Introduction

The origin of life is generally presumed to require a local concentration of life-forming biomolecules (e.g. amino acids, nucleic acid bases, phosphate and sugars etc.), which were either supplied abundantly or sufficiently stable to accumulate under primitive Earth conditions. Prebiotic syntheses of biomolecules from chemical precursors often suffer from low yields (Lucrezia et al. 2007; Orgel 2004; Lambert 2008; Shapiro 1999), thus the steady-state concentrations of amino acids (AAs) in the primitive oceans has been estimated to be on the order of 4 to 10−7 mM (Lahav and Chang 1976; Stribling and Miller 1987).The stability of these molecules under the conditions (viz. ultra-violet, thermal, electric discharge etc.) prevalent on Earth when life started ~4.4–3.5 Ga ago may also have been an impediment to their accumulation (Cleaves et al. 2012).

Bernal (1951) suggested a possible solution to these problems and postulated concentration of biomolecules on mineral surfaces like metal oxides. Metal oxides and silicates are widely distributed on the Earth’s surface and also reported on another planet like Mars (Stephenson et al. 2013; Carr and Head 2010). Bentonite (BNT) and montmorillonite (MMT) clays, well known for their cation exchange capacities, had been widely investigated at different conditions of pH, temperature and concentrations to explore Bernal’s ideas regarding their potential roles as concentrating surfaces and catalysts for polymerization (Zaia 2012; Lambert 2008 and references therein it). The sorption behavior of SMT surfaces is primarily governed by variables such as the interlayer conditions, presence of cations, water, etc. (Prabhakar et al. 2007).

Metal ions are known for their affinity to form peptide bonds in relevance to the prebiotic evolution of biomolecules (Rode et al. 1993; Pant et al 2009; Gururani et al. 2012a; Remko and Rode 2000). Metal ion-exchanged SMTs are good adsorbents for amino acids, nucleic acid bases, sugars etc., and also catalyst for biomolecule formation (Lambert 2008; Kalra et al. 2003; Gururani et al. 2012b). A detailed mechanism behind prebiotic mineral-organic interfacial processes and its potential implications for the origins of life have been reviewed by Cleaves et al. (2012).

Recently, the role of metal ions in chemical evolution has been bolstered with advances in computational chemistry (Remko and Rode 2000; Rode et al. 1993: Merz et al. 2014; Cleaves et al. 2012). The possible interaction of metal ions with biomolecules is often investigated with the help of high-pressure mass spectrometry along with guided ion beam mass spectrometry (Kebarle and Hogg 1965; Rodgers and Armentrout 2000). However, quantum mechanical calculations give better insight into their possible complex formation mechanism, solvation and counter ions effects of the different complexes (Marino et al. 2001). Hydrated biomolecular complexes serve as simpler models for more complex protein─metal systems in an aqueous environment (Remko et al. 2011).

Among other prebiotically formed AAs, Leu has been reported from exogenous (Pizzarello 2009) as well as endogenous (Zaia 2012) sources. However, to the best of our knowledge, a systemic experimental study of the surface interactions of Leu on Ca2+-, Mg2+- and Cu2+- exchanged SMTs (BNT and MMT), separately, has not been realized yet. In this communication, we investigate the adsorption behavior of different metal ion exchanged SMTs because such studies related to the biomolecular interaction on prebiotically available solid surface and their affinity towards silicates, are relevant to astrobiology (Lambert 2008; Cleaves et al. 2012). The stability of the adsorbate under different experimental conditions is also a major concern for astrobiological investigation as silicates are also known for their catalytic role in polymerization and decomposition of biomolecules (Zaia 2012; Marshall-Bowman et al. 2010; Ferris et al. 1996). Thus, the stability of Leu was also investigated under different sorption conditions.

Computational studies of metal-amino acid complexes (Leu—M2+) were conducted using Gaussian 03 (Frisch et al. 2003) to understand the affinity of metal ions with biomolecules and the effect of hydration on Leu—M2+ × (H2O)n, (n =2 and 4) complex formation, which has not been studied yet theoretically. The details of local interactions between metal ions and AAs are of prime importance in living systems (Marino et al. 2001).

