An experimental and numerical model of the behavior of cytosine in aqueous solution under gamma radiation. Relevance in prebiotic chemistry

Abstract

Cytosine is an essential chemical molecule in living systems, such as DNA and RNA, it is essential in astrobiology to study how it behaves under probable primitive conditions. We looked at how cytosine broke down in aqueous solutions exposed to high radiation levels to learn more about how stable it might have been on the early Earth. We conducted various types of analysis, such as ultraviolet–visible spectroscopy and high-pressure liquid chromatography. We also developed a computer model to describe the kinetic processes and learn more about the molecules involved in the system. This model fits the results of experiments and lets us study cytosine's stability when it is exposed to gamma radiation. It enables researchers to theorize processes that are hard to test in the laboratory and is essential for studying how stable cytosine behaves in high-radiation settings.

1. Introduction

Studying cytosine's behavior in diverse environments is important in various fields because it is a fundamental part of DNA and RNA, and therefore an important molecule in chemical evolution studies. The prebiotic synthesis of cytosine has been proposed through several pathways. The participation of a three-carbon fragment, such as cyanoacetylene or cyanoacetaldehyde, with other molecule with one-carbon, such as cyanate, is one of the most likely pathway proposed [[1], [2], [3], [4]]. These precursors can be formed by an electric discharges in a mixture of methane and nitrogen [3,4]. Also, these molecules are relatively abundant in interstellar media [[5], [6], [7]]. Related with the stability of cytosine in prebiotic environment, few studies have been conducted on how stable nucleic bases, especially cytosine, are in the conditions of the early Earth, at high temperatures, or in high-radiation fields.

Ionizing radiation is an energy source that promotes molecular changes by activating of chemical reactions [[8], [9], [10], [11], [12]]. These reactions are of interest in various areas of knowledge, such as dosimetry [[13], [14], [15], [16]], food chemistry [17], and particularly, in chemical evolution [18], for their efficiency in the synthesis and decomposition of organic compounds and, its ubiquity [12,19,20]. The presence of radionuclides since the formation of the solar system, with long half-lives such as ²³⁸U, ²³⁵U, ²³²Th, and ⁴⁰K [21], provides a continuous source of energy to the early Earth. For this reason, this source has been proposed to contribute to the reservoir of important compounds. This energy can continuously synthesize organic molecules in aqueous media or in solid phase [20]. In this likely scenery, the organic molecules were exposed to ionizing radiation, which caused reactions that enriched the chemical media [19,22]. In this type of scenario, cytosine could form and react to synthesize other molecules that are important for chemical evolution [23]. The reactions series of cytosine and other prebiotic molecules could have occurred in a variety of environments on primitive Earth [24], including alkaline [25] or acidic lakes [26], hot spring systems [27], shallow fresh-water [28,29], seawater [[30], [31], [32]], or glacial brines [33]. In this study, the synthesis of cytosine and its exposure to ionizing radiation in aqueous media can be analogous to an acidic lake, seawater, or a shallow fresh-water environments. Actually, the era of prebiotic chemistry and the origin of life is limited to a relatively short period of time around the 4.2 to 3.8 Gya [34,35].

The interaction of organic molecules with ionizing radiations produces free radicals that can react with other molecules before being converted to a chemically stable form, and they generate complex radiolysis mechanisms [36]. The reproductions of experimental results with numerical models can help validate the proposed reaction mechanism for some aqueous organic reactions under ionizing radiation.

Due to the large number of variables in the radiolysis of aqueous solution of an organic compound and the fact that some information (such as rate constants and diffusion constants) are unknown, few studies have focused on the kinetic resolution of systems for the behavior of aqueous organic-molecules important in chemical evolution studies. Mathematical modeling are one of the alternative methods to formulate hypotheses about organic molecules’ behavior under various physical and chemical conditions [[37], [38], [39], [40], [41]], which can be applicated to environments like the primitive Earth, comets, icy planets, and interstellar media [[42], [43], [44]]. Asteroids provide a potential environment for prebiotic chemistry, as a wide variety of organic molecules have been found in meteorites, particularly in carbonaceous chondrites [[45], [46], [47], [48]], and therefore this is a very important topic for chemical evolution and astrobiology approaches.

