Norstictic acid: Correlations between its physico-chemical characteristics and ecological preferences of lichens producing this depsidone

The depsidone norstictic acid is a widespread secondary metabolite produced by lichen-forming fungi. Like other lichen substances, most of the norstictic acid is deposited as crystals in the apoplast of lichens, whereas only a small proportion is soluble in water. Since recent studies suggested a role of lichen substances in shaping the preferences of lichens for specific pH conditions and metal availabilities in their environment, we studied physico-chemical properties of norstictic acid to test the hypothesis that the dissociation and metal-binding behavior of this substance could explain the ecological characteristics of norstictic acid-producing lichens with respect to pH and metal availability. UV spectroscopy was used to study the dissociation and the pH dependence of the metal-binding behavior of isolated norstictic acid in methanol. Metals applied were selected macro- and micro-nutrients (Cu2+, Fe2+, Fe3+, Mg2+, Mn2+, Zn2+). The first dissociation constant (pKa1) of norstictic acid amounts to 4.0. It is higher than that of other lichen substances produced by lichens at strongly acidic sites but equals that of the dibenzofuran usnic acid. In solutions containing norstictic acid and Cu2+, Fe3+, Mn2+ or Zn2+ increasing absorbance was observed at pH > 7.5 indicating increasing affinity of the metal ions for the norstictic acid along with pH. Fe2+ and Mg2+ do not form complexes with norstictic acid. The relative high pKa1 value of norstictic acid is probably responsible for the lack of norstictic acid-producing lichens on strongly acidic substrata. At pH ≪ 4, norstictic acid is thought to damage lichens by shuttling protons into the cytoplasm. Moderate affinity of norstictic acid to Cu2+, Fe3+, Mn2+ and Zn2+ suggests a potential role at controlling the uptake of these ions at slightly alkaline conditions.