Experimental

All reagents were of analytical grade. BNT and MNT K10 clays were purchased from E. Merck, India and Sigma Aldrich, Germany respectively, while L-leucine (Leu), H2O2, MgCl2, CuCl2 were from Qualigens, India and CaCl2 was obtained from CDH India. Triple distilled-deionized water (pH 6.8 ± 0.1) was used throughout the study. All containers, measuring equipment and assemblies were of borosilicate glass. A JascoV-550, UV/VIS/NIR spectrophotometer was used for the absorbance measurements. The pH was recorded on Hanna pH meter (model pHep, accuracy: ±0.1 pH unit).

The sequential steps involved in the preparation/purification of adsorbents during analysis are shown in scheme I, and described by Prabhakar et al. 2007 and Kalra et al. 2003. Ca2+, Cu2+ and Mg2+ exchanged BNT/MMT were prepared by the saturation method as mentioned earlier (Kalra et al. 2003). All the adsorbents (Scheme I) were ground, sieved using a 100 mesh size sieve (SETHI BSS 100) and then dried in vacuum desiccators. The color of the respective SMTs have been found to be 10R8/1 (white) and 7.5YR7/2 (light brown) on Munsell Notations, for MMT and BNT respectively.

Scheme I
scheme 1

Graphical representation of different steps involved in the preparation/purification of adsorbents and different analysis

Full size image

The purity of adsorbents (BNT/MMT and their divalent cation-exchanged forms) were determined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) using a Hitachi S-3700 N Instrument using an acceleration voltage 15.0 kV with a take-off angle 44.1°,and a ZEISS EHT 20 kV instrument. Purity was further checked by X-ray diffraction (relative intensity and inter-planar spacing) data, which were in good agreement with reported values (Caglar et al. 2009). X-ray powder diffraction analysis of adsorbent and adsorption adducts were performed on a Thermal X-TRA ARL diffractometer equipped with a ‘Peltier’ detector, using Cu Kα radiation (wavelength = 1.5406 Å) operated at 30 mA and 40 kV and graphite monochromator detection between range 0-40° 2θ angle. Vibrational studies were carried out using a PerkinElmer DX-II FT-IR spectrophotometer equipped with DTGS detector. The IR spectra were collected at 4 cm−1 resolution and 100 scan with frequency range from 400 to 4000 cm−1.

The adsorption of Leu on different adsorbents in aqueous medium was studied as function of pH (4.0–9.2), time (1 min – 72 h) and concentration (1 mM to 10 mM) of adsorbate. Adsorption of Leu in varying concentration over different pH was studied in order to obtain saturation point by adding acetate-acetic acid (pH 4.0–6.0) or borax-boric acid (pH 6.6–9.2) buffer to the AA solution containing adsorbents, keeping in mind that the buffer solution should be a very poor ligand so that stable complex formation with the mineral could be avoided and this was confirmed by using different buffer concentration on Leu solution.

Buffer solution of Leu (5 ml) at different pH values was added to the adsorbents (50 mg) in separate conical flasks (10 ml) under air, stirred mechanically using magnetic stirrer and allowed to stand at room temperature (298 K) for different time intervals to determine the optimum conditions for adsorption. The solutions were then centrifuged at 2000 rpm for 25 min. The supernatant was decanted leaving the SMT as a residue. The supernatant in each experiment was used for quantitative estimation of AA content after adsorption and the SMT residue was separated, washed (3 times) and then, dried at ambient temperature under vacuum. Further, it was ground/ sieved and used for SEM/EDS, FTIR and XRD studies (Scheme I). The concentration of adsorbate before and after adsorption was determined using UV difference spectra (at 201 nm) in quartz cuvettes with a 1 cm path length using tdH2O as reference. Same amount of respective buffer had also been added into the tdH2O which was used as reference.

