Adsorption of l-lysine on montmorillonite

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Abstract

Amino acid adsorption on smectite is relevant to prebiotic processes involving possible catalytic reactions in the early Solar System, as implied by the clay-organic correlation found in meteorites, and the generation and modification of organic components essential for the origin of life. Here we report the results of a study investigating the adsorption of l-lysine (0.025–0.4 M) onto montmorillonite. The reaction products were studied using X-ray diffraction, chemical analysis and infrared spectroscopy. We find that lysine adsorption is first dominated by cation exchange and then by adsorption of electrically neutral lysine (as a zwitterion), as indicated by chemical and FTIR evidence. At the maximum concentration, lysine displaces only ∼1/3 of the original interlayer cations. The d-spacing of the smectite–lysine complex increases from 1.2 to 2.1 nm as more lysine enters the interlayer space, and water is expelled. We propose a structural model of lysine sorption in the interlayer in which lysine is oriented at >45–90° to the plane of siloxane O atoms.

Introduction

Life on Earth arose sometime before 3.5 Ga, and was undoubtedly preceded by a period of chemical evolution during which simple organic molecules were transformed into more complex and ordered forms (e.g. [1]). How this chemical–physical evolution proceeded is the subject of great debate and reconstructing how simple, inanimate organic molecules may have been transformed into the raw materials of life is an important goal of Earth and Biological Sciences, as well as of Astrobiology [2], [3], [4], [5].

In an early Earth scenario, clay minerals are considered to provide one of the most likely substrates where organic matter could have been concentrated and possibly transformed by abiogenic catalytic reactions to polymeric organic networks [6], [7], [8] that were the forerunners of biopolymers. Similarly, meteorites contain organic matter in strong association with clay minerals [9]. This relationship supports suggestions that clay minerals may have had an important sequestering and possibly catalytic role in the organic chemical evolution in the early Solar System.

Among the several types of organic compounds of fundamental importance for life, it is perhaps the amino acids that have attracted the most attention because they are the building blocks of proteins. Abiogenic organic matter, and the amino acids in particular, is dominated by monomers and not the polymeric forms found in terrestrial biology. It is therefore likely that the initial adsorption and possible clay–amino acid reactions took place with the monomeric forms. Thus, it is important to characterise amino acid–clay mineral interactions [2], [10].

There have been numerous previous studies involving amino acid adsorption on clay minerals [11]. However, many of them are not directly relevant to the present study as they address problems different from the amino acid mode of adsorption on the clay, which is the focus of the present study. The results from previous work are variable, depending on the amino acid, the clay and the experimental conditions (e.g., pH; [12]). Even if montmorillonite only is considered, the results depend on the interlayer cations present (e.g. [13], [14]). However, over and above the specific experimental differences, the authors have recognized a similar group of processes controlling amino acid adsorption on clay surfaces, namely, cation exchange, amino acid dipole interaction with the interlayer cation and the charged clay surface, hydrogen bonding and physical forces [6], [14]. The most important control on the type or types of force operating is the net electrical charge of the amino acid, which depends on the pH in both the bulk solution and in the proximity of the clay layer or the interlayer space. In the case of montmorillonite, where most of the adsorbed amino acid is in the interlayer space, the action of the above forces provide some indications of the possible structure of the amino acid–clay complex. X-ray diffraction analysis allows one to measure the distance between montmorillonite 2:1 layers and thus to constrain the orientation of the amino acid molecule in the interlayer space [15]. Infrared analysis allows one to determine the protonation state of the amino acid functional groups (i.e., single bondNH2 versus single bondNH3+, or single bondCOO versus single bondCOOH), thus providing information on the interactions between the interlayer water, cations and amino acid molecules [12].

In this paper, we present new data that constrain the extent and mechanisms of the adsorption of the amino acid l-lysine on montmorillonite. The results will provide further insight into amino acid retention and modification at the montmorillonite interlayer space.

Section snippets

Preparation of smectite and l-lysine solutions

Wyoming bentonite (SWy-1) was obtained from the Clay Minerals Society, USA, which contains mainly Na in the interlayer. Calcite was removed using 0.1 M HCl (Aristar grade, Merck), which was added to a clay suspension until pH 4 was reached. The suspension was stirred during calcite dissolution and the pH was monitored continuously and maintained at 4 by adding 0.1 M HCl as needed. The process was continued until no pH increase was observed, indicating complete removal of calcite. After calcite

Results

Total C and N determination revealed that the C data were the most reliable indicators of l-lysine adsorbed by the montmorillonite. Fig. 1(a) shows the amount of lysine adsorbed to montmorillonite as a function of the equilibrium solution concentration along with the amount of displaced interlayer cations. The plot has similar shape and relative adsorption values to others published for other amino acids [12], [17]. Our data suggest that the adsorption maximum has not been reached with the

Discussion

Fig. 1 suggests that there are two mechanisms of adsorption as lysine concentration increases in solution. The first mechanism operates only for the most dilute concentration, 0.025 M, and seems to consist of a cation exchange in the interlayer [23]. The second operates at the higher lysine concentrations and is not a cation exchange reaction. For the cation exchange at 0.025 M, lysine should have a net positive charge as it enters the interlayer. There are three functional groups in lysine that

Relevance to prebiotic conditions

Our study shows that montmorillonite can adsorb large amounts of l-lysine if the concentration in solution is sufficiently high. Given that all α-amino acids are amphoteric, possessing both basic (single bondNH2) and acidic (single bondCOOH) groups which dominate amino acid behaviour at physiological pH in biogeochemical systems, one can reasonably assume that this broad behaviour extends to other amino acids. This generalisation is particularly relevant to the amino acids arginine and histidine, both of which

Acknowledgments

The authors thank Imperial College London and the Natural History Museum, London, for financial support. The help of Andreas Morlock with the FTIR analysis and of Sarah James with various aspects of the experiments is gratefully acknowledged. We also thank two anonymous referees for insightful comments.

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