The polymerization of amino acid adenylates on sodium-montmorillonite with preadsorbed polypeptides

We studied the spontaneous polymerization of amino acid adenylates on Na-montmorillonite in dilute, neutral suspension, after polypeptides were adsorbed on the clay. This led to the unexpected finding that the degrees of polymerization (DP's) of the oligo- and poly-peptides obtained depended on the amounts of polypeptides that were preadsorbed. Plotting average molecular weights obtained against c-spacings of the clay platelet aggregates which widened as a result of polypeptide addition and adsorption before the polymerization, does not permit an obvious explanation of these observations. The best correlation assigns a role to the fractional occupation of the individual intercalation layers of the polypeptides, as the adsorption increases towards a first complete mono-interlayer, then to an incipient and eventually to a complete double layer on to a third interlayer, after which the clay stacking breaks up. Spacings which correspond to an intermediate occupation of any of the three successive interlayers favor amino acids self-addition to polymers. The opposite is true for nearly empty or filled intercalation layers. We hypothesize and describe, how a catalytic activity may derive from c-spacings that offer adsorption sites for the reagent amino acid adenylate within the peripheral recesses of irregularly stacked clay platelets by bringing the anhydride bonds and neutral amino groups into favorable reaction distances. Moderately filled intercalation spaces may also act as sinks for the newly formed oligomers and facilitate the freeing of reaction sites for the occupation by fresh reagent. The c-spacings required for these mechanisms are the result of the intercalation of the preadsorbed polymer, but similar conditions prevail when polymers are adsorbed as they are generated during polymerization.