The amount of AA adsorbed under different conditions of pH, time and concentration was calculated from the difference between the initial AA concentration and the concentration after adsorption in each case. The initial concentration of AA and the quantity adsorbed were used to obtain the adsorption isotherms (Pandey et al. 2013). Percentage binding was calculated as follows-

$$ \%\; binding=\frac{C_e-{C}_f}{C_e}\times 100 $$
(1)

Where, Ce is the initial concentration of adsorbate and Cf is the final concentration after adsorption. The theoretical limit of the concentration of adsorbate (Leu) available for adsorption is given (Fig. 1a).

Fig. 1
figure 1

Graph showing, (a) theoretical limit of availability of Leu for complete adsorption by 50 mg of adsorbents, (b) adsorption behaviour as a function of quantity adsorbed versus equilibrium molar concentration, and (c) Langmuir adsorption isotherm for different adsorbents for equilibrium time 8 h and pH 6.0

Full size image

Chromatographic techniques (Paper chromatography and HPLC) were used to measure the degradation products (if any) of Leu after adsorption studies as described in our previous work (Pandey et al. 2013).

Computational Study

Density functional theory (DFT) was used to investigate the interaction of Leu with metal cations (Ca2+, Mg2+ and Cu2+). Becke’s three parameter exchange functional with a Lee, Yang and Parr type nonlocal correlation functional hybrid Hartee-Fock DFT method (B3LYP),was applied for metal ions and Leu with the all-electron 6-31G+(dp) internal basic set in Gaussian 03w (Becke 1993; Lee et al. 1988; Hohenberg and Kohn 1964). All the geometries were optimized and frequency calculations were used to cross-check the global minima of all the chemical species under study, where no imaginary frequencies were observed. Thermodynamic parameters are also obtained through frequency calculation. Open shell calculations using a spin-unrestricted formalism have been used for Leu—Cu2+complex as Cu2+ is an open shell system with a d9(2D) ground state (Remko et al. 2011). All the computational analysis were carried out with Intel(R) Core (TM) i5-3230 M CPU@2.60GHz PC on Windows-7 operating system. The formation of Leu—M2+ × (H2O)n, (n = 2 and 4) complexes can be described following, which has been used to compute the metal ion affinity as a negative enthalpy variation (ΔH) for the reaction-

$$ \begin{array}{l}Leu + {\mathrm{M}}^{2+}+{\left({H}_2O\right)}_n\to Leu\hbox{---} {M}^{2+}\times \left({H}_2O\right)\hfill \\ {}M=M{g}^{2+},\ C{a}^{2+} and\ C{u}^{2+},\ n=0,\kern0.24em 2\ and\ 4.\hfill \end{array} $$
(2)

Results and Discussion

The adsorption of Leu on MMT/BNT and cation exchanged MMT/BNT, in aqueous medium was studied as a function of pH (4.0–9.2), temperature (298 K) and concentration (1 mM – 10 mM) of the adsorbate in order to find out the conditions of maximum adsorption. Moreover, the main aim is to study the role of divalent cation (Cu2+, Mg2+ and Ca2+) exchanged SMT as an adsorbent under prebiotic conditions.

The amount of adsorbate was determined from the standard concentration curve obtained from the optical density of the different Leu solutions monitored at their respective peak (λmax) of absorption spectrum. Linear behaviour (r2 = 0.97) was observed between the absorbance and concentration of Leu (Supplementary information S.1). Moreover, Leu adsorption was confirmed using EDS which showed the presence of C and N in addition to the elements present in the SMT.

The optimum conditions of surface interaction with respect to time and pH showed that the amount adsorbed depends on the contact time, and pH, where adsorption equilibrium is attained within 8 h (Supplementary information S.2). The optimum pH was found to be 6.0 which is also, isoelectric point of Leu (Supplementary information S.3).