In the search for a simpler and manageable model to describe an organic compound's behavior in aqueous solution exposed to ionizing radiation, some authors have proposed resolving ordinary differential equations (ODEs) focused on mass balance [37,49,50] and artificial intelligence methods to approximate solutions to these equations [51,52].

The aim of this work is to develop a computer model that shows how cytosine, as a prebiotic compound, and its products behave in aqueous solution when exposed to high-energy radiation fields. We validated the model by experimenting with the irradiation of cytosine samples with a⁶⁰Co gamma source.

This study explores issues related to chemical evolution, radiation chemistry, chemical kinetics, differential equations, and numerical modeling. The research concludes with an interdisciplinary and multidisciplinary approach.

2. Materials and methods

2.1. Laboratory reagents

All experiments used deionized water and reagents of the highest purity available (Sigma®, Co., USA). Cytosine (C₄H₅N₃O) was prepared in an aqueous solution at 5x10⁻⁴ mol/L oxygen-free at a pH 2.

2.1.1. Irradiation of samples

The solutions were placed in glass tubes and irradiated at various doses (1.5–5 kGy) in a gamma radiation source (Gamma-beam 651-PT, located at the Instituto de Ciencias Nucleares, UNAM), at room temperature (293 K) and 585 mmHg of pressure (mean atmospheric pressure of Mexico City). The dose rate was 235 Gy/min, evaluated with a Fricke-copper sulfate dosimeter [53].

2.2. Analysis

2.2.1. UV–Vis spectrophotometry

The aqueous solutions were analyzed using UV–Vis spectrophotometry at a wavelength of 275 nm with a Varian 100® Scan Cary UV–Vis spectrophotometer and a 1-cm quartz cell. Fig. 1 shows the calibration curve of cytosine.

Fig. 1.

Calibration curve of UV-spectroscopy for cytosine at 275 nm.

2.2.2. High-performance liquid chromatography (HPLC)

A Varian 9010® (Walnut Creek, CA, USA) liquid chromatograph was used with a variable UV–Vis wavelength detector Varian UV 9050® (Walnut Creek, CA, USA). For analyses, reverse phase chromatography was used on an Supelcosil™ LC-18, 25 cm × 4.6 mm, 5 μm (Belefonte, PA.), the elution was performed with a mixture of methanol-water (80:20 v/v), with a flow rate 0.6 mL/min. The peak assignments were based on the co-injection of standards, the comparison of retention times at various pH, and the ratio of absorbance, which was recorded at 260 nm.

2.3. Numerical model

This model, which describes the kinetic chemistry reactions, is formed by a system of coupled non-linear ordinary differential equations. Each equation contains information about the mass balance of each chemical species that changes its molar concentration over time. Each differential equation comprises three terms: the source term, which contains the molecules that are formed; the sink term, containing the molecules that decay; and an external energy source [37]:

$$\frac{d{X}_i\left(t\right)}{dt}=f_i+\sum _{m=0}^N\sum _{n=0}^N{k}_{m,n}^{\left(i\right)}{X}_m\left(t\right)X_n\left(t\right)-\sum _{j=0}^N{k}_{i,j}^{\left(i\right)}{X}_i\left(t\right)X_j\left(t\right)\text{,}$$ (1)

where $X_i$ is the molar concentration of i-species at time t, $k_{i,j}^{\left(i\right)}$ is the rate constant for the j and k-species produced by the i-species, N is the total molar concentration in the system, and $f_i$ is an external source.

