Freeze-dried cells, contacted with

Fig. 26. The ESCA C (Is) spectra of freeze-dried cummingtonite contacted with Ehrlich cells [146] and the Ph.D. dissertation of S. Seal (1996).

This treatment is especially recommended if the cells are lyophilised for deuterations. The cells can be lyophilised without loss of hydrogenase and enoate reductase activity. Without this treatment usually 60-80 % of the enzyme activity can get lost during freeze drying, even if contact with oxygen is tried to omit. Freeze dried cells are used for reductions in H20. 

Dissolved oxygen and other gases can be removed from the solvent by the freeze-pump-thaw (FPT) method, or by the expansion of gases above the solvent into an evacuated container on the vacuum line. It is customary to freeze-pump-thaw the solvents before vapor transfer from the storage container to the vacuum electrochemical cell. Such FPT cycles should be done as soon as the solvent is brought into contact with the drying agent. The FPT method involves three basic steps (i.e., one cycle), which are often repeated ... 

Besides that, Co EMS was earlier applied for monitoring the state of cobalt(ll) in roots of water hyacinth Eichhomia crassipes [28], as well as in cells of a cyanobacterium (the blue-green alga Synechococcus vulcanus) [29] and Gram-negative bacteria (Escherichia coli [30] and, more recendy. Azospirillum brasilense in the freeze-dried state [27,32] or in frozen aqueous suspensions (FAS) [31,32] after their contact with Co" in solution). 

In the soil diazotrophic rhizobacterium A. brasilense (strain Sp245, reported to be tolerant to submillimolar concentrations of heavy metals, including cobalt(ll), in the culture medium [32]), EMS studies were first performed on freeze-dried bacterial samples (measured at T = 80K Fig. 17.2) [27]. The following experiments with the same bacteria were performed with live cells rapidly frozen after certain periods of time (2-60 min) of contact with Co", and EMS spectra were measured for frozen suspensions (without drying), which more closely represent the state of cobalt in the live cells [31,32] (Fig. 17.3). Mossbauer parameters calculated from the experimental data are listed in Table 17.1. 

The third method, dry ice (Fig. 4.1, pathway No. 3), is readily available in most labs and does not require much additional equipment. Dry ice melts at -56°C. Freezing done at this temperature is warmer than isopentane which has slower heat transfer out of the tissue. Dry ice is used as block or as pulverized with a hammer. The tissue contacts the dry ice and freezes in several seconds, which is relatively slow freezing. As such, the process generates ice crystals in the cells and can reduce the quality of the cellular detail seen in the microscope (Rosene et al., 1986). For... 

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