The theoretical limit of the availability of total adsorbate (Leu) in 5 ml for 100 % adsorption by the 50 mg of SMT is shown in Fig. 1a. The adsorption isotherms (plot between equilibrium concentrations (Ceq) versus quantity adsorbed) are provided in Fig. 1b. The initial portion of the isotherm (From 1 to 6 mM of Leu) represents a linear relationship between the quantity adsorbed and Ceq of adsorbate, while at higher concentration (6 mM─10 mM), the isotherm saturates and adsorption becomes independent of Leu concentration. Langmuir type adsorption is observed (Fig. 1c) assuming the formation of a monolayer of adsorbate molecules on the surface of the adsorbent. The Langmuir parameters were determined form the equation below (Pandey et al. 2013).

$$ \frac{C_{eq}}{X_e}=\frac{1}{K_L.{X}_m}+\frac{C_{eq}}{X_m} $$
(3)

Where Ceq is the equilibrium concentration of Leu.KL is a constant which is a function of the adsorption energy i.e. the enthalpy of adsorption coefficient (KL = Ae–ΔH/RT); Xe is amount of amino acid (mg) adsorbed per gram of SMT;Xm is the amount of Leu required for a definite weight of SMT for complete surface coverage.

The adsorption parameters (Xm and KL) were calculated from the slope and intercept obtained from the graph of Ceq/Xe versus Ceq (Fig. 1c and Eq. 3). The goodness of linear fit of the Langmuir adsorption isotherm (Fig. 1c) was judged by the linear regression coefficients (r2) which were ~0.99 (Table 1). Adsorption data were collected in triplicate and ~3.0─7.8 % error was observed in this study. Mean values are used to obtain isotherm (Fig. 1b, c).

Table 1 Percentage binding and Langmuir isotherm parameters of Leu on the surface of different adsorbents
Full size table

Percentage binding of Leu on the two SMT is shown in Table 1, using eq. 1. Leu is ~24 % adsorbed on BNT while ~34 % is adsorbed for MMT, and same trend of the percentage adsorption with the divalent exchanged SMT has also been observed. Cu2+ exchanged SMT enhance the adsorbing ability of the both SMT (i.e. ~39 and ~26 % for MMT-Cu2+ and BNT-Cu2+ respectively) while in both the cases for Mg2+ exchanged SMT has apparently decreased ( ~24 and ~34 % respectively) the affinity of Leu for SMT(Table 1). SEM images of BNT-Cu2+ and its adduct with Leu are shown Fig. 2, clearly indicating the surface morphology of the BNT-Cu2+, as Leu has finely covered its surface after adsorption (Fig. 2b).

Fig. 2
figure 2

SEM images of BNT-Cu2+ (a), and (b) adsorption adduct of Leu with BNT-Cu2+

Full size image

Vibrational studies using FTIR were used to analyse the possible sites of interaction. FTIR spectra of Leu, MMT and Leu─MMT complex are provided in Fig. 3. The characteristic infrared spectral frequencies have been summarized in supplementary material (S.t.1).The bands at ~1035 cm−1and ~3622 cm−1corresponds to Si─O─Si stretching vibration and ─OH stretching, respectively, while 3431 cm−1 and 1638 cm−1 are from the stretching and bending vibration of adsorbed water in MMT (Fig. 3b). In the case of Leu, no band corresponds to ─NH2 bending and absence of protonated ─COOH (i.e. ─OH stretching at ~3500 cm−1, ─C = O stretching at ~1700 − 1780 cm-1, ─OH bending at ~1200–1300 cm-1), confirming amino group is protonated (—NH3 +) while carboxylic group is deprotonated (—COO−) (i.e. zwitterion) (Fig. 3a). Polymerization of Leu is not found in starting adsorbate as well as adsorption adduct as no peak correspond to amide (1640 cm−1 and 1550 cm-1) is found (Fig. 3a, c).

Fig. 3
figure 3

IR spectra of (a) Leu, (b) MMT, and (c) adsorption adduct, Leu + MMT after adsorption at pH 6.0 ± 0.1

Full size image

A band appears at 3073.17 cm−1 which is assigned to an leu-NH3 +stretch (Fig. 3a) which changes to 3121.95 cm−1 after adsorption. Further, shift in the frequencies (i.e.—COO− symmetric stretching, —COO− anti-symmetric stretching) also suggest possible interaction via the COO− moiety of Leu. A lower shift in the vibrational frequencies in adsorption adduct (Leu + MMT) also indicates, weak forces are applied in the course of surface interaction (Stevens and Anderson 1996).

