Adsorption of Insecticidal
Cry1Ab Protein to Humic
Substances. 2. Influence of Humic and Fulvic Acid Charge and Polarity
Characteristics
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Abstract
Assessing the fate and potential risks of transgenic
Cry proteins
in soils requires understanding of Cry protein adsorption to soil particles.
The companion paper provided evidence that patch-controlled electrostatic
attraction (PCEA) and the hydrophobic effect contributed to Cry1Ab
protein adsorption to an apolar humic acid (HA). Here, we further
assess the relative importance of these contributions by comparing
Cry1Ab adsorption to seven humic substances varying in polarity and
charge, at different solution pH and ionic strength, <i>I</i>. Cry1Ab adsorption to relatively apolar HAs at <i>I</i> = 50 mM exhibited rapid initial rates, was extensive, and was only
partially reversible at pH 5–8, whereas adsorption to more
polar fulvic acids was weak and reversible or absent at pH >6.
The
decrease in adsorption with increasing HS polarity at all tested pH
strongly supports a large contribution from the hydrophobic effect
to adsorption, particularly at <i>I</i> = 50 mM when PCEA
was effectively screened. Using insect bioassays, we further show
that Cry1Ab adsorbed to a selected HA retained full insecticidal activity.
Our results highlight the need to consider adsorption to soil organic
matter in models that assess the fate of Cry proteins in soils