Interaction of Aromatic Amines with Iron Oxides: Implications for Prebiotic Chemistry - Discover Life

Iron oxides are present in almost all the different compartment of global system, including the pedosphere, biosphere, lithosphere, and hydrosphere (Cornell and Schwertmann 2003). The large surface area of goethite (57.40 m²/g) and akaganeite (27.37 m²/g), classify them as strong adsorbents for biomonomers and other species.

The dominant theory of Earth’s earliest atmosphere (Urey 1952) until quite recently predicted that CH₄, NH₃ and H₂ were initially common constituents and consequently that the first phases of chemical evolution and origin of life took place in a highly reducing environment. The general view of the early atmosphere has shifted from a reducing one to a redox ‘neutral’ or slightly oxidizing one (Levine et al. 1982; Towe 1990). A reducing atmosphere would, however, allow the existence of fewer catalytically active minerals on the Earth’s surface, primarily silicates and sulfides, than would a redox neutral one Holland 1984). A more oxidizing early atmosphere increases the chances for the existence of oxidized iron minerals like FeOOH and perhaps even manganates. Braterman and coworkers (1983) showed that the Archean Banded Iron-Formations (BIFs) may have formed in such an atmosphere due to photochemical reactions caused by UV light. Oxidized iron, mainly in the form of hematite (α-Fe₂O₃) is present in BIF already in the oldest sedimentary sequence on Earth. Thus far the maximum determined age of sedimentary rocks is 3.8 Ga (Moorbath 1977). That age has been measured at Isua on Western Greenland. Obliviously oxidized iron minerals have been abundant through the whole known geologic history of the Earth. The binding and reactions of nucleotides and polynucleotides on iron oxide hydroxide polymorphs has been studied by Holm et al. and it was concluded, these might have adsorbed organics and catalyzed the condensation processes which led to the origin of life (Holm et al. 1993).

Synthetic ferrihydrite was found to act as amino acid adsorbent and promoter for peptide bond formation (Matrajit and Blanot 2004). The iron oxide hydroxide minerals, goethite and akaganeite were likely constituents of sediments present in, for instance, geothermal regions of the primitive Earth. Condensation of DL–glyceraldehydes to ketohexoses in the presence of iron (III) oxide hydroxide has also been reported (Weber 1992). Recently iron oxides (Goethite, Akaganeite and Hematite) were proved to catalyze the formation of nucleobases from formamide (Shanker et al. 2011).

Amines are one of the important classes of organic compounds which are widely distributed in nature in the form of amino acids, alkaloids and vitamins. The presence of amino acids containing aromatic rings on the primitive Earth has been proposed (Friedmann and Miller 1969). One could, therefore, reasonably postulate the presence of aromatic amines in the primitive Earth environment. During the last few years, experimental studies on the catalytic role of cyano complexes and their possible role in chemical evolution have been carried out. A number of metal hexacyanoferrates (II) have been prepared and their interaction with aromatic amines have been studied by Alam and Kamaluddin (1999 1999, 2000), Alam et al. (2000a, b, 2002) and Viladkar et al. (1994). It has been established by the above study that metal hexacyanoferrates (II) are highly efficient in adsorbing amines in neutral as well as alkaline media (pH ~ 9) to bring about oxidation of the amines.

In order to further investigate the importance of various forms of iron oxides during the course of chemical evolution in catalyzing different reactions, aromatic amines were interacted with goethite, akaganeite and hematite. Spectral studies have indicated that surface acidity of iron oxides is responsible for interaction.

Materials and Methods

Ferric nitrate (Merck), potassium hydroxide (Merck), ferric chloride (Merck), Aniline (Merck), p-Anisidine (Merck), p-chloroaniline (Merck), p-toludine (Merck) and all other chemicals used were of analytical reagent grade. Aniline was distilled and anisidine was recrystallized each time before use.

Synthesis and Characterization of the Iron Oxides (Goethite, Akaganeite and Hematite)

We prepared appreciable amounts of iron oxides (goethite, akaganeite and hematite) as described by (Shanker et al. 2011). It was characterized as a pure material by X-ray diffraction (XRD), FE-SEM (Field emission scanning electron microscopy) and analytical TEM (Transmission electron microscopy), by comparing the spectra and diffraction lines to the ones already published. Further, details concerning the characterization (XRD spectra, diffraction lines, diffraction rings, FE-SEM and TEM images) are same as reported earlier by Shanker et al. 2011).

Adsorption of Aromatic Amines on Iron Oxides

Evaluation of Parameters Necessary for Investigating the Sorption Equilibria

The conditions which were studied for investigating the adsorption equilibrium for the amine and metal oxides were:

1. i)

   Concentration of adsorbate

2. ii)

   Particle size of adsorbent

3. iii)

   Equilibrium time for adsorbate and adsorbent

4. iv)

   Quantity of adsorbent

5. v)

   Effect of pH and effect of temperature

For determining the adsorption isotherm of amines on iron oxides a moderate concentration range of amines (1 × 10⁻⁴ to 1.5 × 10⁻³ M) has been chosen in order to get absorbance of amines in the suitable range of the absorbance scale on the spectrophotometer.