A proton sandwiched between two electronegative atoms (i.e. N of —NH3 +and O of ─Si─O− and ─Al─O− of SMT) after adsorption require more energy to vibrate and thus, a shift in the hydrogen bond donor (—NH3 +) as well as acceptor (—COO−) sites of the zwitterion of AAs after adsorption on mineral surface found to be the crucial forces to stabilize AAs on SMT (Fig. 3 and S.t.1). The protonated amino group is an excellent hydrogen bond donor (Kollar et al. 2003; Fuchida et al. 2014; Marsh and Donohue 1967). The XRD of adsorption adducts have also, shown a shift in the basal spacing (001). The basal spacing of MMT is 15.56 Å which increases upto 17.18 Å after adsorption (Supplementary material S.4-S.6). The intercalation of AAs between the layers of SMT during the adsorption is more prominent than interaction on surfaces and edges of the SMT (Friebele et al. 1980) and an increment in the basal spacing (001) indicates towards the interlayer adsorption where AAs occupy the interlayer space of SMTs because AAs adsorbed on the surface may easily desorbs when SMTs are washed with tdH20, prior to the FTIR and XRD analysis, due to the low activation energy of desorption (Lambert 2008). This interlayer acquisition of AAs in SMT may occur via cation exchange (Greenland et al. 1965) which has recently been shown by Fuchida et al. 2014.

Considering the SMT surface, previous reports have also shown surface interactions are largely dependant on interlayer cations, hydrogen bonding, physical forces and the clay’s surface charge. (Ponnamperuma et al. 1982; Greenland et al. 1965) while net electric charge of the AAs, which depend on the pH in both the bulk solution and in the proximity of the clay layer or interlayer spaces, is another factor deciding adsorption (Prabhakar et al. 2007). Further, no decomposition or oligomerization of Leu was observed in our experiments.

Computational study

Complexation modes between M2+ and Leu with attractive interactions between electron rich (─COO−) and electron deficient sites (M2+) have been considered for zwitterionic amino acids as reported by Remko et al. (2011) and Marino et al.(2001).The respective atoms of zwitterionic Leu are numbered (not IUPAC) according to the scheme presented in Fig. 4a, and optimized geometric parameters of the Leu considered for theoretical studies along with hydrated or non-hydrated metal complexes Leu—M2+ × (H2O)n,(where n = 0, 2 or 4) are provided in supplementary material S.t.2-S.t.4. All of the output files can also be made available by contacting the corresponding author directly. For the hydrated system Leu—M2+, metal ions are coordinated with two water molecules (Figs. 5a, c and e) using a supermolecule modelling of hydration while other two water molecules were placed near the polar part of the AA (Figs. 5 b, d and f), where the water molecules form intermolecular hydrogen bonds with the proton donor -NH3 + group (Remko et al. 2011).

Fig. 4
figure 4

Chemical structures of (a) Leu with numbering, and (b) B3LYP/631 + G(d,p) optimized distances (Å) for different bicoordinated Leu—M2+ complexes with Mg2+, Ca2+ and Cu2+ reported without enclosures, within parentheses, and with square brackets, respectively

Full size image
Fig. 5
figure 5

Overall structure of the B3LYP/631 + G(d,p) optimized complexes of Leu with Mg2+ (a and b), Ca2+ (c and d) and Cu2+ (e and f) in presence of water molecules (n = 2 or 4)

Full size image

The bond lengths, bond angles and dihedral angles of the computed Leu—M2+complexesaregiven in Table 2, while calculated net charges on the metal, oxygen atoms, nitrogen atoms, dipole moment, total energies, relative energies including ZPE and thermodynamic parameters of nine different complexes of Leu—M2+ × (H2O)n(M = Cu2+, Mg2+, Ca2+ and n = 0, 2 and 4) are summarized in Tables 3 and 4.

