The dielectric response of hydrated water as a structural signature of nanoconfined lichen melanins

Abstract

Lichens are unique symbiotic organisms from a mutually beneficial alliance of fungi and algae/cyanobacteria that successfully survive extreme temperatures and drought conditions. Most probably such extraordinary vitality of lichens is underlain by melanins, one of the main structural and chemical lichen components, and their mutual relationship with residual water. In this paper, we propose mechanisms, which allow lichens to store up the extra water in their structure. Melanins that are constituents of the cortical lichen layer and presumably contribute to unique water–lichen interactions are chosen for physical experiments in a wide temperature domain. Two melanin pigments extracted from different lichens are studied here – eumelanin from Lobaria pulmonaria and allomelanin from Cetraria islandica. To investigate the inner melanin structure and water–melanin interactions, FTIR and BDS techniques are applied. The BDS technique was used in a wide temperature region of 123–293 K for melanins with various hydration levels. The relaxation processes related to the confinement of supercooled water - in melanins are observed and discussed in details. At medium and high hydration levels, the relaxation process in two melanins of different chemical compositions and supramolecular structures exhibits a well-known crossover that was already observed in many types of confinements. The analysis of FTIR and BDS results helps to clarify the lichen–water interaction processes.