Operacionalización de variables

heterocycle) for all seven purines. The percent recovery of the irradiated nitrogen heterocycles was initially calculated in two ways based on (1) integrated peak area (Equation 1) and (2) the mole ratio using the calibration curves (Equation 2).

𝐴𝑟𝑒𝑎 % = 𝐴𝑟𝑒𝑎 (𝐼𝑟𝑟𝑎𝑑𝑖𝑎𝑡𝑒𝑑 𝑆𝑎𝑚𝑝𝑙𝑒)

𝐴𝑟𝑒𝑎 (𝑈𝑛𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑡𝑒𝑑 𝐶𝑜𝑛𝑡𝑟𝑜𝑙) × 100 Eq. 1

𝑀𝑜𝑙𝑒 𝑅𝑎𝑡𝑖𝑜 = 𝑀𝑜𝑙𝑒𝑠 (𝐼𝑟𝑟𝑎𝑑𝑖𝑎𝑡𝑒𝑑 𝑆𝑎𝑚𝑝𝑙𝑒)

𝑀𝑜𝑙𝑒𝑠 (𝑈𝑛𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑡𝑒𝑑 𝐶𝑜𝑛𝑡𝑟𝑜𝑙) Eq. 2

However, when these data were plotted against the increasing total radiation dose or dose rate, the trend was the same (see data plotted for purine in Figure 3.10 for an example of total dose data) and the choice was made to report data as percent recovery based on the integrated peak area for both studies.

The first study performed was a total dose study on the seven nitrogen heterocycles in order to evaluate their radiolytic stability with increasing total dose (up to 0.992 Mega- grey (MGy) at a constant dose rate (~350 kGy/hr). Figures 3.10-3.16 show the percent recovery vs. increasing total radiation dose for all of the irradiated nitrogen heterocycles (note that the data points at 0 Gy represent four non-irradiated control samples which were made at the same time and underwent the same preparation as all of the other samples). These plots show that these nitrogen heterocycles in the solid state display a resistance to decomposition from γ-radiation.

Figure 3.10. Total dose decay curve for purine. Decay curves are in terms of percent recovery based on peak area (top) and mole ratio (bottom). All further plots show percent recovery based on peak area. Purine undergoes 20 ± 3 % decomposition over the course of ~1 MGy of γ-radiation exposure. The majority of this decomposition occurs at higher doses (>800 kGy).

No degradation products were detected in UV chromatograms at 260 nm (or in the full spectrum scan from 200-400 nm). In the solid state, purines are remarkably stable with a maximum degradation of 53 % for guanine at radiation doses up to nearly 1 MGy of total

dose, as compared to the non-irradiated control (see Table 3.3). Virtually no degradation occurred in adenine and hypoxanthine. Xanthine remained nearly stable with the exception of the last data point showing a 29 % decrease, compared to the control. Xanthine’s radiolytic stability is consistent with other methylated xanthines (theobromine, theophylline, and caffeine), although the latter studies used an electron beam (9.96 MeV) and doses to only 400 kGy.60

Figure 3.11. Total dose decay curve for adenine. Adenine was determined to have no discernable decomposition over the course of ~1 MGy of γ-radiation exposure.

Figure 3.12. Total dose decay curve for hypoxanthine. Hypoxanthine is a deamination product of adenine. This replaces adenine’s amine group with hypoxanthine’s carbonyl group at the 6-position. Both adenine and hypoxanthine revealed no discernable decomposition over the course of ~1 MGy of γ-radiation exposure.

Figure 3.13. Total dose decay curve for guanine. Guanine gradually decomposes 53 ± 3 % of the initial amount over the course of ~1 MGy γ-radiation exposure. A large fraction of this loss occurs at high doses (>800 kGy).

Figure 3.14. Total dose decay curve for xanthine. Xanthine is the deamination product of guanine. Replacement of an amine with carbonyl at the 2-position appears to provide radiolytic stability. Xanthine decomposes by 29 ± 3 % over the course of nearly 1 MGy γ-radiation exposure; most of which occurs after 800 kGy of exposure.