Table 2 B3LYP/6-31 + G(d,p) optimized relevant bond lengths (Å), bond angles (deg) and dihedral angles (deg) for Leu—M2+ complex
Full size table
Table 3 B3LYP/6-31 + G(dp) calculated net charge of metal (q M+), two oxygen (q O(3) and q O(4)), nitrogen (q N(5)) in |e|, dipole moments (μ in Debye) and amount of metal ion charge transfer (CT) for different Leu—M2+ × (H2O)n, (n = 0, 2 and 4) systems calculated at 298.15 K and 1 atm. pressure
Full size table
Table 4 B3LYP/631 + G(d,p) calculated absolute energy (E in au), energy change (ΔE in k.cal/mol), metal ion affinities (ΔH in k.cal/mol), Gibbs free energies (ΔG in k.cal/mol) and entropies (ΔS in cal/mol.K) for different Leu—M2+ × (H2O)n, (n = 0, 2 and 4) systems calculated at 298.15 K and 1 atm pressure
Full size table

Preliminary computations of Leu suggest it forms stable complexes with divalent metal cations that may form in two different ways. First, binding of M2+ with the O3 of carboxylic acid group and the N5 of amine in neutral AA while, another, its interaction with both the oxygen (O3 and O4 ends) of the carboxylate ion of the zwitterionic form of the AA. The bicoordinated complexes of M2+via simultaneous interactions of carbonyl oxygen and amine nitrogen is not possible as the lone pair of electron have already been associated with the proton (H+) liberated from –COOH group in forming the zwitterion. The zwitterionic form of the AA may be stabilized by the presence of counter-ions and/or water molecules (Remko et al. 2011; Bush et al 2008). Further, harmonic vibrational frequencies of the optimized species have shown that all nine complexes are in their minimum energy state as no imaginary frequencies have been observed in their respective frequency calculations using the level of quantum theory as described in experimental section.

The selected geometrical changes for the Leu—M2+ complexes indicate that the O(4) ....M(9) bonds are always shorter (0.025 Å, 0.041 Å and 0.032 Å for Mg2+, Ca2+ and Cu2+ respectively) than O(3)....M(9) bonds, indicating that these metal ions form stronger bonds with O(4) which are farther away from the ammonium group (─NH3 +) of Leu. The equilibrium distance of O(4)....M(9) and O(3)....M(9) increases in the order: Cu2+ < Mg2+ < Ca2+ (Fig. 4b). The M2+ in both the non-hydrated (Leu—M2+) as well as hydrated (Leu—M2+ × (H2O)n, n = 2 or 4) lie in the same plane of carboxylate groups of Leu (Fig. 5).

Table 4 summarizes the different energies and thermodynamic parameters which reflect that the stability of Leu-M2+can be substantially modified by the specific metal ion and hydration. Energy parameters were calculated using eq. 2 and all nine complexes studied were found to form exothermically (Table 4). Leu—Cu2+was found to be the most stable complex while its hydration further increases its stability with a maximum enthalpy of -386.28 kcal/mol for Leu—Cu2+ × (H2O)4, among all nine complexes studied(Table 4). Metal ion affinities as a negative enthalpy variation (ΔH) for Leu—M2+complexes has found minimum with Ca2+(Table 4). Gibbs energies, which measure the tendency to associate in real molecular complexes, are found to be negative and have energies from -114.27 to -342.61 kcal/mol (Table 4). The energetic differentiation of all the complexes studied based on the type of different metal ions (Mg2+, Ca2+ and Cu2+) with Leu, a decreasing order of Gibbs energy is Leu—Cu2+ > Leu—Mg2+ > Leu—Ca2+has been observed. The ΔG for the non-hydrated leu-M2+complexes were calculated to be -173.09 k.cal/mol, -114.27 k.cal/mol and -251.34 k.cal/mol for Mg2+, Ca2+, and Cu2+ complexes respectively. To analyze the affinity of hydration with the Leu—M2+ complexes, first two water molecules were coordinated to M2+ ions (Figs. 5a, c and e), which lowered the ΔG values for the complexes with Mg2+, Ca2+, and Cu2+ by-79.85 k.cal/mol, -56.54 k.cal/mol and -72.14 k.cal/mol respectively (Table 4). Addition of another two water molecules to give an Leu—M2+ × (H2O)4 complex further lowered the ΔG values by -18.31 k.cal/mol (Fig. 5b), -16.87 k.cal/mol (Fig. 5d) and -19.13 k.cal/mol (Fig. 5f) for Mg2+, Ca2+, and Cu2+ complexes respectively.

