Freezing of water bound in lichen thallus as observed by ~1H-NMR. I. Freezing of loosely bound water in Cladonia mitis at different hydration levels

摘要

Proton NMR spectra for Cladonia mitis, hydrated to Δm/m_0 = 0.193, 0.126 and to 0.076, were recorded at different temperatures between room temperature and -45℃. The loosely bound and free water fractions wee selected for the observation using very long pulse length (π/2 = 35μs). For the thallus hydrated to Δm/m_0 = 0.193 the stepwise increase in linewidths and decrease of area under peak caused by free water freezing was observed at t = -20℃, whereas for thallus hydrated to lower level the decrease in area under peak proceeded continuously with decreasing temperature. Chemical shifts of proton NMR line vary linearly with decreasing temperature with the slope dδ(T)/dT linearly depending on sample hydration level. The estimated hydration level for which dδ(T)/dT equals that for bulk water was Δm/m_0 = 0.267, which exceeds the hydration level sufficient to initialise ice nucleation in Cladonia mitis. The role of biological ice nuclei in promoting the initialization of ice crystallite growth within lichen thallus is discussed.