The adsorption and reaction of adenine nucleotides on montmorillonite

Abstract

The binding of AMP to Zn²⁺-montmorillonite was investigated in the presence of buffers and salts. Good's buffers, piperazine-N,N′-bis(2-ethanesulfonate) [PIPES] and morpholine-N-2-ethanesulfonate [MES], perturbed the exchangeable cations to a lesser extent (only 9% of Zn²⁺ displaced by 0.2 M buffer) than was observed with imidazole and lutidine buffers or NaCl and KCl salts (up to 80% of Zn²⁺ displaced). AMP adsorption isotherms measured in the presence of 0.2 M PIPES, MES or Na₂SO₄ exhibited normal Langmuir-type behavior. The adsorption coefficient, K_L, is 3-fold greater in the presence of HEPES or PIPES than it is in the absence of buffers. Basal spacings measured by X-ray diffraction for Zn²⁺-montmorillonite are 13 and 15 Å in the presence of PIPES, while a value of 12.8 Å was determined in the absence of PIPES. These data are interpreted in a model in which the adsorption of AMP is mediated by a Zn²⁺ complex of PIPES in different orientations in the interlamellar region of the montmorillonite. The type of exchangeable cation does not affect the ability of the lattice-bound Fe³⁺ in the montmorillonite to oxidize diaminomaleonitrile (DAMN). Exchangeable Cu²⁺ oxidizes DAMN, but exchangeable Fe³⁺ is nearly ineffective as an oxidant. The addition if DISN to 3′-AMP bound to Zn²⁺-montmorillonite in the presence of 0.2 M PIPES resulted in a higher yield of 2′, 3′-cAMP than is observed with a comparable concentration of Zn²⁺, a result which implicates surface catalystis by the montmorillonite.