At room temperature, the external energy source term is determined by Ref. [40]:

$$f_i\left(I_d\right)=\frac{{6.2*10}^{11}}{3.6N_A}\frac{M_i}{M_{H_{2}O}}{G}_i\left[I_d*\left(6*{10}^3\right)\right]\text{,}$$ (2)

where N_A is the Avogadro number (6.022 x 10²³ molecules), M_i is the molecular mass of i-species, $M_{H_{2}O}$ is the water molecular mass (18.02 g/mol), G_i is the radiochemical yield of i-species when the system absorbs 100 eV (particular for each chemical species), and I_d is the dose intensity (Gray/min) [54].

We developed code in Python 3.7.9 to solve the coupled non-linear ODEs using the odeint library of the scipy_integrate package from Numpy [55]. Odeint uses the LSODA method that use the linear multistep method (Adams method) for non-stiff problems and the backward differentiation formula for stiff problems [56]. The initial conditions are given by the rate constants (k_i) and the molar concentration of the cytosine aqueous solution (5.0 x 10⁻⁴ mol/L). The other molecules in the model have an initial concentration of 0 mol/L, as an initial condition.

For model validation, we computed the Pearson correlation coefficient, R², and the root mean square error:

$$RMSE=\sqrt{\frac{{\sum }_{i=1}^N{\left(x_i-y_i\right)}^2}{N}}\text{,}$$ (3)

where $x_i$ is the observed value, $y_i$ is the estimated value, and N is the numbers of observations. Also, we computed the residuals, their sum, mean, standard deviation, and a Q-Q plot to evaluate their normality.

We conducted iterations of the model, varying the order of magnitude of each rate constant (k_i) in an interval from 1 to −1 (+0.1, +0.5, 1, −0.1, −0.5, −1), to evaluate the ODE system's sturdiness.

3. Results

Our experimental results show a progressive decomposition of cytosine in aqueous solution when it is exposed to gamma radiation (Fig. 2). Among the irradiation products, uracil was identified by HPLC. Other products appeared in minor proportion and have been reported previously [8].

Fig. 2.

Decrease in cytosine molar concentration in aqueous solution vs. radiation dose. The solid line is the numerical model, and the dots are the experimental results.

3.1. The model

The proposed reaction mechanism involves 14 chemical species for the decomposition of cytosine aqueous solution induced by gamma radiation. The mechanism was cut until the formation of small molecules with known rate constants.

Reaction	Rate constant (s−1)	Reference	Eq. #
$H_{2}O\stackrel{\gamma -radiation}{\to }OH•,e_{aq}^{-},H•,H_2$		[36]	(4.0)
	6.3*109	[57]	(4.1)
	3.1*109	[58]	(4.2)
	1.3*1010	[59]	(4.3)
	∼6.3*109	[58]	(4.4)
	∼4.7*108	[58]	(4.5)
	5.7*109	[59]	(4.6)
	3.0*109	[58]	(4.7)
	∼6.5*108	[8]	(4.8)
		[60]	(4.9)
		[60]	(4.10)

The products of Equation 4.1 have a probability of 87 % for 5-hydroxycytosine and 13 % for 6-hydroxycytosine [57]. The formation of dimers is considered in the reaction 4.8. The reaction between 5-hydroxycytosine and 6-hydroxycytosine produces several dimeric species, but the yield of their formation is low. For the purposes of this work, we only consider dimers as products of this reaction, and not consider particular cases.

The chemical reaction mechanism of Equations 4.0–4.10 is rewritten as a coupled nonlinear ordinary differential equations system (Equations (1), (2))). One equation per molecule:

$$\frac{d\left[H•\right]}{dt}=f_{H•}-k_5\left[uracil\right]\left[H•\right]$$ (5.1)

$$\frac{d\left[e_{aq}^{-}\right]}{dt}=f_{e_{aq}^{-}}-k_3\left[cytosine\right]\left[e_{aq}^{-}\right]$$ (5.2)

$$\frac{d\left[OH•\right]}{dt}=f_{OH•}-k_1\left[cytosine\right]\left[OH•\right]-k_2\left[5-hydroxycytosine\right]\left[OH•\right]-k_4\left[c{ytosine}^{-}\right][OH•]-k_6^{}\left[uracil\right][OH•]-k_7^{}[5-hydroxyuracil][OH•]$$ (5.3)

