# Natural Clays as Potential Amino Acids Carriers for Animal Nutrition Application
## 1 Introduction
The positive effects of dietary supplementation with clays, such as, for instance, the evidence of improved weight gain and feed conversion in pigs, can counteract the reported negative effects [12]. In these last years, growing attention has also been paid to the use of incorporated biological molecules and structural peptides in clay-based synthetic materials, not necessarily for feed application. The advantage is related to the synergic combination of the physical properties of the biological part and the synthetic domains [13]. For this purpose, among other biomolecules, \(\alpha\)-amino acids have been the most considered by synthetic organic chemists, because they are cheap and readily available optically pure. The use of these compounds in natural or synthetic polymer-based hybrid materials increases the biocompatibility of these systems. In this respect, examples are bioresorbable sutures, screws or plates, tissue engineering scaffolds, and drug delivery systems. Such nanohybrid materials can combine the structural and textural properties of the inorganic host with the functionality of the organic guest. For this application, the smectite group, including montmorillonite (MMT), is reported as the best choice, in view of its swelling capability and the cationic exchange capacity. For example, Na-Cloisite has been modified with L-alanine, L-valine, L-leucine, L-isoleucine, L-phenylalanine, and L-methionine by a cation exchange method to synthetize improved organoclays, greener and more environmentally friendly [13]. Na-Cloisite has also been reported as the matrix of choice to synthesize novel optically active organo-modified layered materials, by reaction of protonated L-methionine and the mineral clay through ion exchange [14]. The production of an optimized polymer-based nanocomposite, based on the methionine-montmorillonite system, has been described and discussed in the literature. The presence of the intercalated amino acid in the mineral nanoclay increased the clay dispersibility in the polymer matrix, thus improving the mechanical properties of the polymeric materials [15]. Moreover, the use of montmorillonite, modified with protonated L-methionine via a cation exchange process, has been proposed for fast and efficient adsorption of amoxicillin from aqueous solutions. The improved clay spacing has been reported to facilitate the amoxicillin adsorption process, and an enhanced adsorption capacity has been reported for the modified montmorillonite, compared to the pristine clay [16]. Finally, the clay-methionine system has also been reported recently for the effective adsorption of lead ions in aqueous solutions. A methionine-montmorillonite composite was successfully fabricated by L-methionine mixed with Na-montmorillonite. The improved adsorption capacity of the composite was correlated to the carbonyl oxygen atom of methionine which, due to the lone pair of electrons, is able to complex Pb\({}^{2+}\) ions [17]. Despite the studies reported above on the use of mineral clays for adsorption purposes, or on the use of methionine-clay system for synthetic purposes, nothing, to the authors' knowledge, has been reported on the use of bentonite as carrier for methionine protection and for methionine-controlled delivery in feed applications. Moreover, nothing is reported about the intercalation process of methionine in its neutral form. In the past, the possibility to intercalate organic molecules, without ion exchange, in montmorillonite natural clays has been investigated. Polyethylene-glycols (PEG), characterized by different molecular weight 1500 and 4000, as well as (PA), characterized by different length and structures, representative of large organic molecules, were intercalated without exchange in two different standard Ca- and Na-montmorillonite mineral clays.