The net atomic charges on the different atoms of the complexes studied, have been investigated by the natural population analysis using the NBO program (Reed and Weinhold 1985; Reed et al. 1985; Glendening et al. NBO program, Version 3.1). The amounts of charge transfer (CT) for the M2+ along with dipole moments are summarized in Table 3. The stronger bond formation between O4....M2+of different Leu—M2+ × (H2O)n complexes can further be analyzed through the charges on O4 along with O4....M2+distance (Å) in comparison to charge and bond distance (Å) of O3 and O3....M2+ respectively. The charge on O4 of different complexes is always in lower side than O3 which suggest the transfer of more of it charge towards M2+ from this end contrary to O3. The magnitude of CT contribution in Leu—Ca2+is much lower (0.076 to 0.098e) for its non-hydrated as well as hydrated complexes while CT for all Leu—Cu2+ complexes are maximum from 0.559 to 0.668e (Table 3). The magnitudes of charge transfer on the metal ions studied are in good agreement with their metal ion affinities (Table 3 and 4). The interactions of glycine with 15 different metals at theoretical level, also showed the magnitude of CT correlates well with metal ion interaction energies (Hoyau et al. 2001).

Looking at the bigger picture, the experimentally studied adsorption processes of Leu on SMT and their Cu2+ and Ca2+ ion exchanged counterparts, essentially represents the enhanced affinity of biomolecules on metal-exchanged SMT, which have been suggested to have played a role during chemical evolution and the origin of life (Lambert 2008). The present study shows significant affinity of non-polar amino acid Leu with M2+ ion exchanged SMT surfaces, specially Cu2+ exchanged SMT, which might be helpful to give insight into the higher percentage of neutral and non-polar AAs in proteins of present day living systems as modern proteins comprise of total 74 % non polar amino acids (Zaia 2004). Metal exchanged minerals could possibly help to accumulate the non-polar amino acids via adsorption on their surfaces which may, facilitate their polymerization to get complex proteins. Further, this study may help to understand the specific sites of attachments of amino acids, peptides or proteins onto different field of adsorption (peptide sequencing identification, nano technology, drug targeting/delivery and chemical evolution) (Roman et al. 2006) and the physico-chemical parameters (i.e. change in bond length, bond angles, electron density, charge and thermodynamic parameters) of M2+─AAs complexes may also help in better understanding of the biochemistry of living systems and also serve as a simpler model for the AAs—M2+ interaction found in the more complex protein, enzyme-metal systems related to living systems such as Cu+ coordinated with peptides (viz. glycylglycine) has been reported to stabilize this peptide structure (Constantino et al. 2005).

Conclusion

The interaction of the Leu on BNT/MMT and its divalent metal ion (Mg2+, Ca2+ and Cu2+) exchanged forms was investigated with respect to the optimal conditions of pH, time and concentration at 298 K. Adsorption parameters such as percentage binding show that Leu is adsorbed more on the MMT than BNT. Further, Cu2+ exchanged BNT/MMT adsorbs Leu the most strongly, while Mg2+exchanged SMTs showed the weakest sorption. Experimentally, interaction of biomolecules with metal exchanged clays and theoretically determined physico-chemical characteristics of different metal ion-biomolecular interactions, may shed light on the prebiotic formation of macromolecules and their significance in living systems such as affinity of Cu2+ for the biomolecular selectivity for the enhancement of peptide formation (Sakata et al. 2014; Constantino et al. 2005). A better surface interaction of AA in presence of Cu2+ incorporated minerals might possibly enhance the proximity effect of functional groups to form peptides. Moreover, it may be concluded that in prebiotic era of the earth adsorption processes involving metal ions might have played a key role for local concentration of biomolecules and thus facilitated further condensation and polymerization on their surfaces into more complex molecules essential for life.