Pulsed EPR spin-probe study of intracellular glasses in seed and pollen

EPR spectra of 3-carboxy-proxyl (CP) in dry biological tissues exhibited a temperature-dependent change in the principal value A'(zz) of the hyperfine interaction tensor. The A'(zz) value changed sharply at a particular tempe rature that was dependent on water content. At elevated water contents, the break occurred at lower temperatures and appeared to be associated with th e melting of the cytoplasmic glassy state. To investigate the reason for th e change in A'(zz) we employed echo-detected EPR (ED EPR) spectroscopy. The shape of the ED EPR spectrum revealed the presence of librational motion o f the spin probe, a motion typically present in glassy materials. The simil arities in temperature dependency of A'(zz) and librational motion of CP in pea seed axes indicated that the change in A'(zz) arose from librational m otion. ED EPR measurements of CP as a function of water content in Typha la tifolia pollen showed that librational motion decreased with decreasing wat er contents until a plateau or minimum was reached. ED EPR spectroscopy is a valuable technique for characterizing the relation between molecular moti on and storage kinetics of dry seed and pollen, (C) 2000 Academic Press.