Internal conversion to the electronic ground state occurs via two distinct pathways for pyrimidine bases in aqueous solution

Hare et al. 10.1073/pnas.0608055104.

Supporting Information

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SI Figure 6
SI Discussion




SI Figure 6

Fig. 6. Normalized steady-state absorbance spectra of the pyrimidine bases (solid lines), nucleotides (dotted lines), and select derivatives (dot-dash lines). Gray, Cyt; pink, CMP; dark green, Thy; light green, dTMP; purple, Ura; blue, UMP; black, DMU; red, 1CHU.

SI Discussion

Alternative Assignments. Three alternative assignments were considered for the transient absorption signals at 340 nm and eliminated. First, we considered the possibility that different tautomers are responsible for the biexponential signals in Fig. 2. Tautomer populations depend sensitively on solvent polarity, yet similar amplitudes are found for the fast and slow decay components in bleach recovery signals for the intermediate dark state first seen in 1-cyclohexyluracil in a wide variety of solvents (1). Additionally, numerous experiments and calculations indicate that the canonical tautomers of Cyt, Thy, and Ura, and their nucleosides and nucleotides, are significantly lower in energy than the next lowest energy tautomer (2-5). Thus, non-canonical tautomers are present in only trace amounts, and they thus cannot account for the ~10-50% of photoexcited molecules that decay via the dark intermediate state. Additionally, this would require that all of the pyrimidine derivatives studied, but none of the purines had significant populations of rare tautomers.

Second, photoionization was ruled out as an explanation for the t₂ dynamics. In this scenario, the slow recovery of the 250 nm signal would reflect geminate recombination of the ejected electron with the parent ion to re-form the initial molecule. Further, the signal at 340 nm would have to be assigned to decay by the radical cation population since the hydrated electron shows negligible absorption here. While radical ions of DNA bases are known to absorb in this region, they also absorb at visible wavelengths (6), but the t₂ decay is not seen at wavelengths longer than 450 nm. Furthermore, the absence of a slow decay channel by the purine mononucleotides AMP and guanine 5'-monophosphate is the strongest evidence against photoionization as these species are much more readily ionized than pyrimidine bases (6).

Finally, we eliminated higher-lying triplet states from consideration due to the evidence that the intermediate state population decays directly to S₀. This would be inconsistent with a higher-lying triplet state, which should rapidly decay to the lowest energy triplet state rather than follow the spin-forbidden route to S₀. Furthermore, the long lifetimes of the lowest triplet states (7) are incompatible with the ps dynamics in Figs. 2 and 4 (t₂ in Table 1).

1. Hare PM, Crespo-Hernández CE, Kohler B (2006) J Phys Chem B, 110:18641-18650.

2. Shukla MK, Leszczynski J (2002) J Phys Chem A 106: 11338-11346.

3. Shukla MK, Leszczynski J (2002) J Phys Chem A 106:8642-8650.

4. Trygubenko SA, Bogdan TV, Rueda M, Orozco M, Luque FJ, Sponer J, Slavicek P, Hobza P (2002) Phys Chem Chem Phys 4:4192-4203.

5. Houben L, Schoone K, Smets J, Adamowicz L, Maes G (1997) J Mol Struct 410-411:397-401.

6. Steenken S (1989) Chem Rev 89:503-520.

7. Gut IG, Wood PD, Redmond RW (1996) J Am Chem Soc 118:2366-2373.

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