure sample was used for microchemical comparisons with the chemically very closely related but much more common depsides perlatolic acid and divaricatic acid, both of which frequently occur as constituents of species closely allied taxonomically to species producing imbricaric acid. Early in our monographic study of the species traditionally assigned to the genus Cetraria and of some obviously closely related plants in Parmelia, we had difficulty in distinguishing microchemically between the closely related depsides perlatolic acid and imbricaric acid. Although perlatolic acid is relatively widespread in Cladonia and Parmelia, imbricaric acid is absent from Cladonia and in Parmelia is known with certainty from only a single species. Imbricatic acid, however, is not uncommon in species of Cetraria (in the broad sense). 1 This study was supported in part by research grants GM-08345 from the Division of General Medical Science of the National Institutes of Health, U.S. Public Health Service, and GB-1239 from the National Science Foundation. 2 Department of Botany, Duke University, Durham, North Carolina 27706. This content downloaded from 157.55.39.35 on Fri, 02 Sep 2016 04:05:53 UTC All use subject to http://about.jstor.org/terms 1966] CULBERSON & CULBERSON: IMBRICARIC ACID 198 In Cetraria sens. lat. the problem of determining perlatolic and imbricaric acids is restricted exclusively to the broad-lobed species, the parmelioid 'Cetrariae', a series of some two dozen species to be treated in two genera in our monograph (for a taxonomic characterization of these groups, see Culberson and Culberson 1965). (The parmelioid 'Cetrariae also include a few species previously classified in Parmelia, for example P. cetrarioides in which imbricaric acid was originally discovered.) While both perlatolic acid and imbricaric acid are represented among the parmelioid 'Cetrariae', imbricaric acid is more common. Moreover, it was noted that many of the plants suspected of containing imbricaric rather than perlatolic acid also produce a pink coloration when the medulla is treated with KOH followed by Ca(OC1) 2. The problematic chemical indentifications were further complicated by these anomalous spot tests also noted by Krog (1951) and many earlier workers since from the structural formulae neither perlatolic acid nor imbricaric acid should give a positive KC test. One of the parmelioid 'Cetrariae' containing imbricaric acid and giving an anomalous KC test is a new species, Cetrarir a alaskana. In view of the difficulties previously encountered in identifying imbricaric acid and in explaining the KC test that is often associated with species containing imbricaric acid, it was thought desirable to prepare a chemically complete proof of the identity of the major constitutent of the new species in the hope that the information found and the tests used would have an even broader application in the chemical elucidation of the other, related, and obviously chemically similar species in the same group. These aims were only partially realized, however, in that while imbricaric acid was extracted and determined chemically beyond question, the substance or substances responsible for the KC test could not be identified. It seems desirable, however, to record the chemical analysis of the new species with its description because it is usually not practical to undertake chemical tests of the extensive sort carried out here in ordinary taxonomic studies where, if chemical information is sought at all, microchemical tests alone are used for the determinations.