Significance of Solvated Electrons (e_aq⁻) as Promoters of Life on Earth

Solvated Electrons (e_aq⁻)

The existence of e_aq⁻ was first established by studing the conversion of aqueous carbon dioxide into simple organic compounds under irradiation with γ-rays (1), as well as with UV light (1, 2). The absorption spectrum of e_aq⁻ in aqueous solution at room temperature with a maximum at 720 nm was determined by pulse radiolysis (3). The absorption of trapped electron (e_tp⁻) in ice-matrix (77°K) of alkaline aqueous solution was found to occur at 530 nm (4). The e_aq⁻ thus represents a thermalized electron (e_tt⁻; E≤10 eV; 1 eV=23 kcal/mole) embedded in a shell of positive dipoles of water molecules (or other polar liquids) within about 0.3×10⁻¹² s. The e_aq⁻ is a strongly-reducing entity, representing the basic form of an H-atom, whereas the last one is the acidic form of the reduction species (5).

(Eq1)

(Eq2)

(Eq3)

Generation of e_aq⁻ in Seawater

Natural radioisotopes contained in seawater, e.g. ⁴⁰K, and cosmic rays can generate e_aq⁻ in seawater. As reported previously, ⁴⁰K acts as principal radiation source of e_aq⁻ in seawater, having a half-life of 1.3×10⁹ years, where 30% has a maximum β–ray energy of 1.32 MeV (6). Taking the G value as 2.6 (where G is number of species formed or decomposed per 100 eV energy absorbed) at a mass of the oceans of 1.4×10²⁴ g and a production rate of 1.3×10² e_aq⁻/g/s, the global production from ⁴⁰K is 1.3×10² moles of e_aq⁻/s (6). The same author also considered e_aq⁻ production by cosmic rays absorbed by seawater at a rate of 1.5×10³ e_aq⁻/g/s, hence cosmic rays contribute about 4% to that of ⁴⁰K. The total permanent amount of e_aq⁻ in the oceans is estimated on the basis of pulse radiolysis experiments to be 8±2×10⁻⁵ mol (6). Additional e_aq⁻ production is achieved by solar energy (λ>300 nm) absorbed by inorganic ions in seawater, such as Fe²⁺ and HPO₄²⁻.

(Eq4)

(Eq5)

It was also found that natural minerals acting as semi-conductors, e.g. TiO₂ (rutil) as aqueous suspension can photolyse water, forming e_aq⁻, OH and H species (7-10). TiO₂ has a bandgap (EG≅3.1 eV), which corresponds to the absorption of sunlight of λ≅400 nm. The photoaction of TiO₂ is schematically illustrated in Figure 1. Thereby, the electrons of the illuminated part of a TiO₂ particle move to the shaded side and are ejected in water as e_aq⁻. The remaining positivly charged ‘holes’ on the illuminated TiO₂ surface react with water forming OH radicals. Hence, each TiO₂ particle produces reducing (e_aq⁻ /H) and oxidizing (OH) species under the influence of sunlight as well as ionizing radiation. It is conceivable that on the catalytic surface of semi-conductors in seawater primary chemical processes can be initiated, e.g. formation of simple organic compounds by CO₂ reduction (9, 10).

Generation of e_aq⁻ in the Atmosphere

Atmospheric water can be degradated under the influence of the vacuum-UV part of the solar energy (VUV light, λ <200 nm) Table I. In addition, electrical discharges and cosmic rays contribute to the generation of e_aq⁻ in the atmosphere. These processes can in general be expressed by the following reactions: (Eq6) (Eq7) It has been experimentally shown that liquid water is photolyzed by VUV light into e_aq⁻, H and OH species (11). The quantum yields of the primary products resulting from VUV photolysis are shown in Table I.

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Figure 1.

Formation of free radicals on surface of TiO₂ under the influence of sunlight as well as by ionizing radiation (7, 9).

The yield of water degradation products resulting from electrical discharges and cosmic rays in the atmosphere remains unknown.

Formation of Oxygen

It is conceivable that the primitive atmosphere contained different gases, amongst which oxygen was found. Oxygen might also be formed as a product of water photolysis according to the following reactions (Eq8, 9, 10, 11, 12 and 13) (5): (Eq8) (Eq9) (Eq10) (Eq11) (Eq12) (Eq13) Similar reactions can also proceed by water degradation at higher temperatures during formation of the earth incrust and from volcanic lava. Simultaneously with these processes, ozone (O₃) might also be generated.

(Eq14)

(Eq15)

Very probably, photochemical reactions initiated by VUV light, electrical discharges and cosmic rays have also contributed to the O₂ formation by splitting of water contained in the atmosphere.

Nitrogen Oxides and Acids

Nitrogen has a triple bond between the nitrogen atoms and is a very stable molecule (D_l=9.756 eV). However, by the action of VUV light, cosmic rays and electrical discharges in the atmosphere, the N₂ molecule can be split. The resulting N^• atoms can react with O₂ forming a variety of oxides, which by reaction with water result in the corresponding acids: (Eq16) (Eq17) (Eq18) (Eq19) (Eq20) (Eq21) According to similar reactions sulfuric, phosphoric and other acids can be also formed in the atmosphere. Such processes could proceed in seawater, initiated by cosmic rays and by radiation from natural radioisotopes.

It should be mentioned that during the incrustation period of the earth nitrogen was very likely bound as nitrides, which now days are still contained in granite. At the edge of volcanic lava, ammonium chloride is formed, which may have contributed to the initial formation of amino acids.

