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

Proton NMR spectra for Cladonia mitis, hydrated to Deltam/m(0) = 0.193, 0.126 and to 0.076, were recorded at different temperatures between room temperature and -45 degreesC. The loosely bound and free water fractions were selected for the observation using very long pulse length (pi/2 = 35 mus). For the thallus hydrated to Deltam/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 degreesC, 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 ddelta(T)/dT linearly depending on sample hydration level. The estimated hydration level for which ddelta(T)/dT equals that for bulk water was Deltam/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. (C) 2002 Elsevier Science B.V. All rights reserved.