$$\frac{d\left[cytosine\right]}{dt}=-k_1\left[cytosine\right]\left[OH•\right]-k_3\left[cytosine\right]\left[e_{aq}^{-}\right]$$ (5.4)

$$\frac{d\left[5-hydroxycytosine\right]}{dt}=+k_1\left\{\left[cytosine\right]*0.83\right\}\left[OH•\right]+\left[cytosineglycol*0.5\right]-k_2\left[5-hydroxycytosine\right]\left[OH•\right]-k_8\left[5-hydroxycytosine\right]\left[6-hydroxycytosine\right]$$ (5.5)

$$\frac{d\left[6-hydroxycytosine\right]}{dt}=+k_1\left\{\left[cytosine\right]*0.17\right\}\left[OH•\right]-k_8\left[5-hydroxycytosine\right]\left[6-hydroxycytosine\right]$$ (5.6)

$$\frac{d\left[cytosineglycol\right]}{dt}=+k_2\left[5-hydroxycytosine\right]\left[OH•\right]-\left[cytosineglycol\right]$$ (5.7)

$$\frac{d\left[{cytosine}^{-}\right]}{dt}=+k_3\left[cytosine\right]\left[e_{aq}^{-}\right]-k_4\left[c{ytosine}^{-}\right][OH•]$$ (5.8)

$$\frac{d\left[uracil•\right]}{dt}={+k}_4\left[c{ytosine}^{-}\right]\left[OH•\right]-k_5\left[uracil\right]\left[H•\right]$$ (5.9)

$$\frac{d\left[uracil\right]}{dt}={+k}_5\left[uracil\right]\left[H•\right]-k_6\left[uracil\right]\left[OH•\right]$$ (5.10)

$$\frac{d\left[5-hydroxyuracil\right]}{dt}=+k_6\left[uracil\right]\left[OH•\right]+\left[cytosineglycol*0.5\right]-k_7[5-hydroxyuracil][OH•]$$ (5.11)

$$\frac{d\left[uracilglycol\right]}{dt}=+k_7[5-hydroxyuracil][OH•]$$ (5.12)

$$\frac{d\left[dimers\right]}{dt}=+k_8\left[5-hydroxycytosine\right]\left[6-hydroxycytosine\right]$$ (5.13)

The numerical model shows a behavior similar to that of the laboratory experiments (Fig. 2), following the same tendency to decrease their molar concentration when exposed to gamma radiation. Table 1 provides data regarding the molar concentration of laboratory experiments and numerical model at each radiation dose.

Table 1.

Mean of the repetitions of laboratory experiments, the computed solutions of numerical model for 5 radiation doses, and the numerical residuals.

Dose (kGy)	Mean laboratory experiments (mol/L)	Numerical model (mol/L)	Residuals (%)
0	5.0E-04	5.0E-04	0
1.5	2.3E-04	2.7E-04	−7.9
2	1.9E-04	2.1E-04	−3.5
3	1.4E-04	1.1E-04	5.7
4	2.5E-05	4.9E-05	−4.7
5	3.0E-05	1.4E-05	3.1

The root mean square error (Equation (3)) between experimental and model data is 2.6x10⁻⁵, equivalent to 5.28 %; the Pearson coefficient = 0.971, and R² = 0.943. The residuals’ sum = −3.7x10⁻⁵, mean = −7.3x10⁻⁶, and standard deviation = 2.8x10⁻⁵. The Q-Q plot shows an aleatory residuals distribution around the mean line, indicating a normal tendency distribution (Fig. 3).

Fig. 3.

Q-Q plot diagram for residual data. This shows only a general tendency of the residuals data.

Fig. 4 shows the cytosine decomposition and formation of uracil as a radiolysis product through time. The products of cytosine aqueous solution radiation increase progressively with the dose.