Ammonia

The formation of ammonia from high purity nitrogen and triply distilled water (pH ~7.5) under the influence of VUV light and γ-ray has been reported (13). As measured using pulse radiolysis, the rate constant of the reaction e_aq⁻/H with nitrogen is extremely low: (Eq22) These findings, however, emphasize the possible generation of amines, amino acids and simple proteins under primitive conditions. It is also conceivable that other kinds of energy, such as high temperatures, might have contributed in this respect (see also equation 16).

Organic Compounds

It is supposed that the primordial seawater and atmosphere contained CO₂, CO, SO₂/SO₃ etc., which by reactions with the primary products of water radiolysis and photolysis (e_aq⁻, H, OH) are converted into simple organic compounds. As already mentioned, CO₂ acts as a very efficient scavenger of e_aq⁻, which led to the discovery of the existence of e_aq⁻ (1). The reaction mechanisms of CO₂ and CO conversion into organic compounds has been studied in detail (1, 2, 9, 10, 14-16). Some basic reactions in this respect are presented in Table II.

These processes can also proceed in the atmosphere, as well as in seawater, by initiation of the above mentioned energy sources.

CO₂ and CO reduction can also occur on the surface of semi-conductor particles, e.g. TiO₂ (7, 9) as shown by the reactions given in Table III.

Amines, Amino acids, Simple proteins

Based on the above presented results, CO₂ and CO can be reduced under primitive conditions by e_aq⁻ to simple organic compounds, including, but not limited to, aldehydes, carboxylic acids and alcohols. In addition ammonia can be converted to amines and other ammonium compounds. All these substances derived from CO₂, CO and ammonia can lead to the corresponding free radicals and finally to the formation of amines and amino acids. It has been shown experimentally that amines in the presence of CO₂ or carbonate can be carboxylated to amino acids. Furthermore, carboxylic acids in ammonia-containing aqueous media likewise are converted to amino acids (17-21). It has also been found that several amino acids are formed by carboxylation of specific amines. This fact demonstrated that the attack of free radicals may occur at different positions of the amino molecule, resulting in a variety of free radicals and consequently in the corresponding amino acids. For example, the carboxylation of ethylamine results in the generation of seven amino acids depending on the respective radiation dose absorbed, of which α-aminobutyric acid gives the highest yield. In addition to this, norvaline and proline production were also determined (20). Experiments showed that the formation of proline starts with alkylation of alanine to norvaline, followed by cyclization of the norvaline radical to give proline, as shown in Figure 2.

Amino acids can also be formed by carboxylation of amines under the action of VUV light (21). Further experimental results demonstrated that radiation-induced carboxylation of ethylamine in aqueous solutions containing sodium sulfide or hydrogen sulfide leads to the formation of cystine, cysteine and cysteic acids in addition to α- and β-alanine and glycine (22).

It should be mentioned that the free radicals formed by prolonged radiation of primary-obtained amino acids lead to the formation of simple proteins (23). Further reactions of protein free radicals can result in the formation of large protein molecules, which tend to form associates (unstable complexes) and hence may act as precursors of cell components.

The Spark of Life

As established by experiments, organic substances with functional groups such as –OH, –O⁻, –COO⁻, –NH₂, –NHCH₃, can eject e_aq⁻ in aqueous solution when excited into the singlet state (24). The same effect was also observed for hormones (25-27), vitamins (28) etc. It should be mentioned that substances ejecting e_aq⁻ can also consume part of these (rate constant, k≥10⁹-10¹⁰ l/mol/s) as well as transfer these electrons to other substances, and thus act as electron mediators (25, 27). Thereby the emitted e_aq⁻ preserve the specific frequency of the ‘mother molecule’ during the transfer process to, for example, the receiving centres in the brain (26). Based on the specific frequency of e_aq⁻ the brain is able to determine the origin of the message and can act correspondingly. This process enables communication between biological systems.

Supposing that primitive organic substances become electronically-excited upon energy absorption and hence, likewise, are able to eject, consume and transfer e_aq⁻ to other primitive compounds, this would represent the first step in the generation of life.

On the other hand it is known that organic substances with large molecules, including hormones (27), tend to form associates (unstable complexes) at concentrations of ≥10⁻⁷ mol/l. Hence, it can be hypothesized that this tendency is also true for primitive organic compounds, leading to the formation of pseudo-bioassociates, which are involved in functioning equilibria.

As mentioned above, the synthesis of organic compounds, which are assumed to be the basis for generating the scaffold of a primitive cell, can be explained by laboratory experiments. However, the ‘spark of life’ for the appearance of living cells still remains Nature's secret. Very probably the cell scaffold could have been created by chance, originating from associated amino acids that formed proteins and simple enzymes as well as redox systems in which e_aq⁻ played a decisive role. It is then hypothesized that under these primitive conditions, a biosystem similar to nicotine amide-adenine-dinucleotide-hydride (NADH; coenzyme 1) can be formed. NADH is considered the most important co-enzyme, and drives the reduction and oxidation processes in the cell and is the most efficient antioxidant. The equilibria involving e_aq⁻ are shown by the following (Eq23): (Eq23) NAD, nicotinamide-adenine- dinucleotide and R, adenosine-diphosphor-ribose. In other words, based on their unique properties, e_aq⁻ can be considered as the species initiating and driving factors for the start of life in primitive cells.

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Figure 2.

Formation of proline by cyclization of norvaline radical, resulting from ethylamine and alanine.