Effect of amino acid moieties on metal binding in pulvinic acid derivatives and ecological implications for lichens producing these compounds

UV spectroscopy confirmed that the lichen-derived pulvinic acid derivative epanorin forms complexes with metal ions (Cu(2+), Fe(2+), Fe(3+), Mg(2+), Mg(2+), Mn(2+), Zn(2+),) at two binding sites both under acidic and alkaline conditions. Similar observations in rhizocarpic acid suggest that the amino acid moieties in both epanorin and rhizocarpic acid are responsible for metal binding at low pH, whereas pulvinic acids without an amino acid moiety bind only to metal ions under alkaline conditions. Metal complexation at low pH is thought to enable lichens with epanorin or rhizocarpic acid to colonize acidic, nutrient-poor substrata, whereas lichens with pulvinic acids without an amino acid moiety prefer either neutral to alkaline or acidic but nutrient-rich substrata. High dissociation constants of epanorin (pK(a)=4.9) and rhizocarpic acid (pK(a)=4.5) in methanol suggest that the amino acid groups in these substances prevent them from acting as a protonophore. Otherwise the relatively high pK(a) values of epanorin and rhizocarpic acid would result in damage of lichens containing these substances at low pH, which is not observed.