Figure 3.15. Total dose decay curve for 2,6-diaminopurine. Addition of an amine group to the 2-position does not have an effect on the stability of purine nitrogen heterocycles. 2,6-diaminopurine is considered as equally stable to γ-radiation as adenine.

Figure 3.16. Total dose decay curve for 6,8-diaminopurine. Similar to 2,6- diaminopurine, addition of an amine group at the 8-position does not have an effect on radiolytic stability.

Table 3.3. Percent recovery of γ-irradiated nitrogen heterocycles in the total dose study. Dose rate was held constant at ~350 Gy/hr.

Figures 3.17 shows four purine nitrogen heterocycles with amine groups in different positions and the respective decay curves for these compounds. Purine (upper left) displays nearly a 20 % decrease over the course of 1 MGy exposure. In contrast, adenine, which has an added amine group at the 6-position (upper right), is stable over the total dose range with no discernable decomposition. This same trend is observed for 2,6-

Dose

(kGy) Purine Adenine Hypoxanthine Guanine Xanthine 2, 6-diaminopurine 6, 8-diaminopurine

142 97.7 102 99.6 108 98.6 103 98.3 283 93.1 102 96.7 91.1 94.5 95.4 101 425 93.5 102 98.2 88.8 94.5 98 99.6 567 93.4 95.7 98 86.7 99.2 97.9 94.3 709 95.7 97.1 95.8 83.6 99.4 99.1 97.1 850 94.2 95.5 100 76.1 96.7 98.6 89.3 992 80.1 102 103 46.8 69.5 102 99.1

Percent Content Dose Rate 350 Gy/Hr.

diaminopurine and 6,8-diaminopurine (lower left and lower right respectively), which both have an amine group in the same 6-position as adenine with additional amino groups on the purine ring. These purines with amine groups display what appears to be an increase in radiolytic stability and may be resistant to γ-irradiative decomposition, although further experimentation is needed in order to better define radiolytic behavior of these compounds.

Figure 3.17. Amine substitution effects on the γ-radiolysis of purine heterocycles. The position of amine substitution appears to have a stabilizing effect on purine nitrogen heterocycles. Any noticeable decomposition (purine upper-left) occurs at total doses higher than 800 kGy.

Figures 3.18 shows decay data for adenine and guanine with their respective deamination products (replacement of an amino group with a carbonyl group), hypoxanthine and xanthine. The decay curves are shown for each of these compounds. Adenine and hypoxanthine appear to be resistant to γ-irradiation, which suggests that purines substituted at the 6-position are relatively stable. Guanine displays a decomposition of nearly 53% with exposures up to 1 MGy. Therefore, it appears that the addition of an amino group at the 2-position in combination with carbonyl substitution at the 6-position

correlates with increased radiolytic decomposition. For xanthine, the replacement of the amine at the 2-position with another carbonyl group yields some degree of stability; however, a loss of 29% over the course of 1 MGy exposure still occurs.

Figure 3.18. De-amination effect on the γ-radiolysis of purine heterocycles. Adenine and hypoxanthine display the same radiolytic stability while xanthine appears to be stable up to doses as high as 800 kGy, compared to guanine.

The second study performed was a dose rate analysis on the canonical purine nucleobases. The percent of irradiated nucleobase recovered was compared to an increasing dose rate (up to 350 Gy/hr) while the total dose remained constant (~250 kGy) to investigate the nature of irradiation products as controlled by both dose and dose rate (see Table 3.4). No degradation products were detected in UV chromatograms at 260 nm (or in the full spectrum scan from 200-400 nm).

