The¯uidity of chloroplast thylakoid membranes of frost-tolerant and frost-sensitive needles of three-to four-year-old Scots pine (Pinus sylvestris L.) trees, of liposomes produced from the lipids of the thylakoids of these needles, and of liposomes containing varying amounts of light-harvesting complex (LHC) II protein was investigated by means of electron paramagnetic resonance (EPR) measurements using spin-labelled fatty acids as probes. Broadening of the EPR-resonance signals of 16-doxyl stearic acid in chloroplast membranes of frost-sensitive needles and changes in the amplitudes of the peaks were observed upon a decrease in temperature from +30°C to A10°C, indicating a drastic loss in rotational mobility. The lipid molecules of the thylakoid membranes of frost-tolerant needles exhibited greater mobility. Moderate frost resistance could be induced in Scots pine needles by short-day treatment (Vogg et al., 1997, Planta, this issue), and growth of the trees under short-day illumination (9 h) resulted in a higher mobility of the chloroplast membrane lipids than did growth under long-day conditions (16 h). The EPR spectrum of thylakoids from frosttolerant needles at A10°C was typical of a spin label in highly¯uid surroundings. However, an additional peak in the low-®eld range appeared in the subzero temperature range for the chloroplast membranes of frostsensitive needles, which represents spin-label molecules in a motionally restricted surrounding. The EPR spectra of thylakoids and of liposomes of thylakoid lipids from frost-hardy needles were identical at +30°C and A10°C. The corresponding spectra from frost-sensitive plants revealed an additional peak for the thylakoids, but not for the pure liposomes. Hence, the domains with restricted mobility could be attributed to protein-lipid interactions in the membranes. Broadening of the spectrum and the appearance of an additional peak was observed with liposomes of pure distearoyl phosphatidyl glycerol modi®ed to contain increasing amounts of LHC II. These results are discussed with respect to a loss of chlorophyll and chlorophyll-binding proteins in thylakoids of Scots pine needles under winter conditions.
Pulsed EPR measurements of the transverse and longitudinal relaxation times of the C« -anion radical in crystalline C«-tetraphenylphosphoniumchloride were done at temperatures from 4 to 40 K. Above 40 K to room temperature the longitudinal relaxation time was taken from the cw-EPR linewidth. The low-temperature data are explained in terms of local magnetic fluctuations, slow C«motion, and localized two-level states related with the local disorder. The relaxation data at higher temperatures reveal experimental evidence for the Jahn-Teller distortion of the C6p -anion radical and allow to determine the Jahn-Teller splitting between the az"electronic ground state and the excited el"state of the unpaired electron.
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