Adsorption of Transgenic Insecticidal Cry1Ab Protein to SiO2. 2. Patch-Controlled Electrostatic Attraction

Adsorption governs the fate of Cry proteins from genetically modified Bt crops in soils. The effect of ionic strength (I) on the adsorption of Cry1Ab (isoelectric point IEPCry1Ab ≈ 6) to negatively charged quartz (SiO2) and positively charged poly-L-lysine (PLL) was investigated at pH 5 to 8, using quartz crystal microbalance with dissipation monitoring and optical waveguide lightmode spectroscopy. Cry1Ab adsorbed via positively and negatively charged surface patches to SiO2 and PLL, respectively. This patch controlled electrostatic attraction (PCEA) explains the observed increase in Cry1Ab adsorption to sorbents that carried the same net charge as the protein (SiO2 at pH > IEPCry1Ab and PLL at pH < IEPCry1Ab) with decreasing I. In contrast, the adsorption of two reference proteins, BSA and HEWL, with different adsorption mechanism, were little affected by similar changes of I. Consistent with PCEA, Cry1Ab desorption from SiO2 at pH > IEPCry1Ab increased with increasing I and pH. Weak Cry1Ab-SiO2 PCEA above pH 7 resulted in reversible, concentration dependent adsorption. Solution depletion experiments showed that PCEA also governed Cry1Ab adsorption to SiO2 particles at environmentally relevant concentrations (a few ng mL−1). These results imply that models describing Cry1Ab adsorption to charged surfaces in soils need to account for the nonuniform surface charge distribution of the protein.