ABSTRACT
It is already known that kinetic resolution of racemic glycidol (2,3-epoxy-1-propanol) takes place when Acetobacter pasteurianus oxidizes the compound to glycidic acid (2,3-epoxy-propionic acid) with glycidaldehyde (2,3-epoxy-propanal) proposed to be the transient seen in this conversion. Since inhibition affects the feasibility of a process based on this conversion in a negative sense, and the chemical reactivity of glycidaldehyde predicts that it could be the cause for the phenomena observed, it is important to know which enzyme(s) oxidise(s) this compound. To study this, rac.- as well as (R)-glycidaldehyde were prepared by chemical synthesis and analytical methods developed for their determination. It appears that purified quinohemoprotein alcohol dehydrogenase (QH-ADH type II), the enzyme responsible for the kinetic resolution of rac.-glycidol, also catalyses the oxidation of glycidaldehyde. In addition, a preparation exhibiting dye-linked aldehyde dehydrogenase activity for acetaldehyde, most probably originating from molybdohemoprotein aldehyde dehydrogenase (ALDH), which has been described for other Acetic acid bacteria, oxidised glycidaldehyde as well with a preference for the (R)-enantiomer, the selectivity quantified by an enantiomeric ratio (E) value of 7. From a comparison of the apparent kinetic parameter values of QH-ADH and ALDH, it is concluded that ALDH is mainly responsible for the removal of glycidaldehyde in conversions of glycidol catalysed by A. pasteurianus cells. It is shown that the transient observed in rac.-glycidol conversion by whole cells, is indeed (R)-glycidaldehyde. Since both QH-ADH and ALDH are responsible for vinegar production from ethanol by Acetobacters, growth and induction conditions optimal for this process seem also suited to yield cells with high catalytic performance with respect to kinetic resolution of glycidol and prevention of formation of inhibitory concentrations glycidaldehyde.
ABSTRACT
Resting cells of Acetobacter pasteurianus LMG 1635 (ATCC 12874) show appreciable enantioselectivity (E=16-18) in the oxidative kinetic resolution of racemic 2,3-epoxy-1-propanol, glycidol. Distinctly lower values (E=7-9) are observed for the ferricyanide-coupled oxidation of glycidol by the isolated quinohemoprotein alcohol dehydrogenase, QH-ADH, which is responsible for the enantiospecific oxidation step in whole cells. The accuracy of E-values from conversion experiments could be verified using complementary methods for the measurement of enantiomeric ratios. Effects of pH, detergent, the use of artificial electron acceptors, and the presence of intermediate aldehydes, could be accounted for. Measurements of E-values at successive stages of the purification showed that the drop in enantioselectivity correlates with the separation of QH-ADH from the cytoplasmic membrane. It is argued that the native arrangement of QH-ADH in the membrane-associated complex favors the higher E-values. The consequences of these findings for the use of whole cells versus purified enzymes in biocatalytic kinetic resolutions of chiral alcohols are discussed.
ABSTRACT
The convergence angle in 478 full crown preparations was assessed. Of these preparations, 351 had been performed by general practitioners and 127 by dental students. Groups of preparations performed on incisors, premolars, and molars were compared, as were preparations performed by dentists and students. Two different convergence angles were measured for each tooth, buccolingually and mesiodistally. The results showed a mean angle of 21 degrees. The mean values for premolars and molars differed significantly. When a comparison was made of preparations performed by students, a significant difference was found between premolars and molars. The same comparison for general practitioners showed a significant difference both for incisors compared with molars and for premolars compared with molars. A wide range was found for the convergence of the axial walls, especially for the preparations performed by general practitioners.
