Chemical and conformational study of the interactions involved in mycotoxin complexation with beta-D-glucans.

In a previous paper we reported that beta-D-glucans isolated from Saccharomyces cerevisiae could adsorb zearalenone, reduce its bioavailability in the digestive tract, and protect animals against its adverse effects. We have now investigated, in vitro, the kinetics of the interaction between other mycotoxins and beta-D-glucans from several sources at three pH values found along the digestive tract (3.0, 6.0, and 8.0). Acid and neutral conditions gave the highest affinity rates for aflatoxins B1 > deoxynivalenol > ochratoxin A and involved both the (1 --> 3)-beta-D-glucans and the (1 --> 6)-beta-D-glucans. Alkaline conditions, owing to their destructuring action on glucans, were favorable only for the adsorption of patulin. Using molecular mechanics, we found that hydroxyl, ketone, and lactone groups are involved in the formation of both hydrogen bonds and van der Waals interactions between aflatoxins B1, deoxynivalenol and patulin, and beta-D-glucans. Differences in the binding capacity of the mycotoxins are due to their specific physical and chemical characteristics.

Comprehensive conformational study of key interactions involved in zearalenone complexation with beta-D-glucans.

The beta-D-glucans from the cell wall of Saccharomyces cerevisiae have shown in vitro affinity for zearalenone. For this reason, their utilization as dietary adsorbent, to reduce the bioavailability of zearalenone, is of practical interest. Our study used powerful devices to elucidate the spatial conformation and molecular sites of interaction between ZEN and beta-D-glucans. In this respect, 1H NMR spectroscopy implicated the hydroxyl groups of the phenol moiety of zearalenone in the complexation by laminarin, a pure beta-(1,3)-D-glucan. X-ray diffraction determined that laminarin displays the conformation of a single-helix with six beta-D-glucopyranose residues per turn. At this stage, molecular modeling was useful to locate the interaction sites and to propose highly probable complexes of zearalenone with laminarin fragment. Interestingly, the beta-(1,3)-D-glucan chain favors a very stable intra-helical association with zearalenone, nicely stabilized by beta-(1,6)-D-glucans side chains. Both hydrogen bonds and van der Waals interactions were precisely identified in the complex and could thus be proposed as driving interactions to monitor the association between the two molecules.

Alkali extraction of beta-d-glucans from Saccharomyces cerevisiae cell wall and study of their adsorptive properties toward zearalenone.

The isolated cell wall of Saccharomyces cerevisiae has some capacity to adsorb zearalenone (affinity near 30%) and reduce the bioavailability of toxins in the digestive tract. The adsorption process was quantified in vitro, and the data obtained when plotted with Hill's equation indicated a cooperative process. The model showed that the adsorption capacity was related to the yeast cell wall composition. This work focused on the role of various beta-d-glucan types in the efficacy of zearalenone adsorption by yeast cell wall and sought to elucidate some of the adsorption mechanisms. Zearalenone was mixed at 37 degrees C with a constant quantity of alkali-soluble or alkali-insoluble beta-d-glucans isolated from yeast cell walls, and the amount of adsorbed zearalenone was measured. Given that the alkali solubility of beta-d-glucans is a determining factor for their three-dimensional conformation and that the alkali-insoluble fraction had a greater affinity (up to 50%) than the alkali-soluble fraction ( approximately 16%), it was concluded that the three-dimensional structure strongly influences the adsorption process. The alkali insolubility of beta-d-glucans led to the formation of single and/or triple helices, which have been identified as the most favorable structures for zearalenone adsorption efficacy. The beta(1,3)-d-glucan and beta(1,6)-d-glucan compositions of the two alkali-extracted fractions and their involvement in the adsorption process are discussed.

Unusual structure of the dimeric 4-bromocalcimycin-Zn2+ complex.

The X-ray structure of [Zn(4-bromocalcimycin)2 x H2O] complex shows two highly different conformations of the ligand in the dimeric association, unusual in this ionophore family.