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The exchange sites of montmorillonite (M) were made homoionic to calcium (Ca), hexadecyltrimethylammonium (HDTMA) or pyridinium (PY) cations. The clays were used as adsorbents for the enzyme, catalase (CA). Equilibrium adsorption and binding isotherms (i.e., after washing of the clay–CA complexes after adsorption at equilibrium until no CA was desorbed) were of the L 3 -type and fitted the Langmuir equation in the initial, but not in the later, portions of the isotherms. The amounts adsorbed and bound at the plateau, as well as the affinity, were higher for the hydrophobic clays (M–HDTMA and M–PY) as indicated by the Langmuir parameter, B max and K eq . In all three systems, there was additional adsorption after the initial plateau at higher concentrations of CA. In the case of M–Ca–CA this probably resulted from some penetration of CA into the interlayer spaces of the clay, as shown by X-ray diffraction analysis. No penetration of the interlayers was observed in the M–HDTMA–CA and M–PY–CA systems. The additional adsorption that occurred after the initial plateau in these systems may have resulted from the formation of multilayers of CA or from a change in the orientation of CA on these clay-organic surfaces, which may also have occurred in the M–Ca–CA system in addition to intercalation. Most of the CA adsorbed at equilibrium was bound on the clays (85–90%). Fourier-transform infrared difference spectra showed a shift in the Amide I and Amide II frequencies for only M–Ca–CA and M–PY–CA, which was consistent with the hypothesis of a conformational modification of the structure of CA on M–Ca and M–PY. The enzymatic activity of CA adsorbed at equilibrium on the three clays was lower than that of free CA and decreased in the order of M–Ca–CA>M–PY–CA>M–HDTMA–CA. As shown by the values of the overall first-order rate constant, K 1 , there was a further reduction in activity when CA was bound on the clays, especially on M–Ca. The pH optimum for the activity of CA remained essentially unchanged when adsorbed or bound on all clays. CA bound on the clay systems, except on M–PY–CA, was poorly utilized in comparison with the free enzyme as a sole source of carbon or nitrogen. |
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