Flávia Cassiola

Aspergillus terreus CCT 3320 immobilized on chrysotile or cellulose/TiO2 for sulfide oxidation

Abstract The increasing interest in applying chiral sulfoxides in asymmetric syntheses requires their preparation on a large scale, which can be obtained by enantioselective enzymatic oxidation of sulfides. We have focused on the preparation of sulfoxides 1 – 6 using Aspergillus terreus CCT 3320 cells to oxidize the precursor sulfides. These biotransformations lead to enantiomeric excesses (ee) better than 95%. In order to improve the biocatalytic process, the cells were immobilized on two supports, chrysotile and on cellulose/TiO 2 . The immobilized cells showed a similar biocatalytic behavior in the conversion rate and in the sulfoxide enantiomeric excess. Scanning electron microscopy (SEM) micrographs show that the cells are intertwined with the fibers of both supports, allowing fast separation from the reaction media and easing the biocatalyst reuse. Supported cells stored for at least 3 months showed no loss of activity.

Characterization of Saccharomyces cerevisiae immobilized onto chrysotile for ethanol production

Saccharomyces cerevisiae (CCT 3174 and commercial bakers yeast) was immobilized by adsorption onto chrysotile. The adsorbed yeast cells were easily washed out, but cells grown in situ were strongly attached by entrapment by chrysotile microfibres. In fermentation experiments with 30% (w/v) glucose solution, the immobilized cells showed a 1·3-fold increase in initial reaction velocity. For immobilized CCT 3174, the final ethanol yield was 26% higher than that with free cells.

Saccharomyces cerevisiae entrapped in chrysotile increases life-span for up to 3 years

Long-term viability of microorganisms has seldom been reported but is important from the technological and scientific point of view. In this work we show that Saccharomyces cerevisiae, a well known biocatalyst, remains viable and active in fermentation experiments for up to 3 years, in the absence of nutrients, when supported on chrysotile fibers. This long-term viability is ascribed to a latency state which the cells enter after about 4 months storage, induced by entrapment of the cells within the chrysotile fibers. Adhered chrysotile fibers do not penetrate the cell. TEM results show that the fibers are adhered only to the external cellular wall layer, and that no damage is caused to the cell wall structure. No fibers were ever found inside the cells. The entrapping fibers could be observed as a distinctive, well preserved silk nest in preparations in which the cells were not fixed chemically. No degradation of the chrysotile adhered fibers was observed. The entrapment is ascribed to the chrysotile flexibility and the size of the cells, which maximize adhesion by electrostatic and van der Waals interactions between the fibers and the cell surface polysaccharides.