Both tyrosinase, a Cu‐containing polyphenoloxidase, and birnessite (δ‐MnO2) are able to catalyze the transformation of phenolic compounds through oxidative polymerization, a process that leads to humification, but the reaction mechanisms are not fully understood. The objective of this study was to characterize or identify the reaction products generated from catechol (1,2‐dihydroxybenzene) by birnessite or tyrosinase. Birnessite and tyrosinase catalyzed the transformation of catechol to oligomers, polycondensates, and fragments. The reaction products formed after catalysis by tyrosinase were brown colored, while those resulting from the birnessite‐catechol system were green colored; the former had a higher absorbance between 200 and 620 nm than the latter. This indicated a higher degree of aromatic ring condensation in products of the tyrosinase‐catechol system relative to those of the birnessite‐catechol system. In addition, the products of birnessite catalysis contained polycondensates and fragments, including aliphatic components, with lower molecular weights than did the products derived from catalysis by tyrosinase. Fourier‐transform infrared (FTIR) analysis and mass spectrometry indicated that the reaction products formed an organic coating on the birnessite granules.

Soil is a living system in which enzymes are present either free in solution or bound to clay and clay‐humus complexes. Enzyme‐clay interactions play a key role in transforming organic compounds in soil environments where the decomposition and synthetic processes are largely catalyzed by enzymes. Scientific evidence indicates that mineral colloids take part in the catalysis of degradative and synthetic reactions of organic compounds. Such information is essential to understanding the role of mineral colloids, the hidden half of the enzyme‐mineral colloid complexes, in catalytic reactions. Despite the abundant literature on the enzyme interactions with pure crystalline aluminosilicates, the nature of enzyme association with soil constituents, including both clean and coated clay minerals and other mineral colloids, and the effect on soil processes still remain unclear. This study integrates the existing information, including recent findings on enzyme‐mineral colloid interactions and their effect on natural and anthropogenic organic compound transformation in soil. Further, the study focuses on the catalytic role of enzyme‐clay complex surfaces in toxic industrial and agricultural compound bioremediation in soil and water environments.

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A B S T R A C T:Optical isomers deamination of L-and D-glutamic and aspartic amino acids and of their DL racemic mixtures has been achieved in the presence of Na-montmorillonite at pH = 6 and room temperature. The adsorption curves showed that the enantiomer adsorbed depends on the type of amino acid. Nevertheless, deamination reaction kinetics brought about a stereoselectivity of the clay mineral for the L-isomer and implicitly showed an unquestionable "structural chirality character" of the clay mineral.Since Bernal's hypothesis (1949) on the possible implication of clay minerals in the origin of life, several investigators (Paecht-Horowitz et al., 1970; Cairns-Smith, 1974, 1975 Lahav etal., 1978; Coyne etal., 1981; Coyne, 1985;Yamagishi, 1981 Yamagishi, , 1982Yamagishi, , 1983)have attempted to understand the action of clay minerals in the transformation and evolution of organic and bio-organic matter. Many investigators showed that the swelling clay minerals, especially montmorillonite, would be able to assume the role of a pseudo-enzyme (Mortland, 1984; Siffert & Naidja, 1987; Naidja & Siffert, 1989, 1990. The question which persists concerns the stereoselectivity of clay minerals in biochemical reactions, and in particular that raised by Degens et al. (1970) as to the cause of the L homochirality of amino acids in living organisms. It is necessary to note that organisms generally contain amino acids in their L-enantiomer form (Lavollay, 1980). Degens et al. (1970) reported a remarkable finding in connection with clay minerals, namely, that L,D-and D,L-aspartic acids in aqueous solution were polymerized at 90~ to differing extents by the catalytic action of kaolin. Thus the L-enantiomer was reported to polymerize to the extent of 25% and the D-isomer to the extent of only 3%. The authors concluded that an asymmetric synthesis of polypeptides had been accomplished on the clay surface and that polymers containing L-amino acids were formed preferentially to those containing D-enantiomers. A later report by Jackson (1971) purported to confirm these asymmetric polymerization findings, to show that the polymerization of aspartic acid failed to occur in the absence of kaolin as catalyst. Jackson also reported that L-phenylalanine was adsorbed by kaolinite to a higher extent than D-phenylalanine. In contrast to these observations, Bonner & Flores (1973) could find no evidence for the differential adsorption of D-versus L-phenylalanine by kaolin from either pH = 6 or pH = 2 solutions, using optical rotatory dispersion, gas chromatography and thin layer chromatography. However, optical rotatory dispersion measurements are unfortunately approximately within the experimental error of the observed rotations.9 1992 The Mineralogical Society

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FT-IR, UV-visible, and X-ray diffraction analyses were used to investigate the nature of organic matter in three fractions of soil humic substances (humin, HU; humic acid, HA; and fulvic acid, FA) extracted at a depth of 0–10 cm from the soil under rye grass (Lolium perenne) culture. FT-IR and XRD analyses showed that the extracted HAs still contained the clay matrix, but the extracted FAs, which were purified by adsorption-desorption from a macroreticular nonionic resin, were free from the clay mineral constituents. The data obtained in the present study shed more light on the chemical structure of the HU, HA, and FA fractions in the infrared absorption band region of 400–1000 cm−1 where the assignments of bands are still lacking in the literature. The bands characteristic of organic functional groups, especially benzene rings and condensed aromatics, which were masked by the strong bands of the clay matrix in the HU and HA fractions, were clearly shown in the FA fraction, which was free from the clay mineral constituents. While the UV-visible spectra of HAs and FAs were similar, the E465/E665 ratios of HAs were about three times lower than those of FAs. Amounts of carboxyl groups, determined by titration, were about four times higher in the FA than in the HA fraction, which is in agreement with the FT-IR analysis. The HU, HAs, and FAs fractions extracted from the soil under the same culture and exposed to elevated CO2 levels (60 Pa pCO2) under different N fertilizer application rates (100 and 420 kg N/ha) were also studied. The results show that there was no significant change in the spectra of HU, HAs, and FAs extracted from soils exposed to elevated CO2 for 2 years. In contrast, elevated CO2 combined with a high N fertilization rate resulted in a remarkable increase of the acidic OH groups of the FA.