Impact of amino acids on the isomerization of the aluminum tridecamer Al13

The stability of the Keggin polycation epsilon-Al13 is monitored by 27Al NMR and ferron colorimetric assay upon heating aluminum aqueous solutions containing different amino acids with overall positive, negative or no charge at pH 4.2. A focus on the effect of the amino acids on the isomerization process from epsilon- to delta-Al13 is made, compared and discussed as a function of the type of organic additive. Amino acids such as glycine and beta-alanine, with only one functional group interacting relatively strongly with aluminum polycations, accelerate isomerization in a concentration-dependent manner. The effect of this class of amino acids is also found increasing with the pKa of their carboxylic acid moiety, from a low impact from proline, up to more than a fifteen-fold increased rate from the stronger binders such as glycine or beta-alanine. Amino acids with relatively low C-terminal pKa, but bearing additional potential binding moieties, such as free alcohol (hydroxyl group) moiety of serine or the amide of glutamine, speed up the isomerization comparatively and even more than glycine or beta-alanine, glutamine leading to the fastest rates observed so far. With aspartic and glutamic acids, changes in aluminum speciation are faster and significant even at room temperature, but rather related to the reorganization towards slow reacting complexed oligomers than to the Al13 isomerization process. The linear relation between the apparent rate constant of isomerization and the additive concentration points to a first order process with respect to the additives. Most likely, the dominant process is an accelerated epsilon-Al13 dissociation, increasing the probability of delta isomer formation.

Jaar van publicatie:2017

BOF-keylabel:ja

IOF-keylabel:ja

BOF-publication weight:3

CSS-citation score:1

Auteurs:International

Authors from:Higher Education

Toegankelijkheid:Closed