Abstract-The absorption of biologically important purines, pyrimidines, and nucleosides by Li-, N a-, Mg-, and Ca-montmorillonite has been studied in aqueous solutions over a range of pH values 2-12. The initial organic concentrations were about 1 m.molar. The ratio clay to organic compounds was such that only up to 25 per cent of the exchange capacity could be saturated by organic cations, but, depending on conditions, up to 100 per cent of the available organic material was absorbed. Of the nineteen compounds studied, only thymine, uracil, and their nucleosides were not absorbed under the experimental conditions. Absorption occurs primarily as a cation exchange reaction under acid conditions and varies with the basicity of the compounds, their aromatic or non-aromatic character, and the possible extent of their van der Waals interaction with the silicate layers. Nucleosides generally are less strongly absorbed than their purines or pyrimidines because their non-planar structure permits less van der Waals interaction; their absorption is influenced by the differences in swelling behavior of montmorillonite with mono-and divalent cations.

Abstract-The co-absorptions of various purines and pyrimidines from aqueous solutions by Na-and Ca-montmorillonite are studied in the range of pH 1-6. The pyrimidines, thymine and uracil, which are not absorbed from solutions of these compounds alone, are appreciably absorbed from solutions containing also adenine or 2,6-diaminopurine, are weakly absorbed from solutions containing hypoxanthine, and are not absorbed from solutions containing purine, cytosine, and caffeine. The specific co-absorption is tentatively attributed to hydrogen-bond formation between the molecules in solution rather than to an overlapping of the molecular configurations (vertical stacking). The montmorilloniteorganic associations have a basal spacing of about 12.5 -~ which permits only a single molecular layer between successive silicate layers.

Abstract-Absorptions of purine and pyrimidine derivatives by Co-and Ni-montmorillonite at pH < 6 and by Cu-montmorillonite at pH < 3 are similar to their absorption by Ca-montmorillonite and take place primarily by a cation exchange process. In the weakly acidic to weakly a~kaline range, absorption is due to complex formation with the inorganic cations, and decrease in the order Cu ~> Ni > Co >> Ca. Adenine, 7-methyladenine, hypoxanthine, and purine are strongly absorbed, 9-methyladenine, 6-chloropurine, and cytosine are weakly absorbed, and thymine and uracil are not absorbed. At pH < 5, the nucleosides are absorbed by Co-, Ni-, and Cu-montmorillonite in approximately the same manner as by Ca-montmorillonite, but at pH > 6 their absorptions decrease in the order Cu ~> Ni > Co > Ca. Fe(llI)-montmorillonite behaves quite differently from the other montmorillonites studied. With purines and pyrimidines, there is strong absorption from pH 3 to pH 7-8; with the nucleosides, the absorption varies considerably with the compounds considered decreasing in the order adenosine > cytidine ~> guanosine ~> inosine.INTRODUCTION THE EXPERIMENTS reported here extend those previously described on the absorption of pyrimidines, purines, and nucleosides by Li-, Na-, Mg-, and Ca-montmorillonite (Clay-Organic Studies, Part XII, Lailach et al., 1968) to montmorillonites saturated with the transition metal cations, Co, Ni, Cu, and Fe(llI). The previous work will be referred to as Part XII.Attention has been given mainly to the absorption isotherms in the pH range 2-12, and the organic compounds have been selected in part to show the influence of substituent groups on the complexing behavior of the compounds with the inorganic cations.The experimental methods for preparing the montmorillonites and measuring the organic absorptions are the same as those described in Part XII. Only for the alkaline samples of Fe(II1)-montmorillonite was a change made in the procedure for u.v.-spectroscopic measurements. Since under alkaline conditions a Fe(IlI)-hydroxide sol was formed which was not removed from solutions by centrifugation, a pH 2 buffer (HC1-KCI) was added to the supernatant and sedimentation of a gel took place within a few minutes. The clear supernatant solutions were then used for the spectroscopic measurements.