ELECTRON MICROSCOPICAL STUDIES OF FROZEN-DRIED YEAST. IV. SCHIZOSACCHAROMYCES, NADSONIA AND SACCHAROMYCODES.
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Cells of Saccharomyces cerevisiae were dried in vacuum, exposed to oxygen, nitrogen, air, and water vapor, and rehydrated with degassed medium without exposure to air. Drying per se caused few genetic changes, but the exposure of dry cells to oxygen increased the frequency of adenine-requiring colonies.
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Recent work has clearly demonstrated a direct correlation between the amount of trehalose present in the yeast Saccharomyces cerevisiae and its ability to tolerate dehydration, but has failed to elucidate the specific role played by trehalose. By using Fourier transform infrared spectroscopy we measured the transition temperature of phospholipids in both intact S. cerevisiae and isolated plasma membranes dried in the presence and absence of trehalose. Our results show that trehalose lowers the temperature of the dry gel to liquid crystal phase transition in yeast from around 60 degrees C to about 40 degrees C, thus allowing yeast rehydrated above 40 degrees C to avoid the damaging effects of passing through a phase transition. These results explain both the need for trehalose and the observation that yeast must be rehydrated with warm water if they are to remain viable. Only when trehalose is present is the dry transition within a physiologically tolerable range and only when the cells are rehydrated above 40 degrees C will they avoid passing through a phase transition.
It was of interest to obtain long-lived thiyl radicals embedded in organic matrices. Solid thiol compounds including penicillamine, glutathione, and cysteine were UV irradiated under anaerobic conditions at 293 K for 60 min. The formed radicals were identified by electron paramagnetic resonance (EPR) (g = 2.0265 +/- 0.0015) at 293 K as thiyl radicals. The blue-colored radical species were subjected to reflection spectrometry (lambda max = 601 +/- 3 nm). The color and the EPR signal remained unchanged for six months. At the same time, the UV irradiation of lyophilisized yeast Cu(I)6-thionein generated stable EPR detectable thiyl was seen when the Cu(I)-thiolate was used. No EPR detectable thiyl radicals radicals at a g-value of 2.026 +/- 0.001. Unlike irradiated cysteine, a five times higher concentration of thiyl radicals were measured in the Cu(I)-thiolates of penicillamine, glutathione, and thiophenole, indicating that the hexanuclear copper arrangement in Cu(I)-thionein is most suitable for both the formation and stabilization of this sulfur radical species.
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