t. Chirality and Chiral Homogeneity in Nature 2. Biotic Theories 3. Abiotic Theories 3.1. Chance Mechanisms 3.1.1. Models for Spontaneous Symmetry Breaking 3.1.2. Spontaneous Resolution on Crystallization 3.1.3.Asymmetric Syntheses in Chiral Crystals 3.1.4. Asymmetric Adsorption and Catalysis on Quartz 3.1.5.Asymmetric Adsorption and Polymerization on Clays 3.1.6. Conclusions 3.2. Determinate Mechanisms 3.2.1. Regional and/or Temporal Processes 3.2.1.1.Electric, Magnetic and Gravitational Fields 3.2.1.2.Circularly Polarized Eight 3.2.1.2.1.Asymmetric Photoequilibration 3.2.1.2.2.Photochemical Asymmetric Synthesis 3.2.1.2.3.Asymmetric Photolysis 3.2.1.2.4.Sources of Circularly Polarized Eight 3.2.2. Universal Processes 3.2.2.l.The Violation of Parity 3.2.2.2.The Vester-Ulbricht (V-U) ttypothesis 3.2.2.3.Radioracemization 3.2.2.4.Direct Effects of Chiral Particles 3.2.2.4.1 .Electrons 3.2.2.4.2.Protons 3.2.2.4.3.Positrons and Muons 3.2.2.5.Weak Interactions, Neutral Currents and Parity Violating Energy Differences 3.2.3. Conclusions 4. The Amplification of Enantiomeric Excesses

The origin and amplification of biomolecular chirality

The adsorption of uncharged polymers by clays is largely "entropy-driven." Polymer confor- mation changes from a random coil in solution to an extended form at the surface in which adsorbed polymer segments or trains alternate with loops and tails extending away from the surface. Although the net inter- action energy, e, per segment-surface contact is small (--1 kT unit), the total energy of adsorption is large because the fraction of train segments, p, is commonly between 0.3 and 0.5. The adsorption isotherms are typically of the high-affinity type, and there is an apparent lack of desorption on dilution. Positively charged polymers (polycations) are adsorbed largely through electrostatic interactions between the cationic groups of the polymer and the negatively charged sites at the clay surface. Here e >> 1 kT unit and p > 0.7, leading to an almost complete collapse of the polymer chain onto the surface. Indeed, beyond a given level of adsorption charge reversal can occur in that the clay-polycation system effectively behaves as an anion exchanger. Little adsorption occurs with negatively charged polymers (polyanions) due to initial charge repulsion between the polymer and the clay surface. Acid pH, a high ionic strength, and the presence of polyvalent cations in the system enhance and promote polyanion adsorption. Uncharged polymers and polycations can enter the interlayer space of expanding 2:1 type layer silicates but polyanions generally fail to intercalate. The interactions of clays with biopolymers, such as proteins, nucleic acids, and polysaccharides, can be rationalized in similar terms. When intercalation occurs, the interlayer biopolymer is further stabilized against microbial (enzymatic) degradation giving rise to practical applications of clay-polymer complexes as flocculants and soil conditioners. Polyanions are effective as flocculants because of their large "grappling distance," whereas uncharged polymers are better suited as soil conditioners because they can spread over adjacent clay/soil particle surfaces like a coat of paint.

Clay-Polymer Interactions: Summary and Perspectives

In this review we have critically evaluated the application of the diagenetic racemization of amino acids to geochronology and geothermometry. Although there has been enthusiastic support given to this new method, it is our opinion that recent developments suggest a more cautious approach. We have discussed the pitfalls and inhereent complications, while outlining the advances which have been accomplished. We conclude that this is an innovative approach which will add valuable information to the scientific literature. However, since our fundamental understanding of diagnetic racemization is still limited, many of the age and paleotemperature estimates which have been assigned to fossil specimens may be unreliable.

A critical evaluation of the application of amino acid racemization to geochronology and geothermometry

Origins of Chiral Homogeneity in Nature

Due to their unique polarity, pore-size distribution, and high surface areas, pillared and de- laminated clays are potentially useful materials for the adsorption of environmenlal toxicants. To deter- mine their properties for adsorption of chlorinated phenols, alumina-pillared montmorillonite (APM), chromia-pillared montmorillonite (CPM), and alumina-delaminated Laponite (ADL) were reacted with aqueous pentachlorophenol (PCP) solutions in batch equilibrium experiments. An hydroxy-A1 Laponite (HAL) in which the Na + exchange ions were replaced by ions of the type AlI304(OH)(24+x)(H20)(I2 .)(7- was included in the study. With ADL as the adsorbent, the extent of PCP adsorption increased with decreasing pH, and then became constant at pH _< pKa. Thus, the neutral phenol was preferred over the phenolate form. Binding of neutral PCP at pH 4.7 to all adsorbents never reached saturation values, and the loadings achieved were limited by the water solubility of the adsorbate. Among the pillared and delaminated clays investigated, ADL exhibited the largest capacity for physical adsorption of PCP at pH 4.7. Differences in the PCP binding capacities for APM, CPM, and ADL suggested that adsorption was dependent on the pore structure and surface composition of the modified clay adsorbent, not on surface area alone. HAL exhibited quantitative uptake of PCP at the 8 #mole/g level, indicating that a chemi- sorption mechanism may operate for PCP binding to this adsorbent. Adsorption of 3-chlorophenol, 3,5- dichlorophenol, and 3,4,5-trichlorophenol by ADL at pH 7.4 increased as the degree of hydrophobicity and chlorination of the phenol increased; hence, the binding capacity was not limited by the molecular size of the adsorbate. In contrast to the adsorption properties observed for pillared, delaminated, and hydroxy-interlayered clays, Na+-montmoriUonite and Na+-Laponite did not adsorb PCP from aqueous solution.

Modified clays for the adsorption of environmental toxicants; binding of chlorophenols to pillared, delaminated, and hydroxy-interlayered smectites

The oxidative power of a smectite can be measured quantitatively by oxidation of hydro- quinone to p-benzoquinone in a clay slurry. Oxidation takes place in the presence of 02 (air) but not N2 unless Fe 3§ or Cu 2+ are the exchangeable cations. This study examined 26 smectite samples with varying compositions and processing. The oxidative power increases with decreasing Li-fixation and increasing cation exchange capacity. Li-fixation does not depend upon the tetrahedral A1. The cation exchange capacity can decrease markedly by mere storage in water. The oxidation proceeds principally on the surface by adsorbed oxygen molecules or radicals. A mechanism is proposed. With Fe 3§ or Cu z§ present, even under N2, oxidation occurs via electron transfer. With smectites containing Fe z§ both the Fe and the hydroquinone are oxidized in the same reaction.

Oxidative power of smectites measured by hydroquinone

These studies concern the catalytic activity of clays on amino acids, particularly tyrosine. Polyhydric phenols were included to help understand the tyrosine reactions. Below pH 3, tyrosine is adsorbed on clay minerals by cation exchange. Above pH 3, oxidative degradation of tyrosine occurs, th. L-isomer altering more rapidly. The rate of alteration depends upon the particular clay mineral, surface modifications such as polyphosphate treatment, heating, and the presence of copper, aluminium, and mercury. A free radical mechanism is proposed for the alteration.

Thomas D. Thompson

Papers

Oxidative power of smectites measured by hydroquinone

The Alteration of Some Aromatic Amino Acids and Polyhydric Phenols by Clay Minerals