Fig. 4.

Uracil as a product of cytosine aqueous solution in the first steps of radiolysis from 0 to 5 kGy. The Y-axis is in log₁₀ scale.

The modifications of k₁ (6.3*10⁹ s⁻¹) and k₂ (3.1*10⁹ s⁻¹) resulted in the greatest alterations in the final cytosine concentration, as the magnitude order of these rate constants show (Fig. 5a and b). Cytosine's behavior does not significantly change when other rate constants vary, whereas other molecules may undergo changes. For uracil, for instance, the variations of k₆ (5.7*10⁹ s⁻¹) result in changes, increasing its final concentration more than 2.5 times when the magnitude order is decreased (k₆ = 5.7*10⁸ s⁻¹) and decreasing it by 90 % when the magnitude order is increased (k₆ = 5.7*10¹⁰ s⁻¹) (Fig. 5d). On the other hand, for cytosine, the variation of k₆ does not produce a significant change (Fig. 5c).

Fig. 5.

Numerical model with the change in order of magnitude for some rate constants: k1 (a), k2 (b), k6 (c) for cytosine and k6 (d) for uracil.

Another effect is notable when we compare the computed solutions for k₁ and k₂ variations: the cytosine concentration decreases more quickly when k₁ decreases and decreases less when k₂ decreases. This indicates that the system's general kinetic does not accelerate or decrease at the same rate if the reaction constant increases or decreases. Finally, this model is numerically stable with the changes of initial concentration of cytosine. The window of stability for this parameter is from 5*10⁻¹ mol/L to 5*10⁻⁷ mol/L.

The high-performance liquid chromatography analysis of the cytosine irradiated samples shows a peak that corresponds to uracil molecule, comparing with standards (Fig. 6). Under this condition, uracil has a retention time of 5.1 min and cytosine of 4 min. Other studies shown that uracil also is very reactive under irradiation [57].

Fig. 6.

High performance liquid chromatography analysis of an aqueous solution of cytosine irradiated at 3 kGy (a), standards of cytosine (b) and uracil (c).

4. Discussion

We analyzed cytosine's (pH 2) behavior with UV–Vis spectroscopy by scanning from 800 to 200 nm and found a maximum value at 275 nm. Cytosine aqueous solution under gamma-radiation fields decomposes progressively, almost totally at 5 kGy (Fig. 2). Gamma radiation induces the formation of free radicals in water [36], and those radicals react with the cytosine and its products, generating new molecules (Fig. 4). Since the products remained in the solution, they participate in the water radicals competition reaction. This explains the non-linear degradation of cytosine. This behavior is commonly observed during the irradiation of aqueous organic molecules under non-conventional high-radiation doses, such in the radiolysis of aqueous solution of glyceraldehyde [61,62], adenine [63], formic acid [64], or malonic acid [38].

We calculate the dose constant ($\hat{k}$) of cytosine to gain insights into cytosine degradation, based on the work of Criquet and Karpel Vel Leitner [65,66]:

$$\mathit{ln}\left(\frac{C}{C_0}\right)=-kD\Rightarrow \hat{k}=-\mathit{ln}\left(\frac{C}{C_0}\right)/D$$ (6)

Where C is the concentration, C₀ is the initial concentration, D is the applied dose (in Gy), and $\hat{k}$ is the dose constant (Gy⁻¹).

The dose constant for the aqueous solution of cytosine is $\hat{k}$ = 4.54*10⁻⁴ (Fig. 7). We calculate the dose constant (Equation (6)) for guanine at pH 2 using the data from a previous work by Paredes-Arriaga et al. (2021) [67], obtaining a $\hat{k}$ = 4.08*10⁻⁴. Other dose constants have been calculated for thymine and adenine (Table 2). The dose constant parameter expresses the reaction kinetics as a pseudo first order equation and it is descriptive only [66]. The dose constants of cytosine, guanine and adenine are close to each other, suggesting that the degradation of some nitrogenous bases is similar as a function of the applied dose. Compared to some carboxylic acids, such as pyruvic, succinic and α-ketoglutaric [68], the dose constants of guanine and cytosine are small, indicating that cytosine and guanine degrade more slowly than some carboxylic acids under ionizing radiation.

