Effects of Exchange Cations and Layer-Charge Location on Cysteine Retention by Smectites

Abstract

Abstract--This study investigates the complexes formed between amino acids, which are the natural degradation products of organic matter, and smectites. Thus, the adsorption and desorption behavior of cysteine and Na-, Ca-, Cu-homoionic smectites with different layer-charge location, a montmorillonite, and a beidellite, were studied. The clay samples were treated with Na, Ca, and Cu 1 N solutions and then with a 0.2 M cysteine solution. To test smectite-cysteine stability at acidic pH, the solids obtained were repeatedly treated with distilled water acidified to pH = 5. All treated samples were characterized by thermal, X-ray diffraction, chemical, and infrared analyses. The results showed that: 1) Na-and Carich smectites adsorbed and retained small amounts of cysteine, and did not show interlayer cationcysteine complexes, whereas the amino acid was strongly retained in the interlayer by Cu-rich smectites; 2) d(001)-values for Na-and Ca-rich smectites showed little or no expansion, whereas for the Cu-rich smectites the intercalation of the organic molecule produced a swelling of the structure; 3) the interaction mechanism of homoionic smectites with cysteine in an aqueous medium occurs by weak interactions, (e.g., van der Waals interactions, hydrogen bonding, dipole-dipole interactions, and other electrostatic forces such as entropy-driven hydrophobic bonding), and/or by complexes involving interlayer cations and organic ligands. The formation of a stable chelate complex with the saturating ion permits cysteine to be adsorbed by Cu(II)-rich smectites and to be resistant to migration in soils and groundwaters.

Infrared mapping resolves soft tissue preservation in 50 million year-old reptile skin

“…(b) Taphonomy of exceptional skin preservation Interestingly, experimental work shows that the cysteine -SH group may deprotonate and strongly adsorb to Cu 2þ substituted smectite minerals, thus decreasing compound mobility [34] and thereby preserve an original biochemical distribution. Cysteine (or thiol) is shown by both FTIR and XANES to be contained in the fossil skin, and XANES furthermore indicates that precursor disulphide (cystine) is also present.…”

“…Chelate complex formation may act to stabilize these organic compounds over time. An additional bonding mode for cysteine has also been documented, which does not involve deprotonation; in this mode cysteine adsorbs onto phyllosilicate basal plane surfaces while retaining the thiol group [34]. A combination of edge and basal plane adsorption modes for cysteine-terminated degradation products would result in: (i) stabilization of amide groups, (ii) preservation of amide, CH, sulphur and copper distributions, (iii) presence of a weak thiol infrared peak, and (iv) presence of a cysteine band in the XANES spectrum.…”

Preparation and characterization of hybrid materials consisting of high-energy ground montmorillonite and α-amino acids

“…The adsorption of amino acids on montmorillonite depends on several conditions, such as temperature, concentration of amino acids (Ramos and Huertas, 2013), exchangeable cations present in montmorillonite (Brigatti et al, 1999), pH (Benetoli et al, 2007), etc. The adsorbed amount of cysteine or glutamic acid on Na-montmorillonite does not exceed CEC values, even if excess of amino acid was used for the adsorption.…”

“…Many studies on the adsorption of amino acids on montmorillonite were published lately (Naidja and Huang, 1994;Brigatti et al, 1999;Benincasa et al, 2000;Kollár et al, 2003;Benetoli et al, 2007;Han et al, 2007;Parbhakar et al, 2007;Mallakpour and Dinari, 2011;Ramos and Huertas, 2013). The results suggest that the structure and properties of prepared organoclays are influenced also by the charge of the amino acid side-chain (R) and the structure of the used montmorillonite.…”

“…by using montmorillonite with different exchangeable cations -Cu 2+ , Ca 2+ , etc. (Brigatti et al, 1999) or acid-treated clay minerals (Rangel-Rivera et al, 2014). Another possible way is to use ground clay minerals obtained by high-energy ball milling (Tao et al, 2014).…”