On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances.

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.

Effect of hydration in metHb: reversible changes monitored by ESR of iron.

The dehydration of human and bovine methemoglobins was monitored using ESR spectroscopy of the iron signal. The interconversion of the Fe(III) signal between the high spin form (at g approximately 6) in solution and low spin form (at g approximately 2) was quantitatively studied as a function of hydration. The dehydration process leads also to a loss of paramagnetism resulting in the appearance of about 40% Fe(II) below 0.40 grH2O/grHb. The remaining 60% of Fe(III) ESR signal is distributed as the residual high spin form (at g approximately 6, 5%) and low spin form (hemichromes H and P, 55%). The formation of hemichrome P was explained as resulting from the coordination of the cysteine residue at beta 93 with the iron atom which follows the rupture of the proximal histidine bond. Experiments with hemoglobins where the sulphur atom of cysteine beta 93 was blocked (N-ethylmaleimide) did not showed the hemichrome P, confirming the involvement of the sulphur atom. This implies that the dehydration process induces displacements and torsion of the F helix, drastically changing the iron coordination at proximal site. In agreement with this proposition the Fe(II) symmetry is pentacoordinated with the disrupted bond to the proximal histidine at fifth coordination. This is also supported by ESR experiments with nitrosyl complex at low hydrations. All conformational changes were reversibly modulated by hydration degree and partially by lyophilization rate. A one-cycle dehydration of bovine hemoglobin followed by solubilization shows 100% reversibility of hemichrome P. Increasing the number of cycles of dehydration-hydration reduces the reversibility degree. With three cycles a reversibility of 70%-75% is observed. The level of 0.40 grH2O/grHb was the critical hydration for the molecules to return to aquo met form and correspond also to a minimal water content necessary to cover all protein surface as obtained from other techniques.