Fig. 7.

Cytosine aqueous solution at pH 2 degradation: ln(C/C₀) = f(dose), and dose constant $\hat{k}$ (Gy⁻¹).

Table 2.

Dose constants for DNA nitrogenous bases.

Molecule	$\hat{\mathit{k}}$ (Gy−1)	Data obtained by
Cytosine	4.54*10−4	This work
Guanine	4.08*10−4	[67]
Thymine	1.0*10−3	[69]
Adenine	2.0*10−4	[70,71]

The numerical model has a good fit with the experimental results and reproduces the general breakdown behavior of an aqueous cytosine solution under gamma radiation. The statistical analysis validates the fit of the model and laboratory experiments, showing an error rate of <6 % in every proof (RMSE, Pearson coefficient, and R²). The random distribution of points in the Q-Q plot for the residual data (Fig. 3) indicates no bias of the estimation.

This model takes the system's chemistry kinetic (Equations 4.0–4.10) and interprets it as coupled differential equations (Equations (5.1), (5.2), (5.3), (5.4), (5.5), (5.6), (5.7), (5.8), (5.9), (5.10), (5.11), (5.12), (5.13)). The model is a simplification of a multivariable problem. It also is a proposal about a probable series of chemical reactions, and the first steps of the free radical attacks on the cytosine and its products (Fig. 4).

This is an example of a simple and effective way to model complex systems with low/moderate computational power because it takes less than 1.2 s to compute solutions of the proposed ODEs system on a standard computer. It is highly efficient and represents an advance in this research line, since optimizes each necessary test. It is important to recall, that is a simplify model of the complex induced reactions in aqueous solution due that the model do not take into account the diffusion coefficients in a liquid environment.

Given the structure of mass-balance differential equations, they have three groups of parameters under the initial conditions: source term, initial molar concentration, and rate constants (k_i). Some rate constants are found in various data bases, and others are unknown; moreover, experimental measurement can be complex. The model used here has the advantage of a raw estimation of some unknown rate constants. We evaluated ODEs system robustness by varying the magnitude order of the rate constants (k_i). Each rate constant variation demonstrates the significance of an adequate computation because it can accelerate or decelerate the formation or decomposition of the molecules involved, thereby affecting model accuracy. Also, this shows the strong coupling of the ODEs system and the importance of the correct accuracy of reaction rate constants.

Uracil formation is a common product of cytosine radiolysis and thermolysis [23]. The uracil, cytosine and their decomposition products play a fundamental role in prebiotic chemistry, as a building block for DNA, RNA, and other molecules of biological importance. This suggests that the proposed reaction is likely to occur in a prebiotic environment on the early Earth. For example, a shallow lake, where Earth's surface radiation can play the role of energy source for the reactions chain.