Iron oxides of various particle sizes have been tested and it was found that particle size of 80–100 mesh was the most suitable one. Experiments were performed varying the time of contact (30 min to 24 h) and the amount of iron oxides at a fixed adsorbate concentration (5 × 10⁻⁴ M). It was observed that the maximum adsorption took place when the amount used was 100 mg per 10 ml of adsorbent solution for aniline, p-toluidine, p-anisidine and p-chloroaniline for 24 h. Adsorption of p-chloroaniline on a metal oxide was found to be a slow process and equilibrium was found to be established after 72 h.

Effect of pH

Adsorption of amine was studied over a wide pH range (4–9) on all iron oxides. Buffers used to maintain pH of adsorption studies were acetate (pH 4 to 7) and Borax (pH ~ 9). It was found that adsorption of aromatic amines on iron oxide was negligible in acidic and basic media. A neutral pH range (6.8–7.12) was found to be suitable for maximum adsorption.

Spectral Analysis

Electronic spectra of aniline, anisidine, p-toluidine and p-choloroaniline were recorded on a UV spectrophotometer (Model UV-188–240 V, Shimadzu Corporation Kyoto, Japan). The characteristic λ_max values for amines (aniline, p-choloroaniline, p-toluidine and p-anisidine are 280 nm, 290 nm, 286 nm and 295 nm, respectively. Infrared spectra of adsorbates, adsorbent, and adsorption adducts were recorded on a Perkin-Elmer 1600 FTIR spectrophotometer using KBr pellets. A series of 50 ml glass test tubes were employed and each tube was filled with 10 ml of amine solution of varying concentration. Goethite, akaganeite and hematite (100 mg) was added to each tube. The pH of the solution was adjusted to the desired value using acetate or borax buffers. Species of these buffers did not get adsorbed onto the iron oxide hydroxides surface. This was verified by conductivity measurements as there was no change in the inflection point of buffers with and without iron oxides. The tubes were shaken with a Spinix Vortex shaker initially for 1 h and then allowed to equilibrate at 25 °C with intermittent shaking at fixed time intervals. The equilibrium was attained within 6 h. The equilibrium time and concentration range were, however, decided after a good deal of preliminary investigations. The concentration of aniline, p-choloroaniline, p-toluidine and anisidine was measured spectrophotometrically at 280 nm, 290 nm, 286 nm and 295 nm, respectively. The amount of amine adsorbed was calculated from the difference between the amine concentration before and after adsorption. The equilibrium concentration of amine and the amount adsorbed were fitted in the adsorption isotherm (Figs. 1, 2, and 3).

Adsorption isotherm for adsorption of amines on goethite

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Adsorption isotherm for adsorption of amines on akaganeite

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Adsorption isotherm for adsorption of amines on Hematite

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Oxidation of Aromatic Amines in the Presence of Iron Oxides

In alkaline medium (pH > 8) brown-red colored products were deposited on the goethite and akaganeite surfaces within 24 h. These colored products were concentrated for Gas chromatography Mass spectrometry (GC–MS) analysis by extraction in benzene. Analysis of the reaction products was performed on a Perkin Elmer gas chromatograph coupled directly to a mass spectrometer system. Separation was performed on a fused silica capillary column (Elite-5 model) with composition 5 % diphenyl and 95 % dimethyl polysiloxane. The conditions for GC were as follows: injector temperature, 280 °C; transfer line temperature, 250 °C. The capillary column temperature was programmed as follows: 80 °C for 2 min; from 80 to 260 °C at 10 °C min⁻¹, held at 250 °C for 15 min. Helium was used as a carrier gas with a flow rate of 2 ml min⁻¹. The mass spectrometer conditions were ion source 250 °C and ionization energy 40 eV.

A wide pH range (3.0–7.0) was selected for preliminary adsorption studies of aromatic amines on iron oxides (goethite, akaganeite and hematite). A remarkable change in the amount of adsorbate was observed by varying the pH of the solution. Subsequent studies were performed at pH ~ 7.0 which was optimum for the maximum sorption of both the amines. Since amines are basic with lone pair of electrons on nitrogen and also assisted by π-electron clouds on aromatic rings, they easily interact with positively charged surface of iron oxide hydroxides. It was observed that at lower pH range aniline showed greater uptake as compared to anisidine. This can be explained on the basis of the fact that in acidic media anisidine has a greater tendency for salt formation and hence a lesser tendency to interact with iron oxides. On the other hand, aniline has a lesser tendency in comparison with anisidine for salt formation resulting in a greater tendency to interact with goethite and akaganeite. At a neutral pH, anisidine has more electrons available to interact with both the iron oxide hydroxides and thus there is a drastic increase in uptake. Adsorption isotherms of aniline, p-chloroaniline, p-toluidine and anisidine in the present case show that adsorption is fast in both the cases and the isotherms are regular, positive, and concave to the concentration axis. A typical graph of Ce/Xe vs Ce is a straight line (Where C_e Equilibrium concentration of solute in mole/l and X_e amount of solute adsorbed per gram weight of adsorbent (mg) (Figs. 4, 5, and 6).