The plots in Figure 3.19 show the percent recovery (based on integrated peak area) vs. dose rate (Gy/hr). Adenine displays what could be the makings of a hyperbolic curve with an asymptote at nearly 100 % recovery. From a statistical standpoint, the lower recovery of adenine at a dose rate of ~87 Gy/h is calculated as an outlier at 95% confidence

using a Dixon’s Q-test; however, it cannot be completely ruled out that a lower dose rate does not have an effect on adenine. On the other hand, guanine is a different story. The effect of dose rate on the nature of irradiation products is readily apparent as the percent recovery of guanine depends on both dose rate and total dose. Further investigation is warranted since these are only two examples; however, the results suggest that both parameters should be examined with each particular compound for a full understanding of its radiation chemistry.

Table 3.4. Percent recovery of γ-irradiated canonical purines in the dose rate study. The total dose received was ~250 kGy. The boxed X indicates that this sample was not performed for this study.

Dose Rate Results for the Canonical Purines Dose Rate Percent Recovery

(Gy/hr) Adenine Guanine

304 82 207 97.5 71 154 98.6 79.8 123 97.9 83.9 103 97 85.2 87 90.7 98

Figure 3.19. Dose rate analysis of purine nucleobases. Adenine shows stability to dose rate. However, it is not 100 % certain the datum at 87 Gy/hr is a statistical outlier. Guanine displays a strong decomposition correlating to the experienced dose rate.

Conclusion

Three trends can be inferred from the data gathered in the analysis of the γ- irradiation of purine nitrogen heterocycles. First, purine nitrogen heterocycles in the solid state irradiated by a 60_{Co source display remarkable stability up to and including 1 MGy of} total γ-irradiation dose, which is the dose equivalent to that derived by radionuclide decay

in asteroids in ~350 million years (assuming a total radiation dose of 14 MGy as calculated by Urey).41, 42 _{Second, the substitution of the purine ring structure by either amine or} carbonyl groups may also have an effect on the decomposition of purine nitrogen heterocycles. Either the absence of substitution on the ring or the presence of both a carbonyl and an amine in the 2- and 6-positions may induce a state of radiolytic susceptibility. Finally, dose rate seems to have a variable effect, which might also depend on the compound being irradiated.

These results suggest that purine nucleobases and related nitrogen heterocycles are relatively stable to γ-radiation if they exist in the solid state (like in asteroids after their initial aqueous alteration period). Given this stability, one may postulate a likelihood for exogenous delivery to seed a primordial world.

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Table A.1. Compound names and structures for the products identified in the thermochemolysis TMAH-GC-MS analysis.

Compound Name Structure Non-derivatized Structure

cyanuric acid, trimethyl-

N,N'-dimethyloxamide N,N-dimethylformamide 1,3-dimethyl-2,4,5- trioxoimidazolidine benzene, 1,3-bis(1,1-dimethylethyl)- (a.k.a. m-di-tert-butylbenzene) aminoacetonitrile

Compound Name and Structrue of Products Identified by TMAH-GC-MS

N N N O O O HN N H NH OH OH HO H N O H NH2 O N H H N O O H2N NH2 O O N N O O O HN H N O O O N NH2

N6,N6',N9-trimethyladenine 1,3,7-trimethylxanthine (caffeine) 2,4-imidazolidinedione, 3,5,5- trimethyl 1-methyl-dihydrouracil urea, tetramethyl- trimethylurea N6,N9-dimethyladenine

Compound Name and Structrue of Products Identified by TMAH-GC-MS

N N N N N N N NH N NH2 N N N N O O HN N H N H N O O N H N O O N H H N O O N NH O O N H NH O O N N O H2N NH2 O H₂N NH₂ O N N H O N N N N HN N N NH N NH2

4,6-dimethyl-3,5-dioxo-2,3,4,5- tetrahydro-1,2,4-triazine

N,N-dimethylurea

4-dimethylaminopyridin-2-amine

6-azathymine, bis(methyl) ether ethylenediamine, N,N'-diethyl-N,N'- dimethyl-

6-azathymine, bis(methyl) ether

N-ethyl-hexahydro-1H-azepine