Freezing-induced removal of water from phospholipid head groups in biomembranes

Abstract

The aim of this study was to investigate the driving forces causing freezing-induced gel phase formation in cellular membranes. Fourier transform infrared spectroscopy (FTIR) was used to study subzero membrane phase behavior of mouse embryonic fibroblast (3T3) cells under dehydrating and non-dehydrating freezing conditions, in the absence and presence of dimethylsulfoxide (DMSO). In addition, suprazero membrane phase behavior and the effects of salt stress and air-drying on lipid disorder were studied. FTIR studies showed that freezing induces a fluid-to-gel membrane phase transition, both in the absence and presence of DMSO, which is reversible upon thawing. With DMSO the phase transition occurs more gradual over a greater temperature range. At suprazero temperatures, membrane conformational disorder was found to be decreased when cells were exposed to high concentrations of NaCl. This effect, however, is minor compared to the effects of freezing on membrane phase state. Membrane conformational disorder at −30 ◦ C in the case of freezing-induced dehydration was lower compared to that of air-dried cells. When cells are air-dried in the presence of 3 M NaCl, however, membrane conformational disorder drastically decreases. This indicates that the freezing-induced removal of water from the phospholipid head groups results from osmotic forces.