Langmuir isotherms for adsorption of amines on goethite

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Langmuir isotherms for adsorption of amines on Akaganeite

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Langmuir isotherms for adsorption of amines on Hematite

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Adsorption data can be represented through a Langmuir adsorption isotherm which assumes the formation of a monolayer of solute molecules on the surface of the adsorbent and given by method reported by Viladkar et al. 1994.

The percent uptake of anisidine, p-chloroaniline, p-toluidine and aniline is represented by (Table 1). The values of X _m and K _L were also calculated (Table 2). X _m values also indicate that anisidine adsorption is more in comparison to that of other amines. It is observed from percent binding that anisidine is strongly adsorbed on iron oxides in comparison to aniline. The observed adsorption trend is also related to the basicities of the amines. This indicates that anisidine is a stronger base than other amines which reflects the greater availability of electrons for the interaction.

The infrared spectra of iron oxides before and after adsorption were recorded and analyzed. The results are given in Tables 1–3 (supplementary information). The change in characteristic frequencies of iron oxides after adsorption was not remarkable. The pronounced change in the characteristic frequencies of amino group indicates that the interaction occurred through the iron oxides with an amino group.

Field Emission Scanning Electron Microscopic studies of the amine and iron oxide was also performed. Figure 7 represents FE-SEM of anisidine, iron oxide and p-anisidine–iron oxide adduct. FE-SEM photographs of iron oxide-amine adduct shows that surface of iron oxide becomes smooth after adsorption. Energy dispersive X-ray (EDX) pattern also supports the adsorption as percentage of C, N, and O increases in the EDX pattern of adduct.

Representative FE-SEM photographs of goethite (a), p-anisidine (b) and goethite-p-anisidine adduct (c)

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Further it was also observed that in alkaline medium pH >8 all the amines reacted with goethite and akaganeite to give the colored products.

Reaction of Aniline with Goethite and Akaganeite

Mass spectra of the reaction products corresponding to peaks at retention time (R_t) 3.31, 7.29, 16.62 and 25.51 min are shown in Figures (1–3 in supplementary information). The formation of benzoquinone (R_t 7.29 min) is clearly evidenced by the peaks corresponding to m/z 108, 82, and 54, in accordance with its fragmentation pattern determined by electron bombardment. However, GC–MS study of the peak with R_t 16.62 min showed the formation of azobenzene confirmed by a peak at m/z 182. Some other mass peaks in the fragmentation pattern of the product at m/z 105, 77, and 51 were also observed. R_t 25.51 peak corresponds to tetramer of aniline, confirmed by their fragmentation pattern. A possible mechanism for the formation of azobenzene, benzoquinone, tetramer of aniline and azobenzene has proposed (Figs. 8, and 9)

Proposed mechanism for the formation of azobenzene

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Proposed mechanism for the formation of tetramer of aniline and benzoquinone

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Reaction of Anisidine with Goethite and Akaganeite

In the case of anisidine the chromatogram showed two major peaks with R_t 3.81 and 15.72 min corresponding to anisidine and its dimer, respectively. Mass analysis of the product with R _t 15.72 min showed major peaks at m/z 242, 135, and 107 which correspond to fragmentation of anisidine dimer (Fig. 10). A possible mechanism for the formation of dimer of anisidine may be proposed as shown in Fig. 11.

Mass spectrum of dimer of p-anisidine

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Proposed mechanism for the formation of dimer of p-anisidine

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Reaction of p-Chloroaniline with Goethite and Akaganeite

In the case of p-chloroaniline the chromatogram showed two major peaks with R _t 3.87 and 30.72 min. Peak with retention time (R_t) 3.87,corresponding to p-chloroaniline whereas the second peak at retention time (R_t) 30.72 min corresponds to its dimer. Mass analysis of the product with R_t 30.72 min showed major peaks at m/z 139, 111, 92 and 77 which correspond to fragmentation of p-chloroaniline dimer (Fig. 12).

Mass spectrum of dimer of p-chloroaniline

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Reaction of p-Toluidine with Goethite and Akaganeite

The oxidation products of p-toluidine gave well defined and well separated peaks in gas chromatogram with R_t 3.25 and 27.59 min. Mass spectra of the peak with R_t 27.59 min showed molar mass 317 (100 %), which corresponds to trimer of p-toluidine Fig. 13. Some high fragments observed were 300, 226, 211, 107, 91 and 77. Above fragment masses resembled with possible fragments of a trimer. Peak with R_t 3.25 min corresponds to the starting material, i.e. p-toluidine.

Mass spectrum of dimer of p-toludline

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Authors

1. Uma Shanker

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2. Gurinder Singh

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3. Kamaluddin

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