Primitive Earth's chemical evolution processes depend on easily accessible, abundant, and effective energy sources to induce chemical reactions. With an annual production of 260,000 cal/cm² [72], ultraviolet radiation is the most abundant form of radiation. However, the primary component affecting an energy source's quality is not just its quantity but also how it is deposited and penetrated—a process that varies depending on whether the item is solid, liquid, or gaseous. UV radiation is an efficient source in the upper atmosphere, its penetration in solids or in aqueous media is limited to some meters depth and therefore its efficiency to induce reactions is limited. On the other hand, ionizing radiation like gamma radiation [12,19,20] is very penetrating, effective in producing chemical changes, and abundant in nature. This characteristic may have been crucial for promoting primordial reactions originating from extraterrestrial, predominantly manifesting as cosmic rays, and terrestrial sources. Primordial radionuclides were produced by nuclear processes in stars and then incorporated into solar system bodies, such as Earth, from the time of its formation. Estimates of the energy released during the disintegration of radioactive elements provide support to the proposal that this source could be involved. Radioactive effects on the Earth's surface are now negligible. Comparing the decay rates of known long-lived radioactive species to the Earth's mantle, estimations of the amount of energy that was available 3.8*10⁹ years ago as heat and ionizing radiation is about 8–10 times actual [18,19,73,74]. As an example, the contribution only from ⁴⁰K, that is widely disseminated throughout the planet and dissolved in the hydrosphere. It is also concentrated in acid-igneous minerals and sedimentary alumino-silicates. From the total K concentration, ⁴⁰K represents 0.0117 %. Although its isotopic concentration is relatively low, ⁴⁰K contributes substantially to the Earth's heat production. Its natural abundance on primitive Earth was roughly eight times greater than it is today [18,19]. The dose-rate for this element had a contribution of 0.132 Gy per year [74] (the unit for absorbed dose is called a gray (Gy) and it is energy deposition of 1 J/kg of material). The consistent distribution of potassium compounds in ancient oceans, lagoons, and tide pools may have been facilitated by the transformation of molecules in solutions, such as cytosine. In extraterrestrial bodies, such comets, it has been estimated the energy from ionizing radiation to have a deposit around 14 MGy [75].

The estimated absorbed dose rate of ⁴⁰K in the ocean water is 1.3*10⁻¹⁰ Gy/s, at 4.55 Gya ago ([20,73]). The dose rate decreases by

$$N_t=N_0{e}^{-\lambda t}$$ (7)

Where N_t is the final number of atoms at time t, N₀ is the initial number of atoms, and $\lambda$ is the decay constant (0.693/T_1/2). Solving Equation (7) at a time of 3.8 Gya ago, gives a dose of 2.703*10⁻³ Gy/y. A system would take ∼370,000 years to be irradiated with a total dose of 1000 Gy, and probably cytosine would be accessible for more complex processes. For experimental reasons, the dose rate used in the experiments is higher than those in the primitive Earth, and it is assumed that the qualitative results are independent of the dose rate [76].

From an astrobiological and chemical evolution perspective, this study contributes to the understanding of the behavior of cytosine under high radiation fields. The possibility of aqueous media being exposed to ionizing radiation is a scenario that could have occurred on early Earth, as well as on other rocky planets, moons, or asteroids. This implies that the synthesis and degradation of cytosine, and its products, can follow the reactions described in this paper, in a variety of scenarios. The agreement between the experimental results and the numerical model helps us to support the validity of this proposed reaction mechanism. The continuous synthesis of organic matter by ionizing radiation in the primitive Earth [19] represents a probable pathway for the formation of nucleobases that form the DNA and RNA and other important molecules for chemical evolution, implying the viability of such environments for the formation of the building blocks of life.

5. Conclusions

In the prebiotic chemistry and astrobiology context, numerical models can provide information about theoretical scenarios that are difficult to recreate in the laboratory. This study shows that the theoretical model and the experimental results from the high-radiation field irradiation of cytosine, as a prototype molecule of prebiotic importance, in an aqueous solution are consistent. It can reproduce the formation/degradation of products in a laboratory experiments. This result suggests that the numerical model is reliable for assessing some radiation-induced chemical processes. This basic, robust model can be used for many chemical reactions. Tracing each chemical species will help us theorize about the behavior of diverse molecules in prebiotic scenarios. The statistical approach provides support for the numerical model and a means of facilitating further research.

Data availability statement

Data included in article/supplementary material/referenced in article.

CRediT authorship contribution statement

A. Paredes-Arriaga: Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. A. Negrón-Mendoza: Writing – original draft, Supervision, Resources, Funding acquisition, Formal analysis, Conceptualization. D. Frias: Writing – review & editing, Supervision, Formal analysis, Conceptualization. A.L. Rivera: Writing – review & editing, Formal analysis, Conceptualization. S. Ramos-Bernal: Writing – review & editing, Resources, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.