The coupling of tautomerization to hydration in the transition state on the pyrimidine photohydration reaction path

Abstract

The ground state reaction path for formation of the pyrimidine hydrates was calculated using a nudged elastic band (NEB) approach, combined with a calculation of the transition state, and implemented using a numerical basis set in the density functional theory (DFT) code DMol(3). The model systems used for study consist of 1-methyl pyrimidines with a H2O molecule as the reactant, and the corresponding C5-hydro-C6-hydroxypyrimidine as the product. The barrier to addition of water across the C5-C6 π-bond ranges from 43-48 kcal mol(-1) in the 1-methylpyrimidines (1-MP) studied. Similar but slightly smaller barriers of 34-45 kcal mol(-1) were found for the tautomers of the 1-MPs, i.e. the enols of uridine and thymine and imine of cytosine. Comparison of these calculations with previous computational and experimental work suggests that a hot ground state formed by the rapid internal conversion of pyrimidines has sufficient energy to permit crossover from the common form to the tautomeric form of the pyrimidine at the transition state. The hot ground state mechanism can account for the experimentally observed yield and thermal reversion of pyrimidine photohydrates, while simultaneously explaining the effect of photohydrates on the mutation rate.

Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

“…This ultrafast photochemistry yields the hot ground state of these molecules or hot valence isomers. These highly excited species are able to react with water molecules of the aqueous environment, yielding the respective photohydrates . This photohydration reaction is also well established for the pyrimidine bases uracil and thymine and is known to be a significant source of lesions in DNA ,.…”

“…These highly excited species are able to react with water molecules of the aqueous environment, yielding the respective photohydrates. [71] This photohydration reaction is also well established for the pyrimidine bases uracil and thymine and is known to be a significant source of lesions in DNA. [72,73] For all these six-membered heterocycles, the photohydrates are metastable and dehydrate to the original chromophore in dark reactions on long time scales.…”

“…This photoproduct was identified as the hydrate of pyridine. As is well known, the 1 ππ* excited states of the six‐membered heterocycles pyridine, pyrazine, pyrimidine and triazine are prone to ultrafast internal conversion to the electronic ground state via 1 ππ*‐S 0 conical intersections which are accessible by out‐of‐plane puckering of the aromatic ring . This ultrafast photochemistry yields the hot ground state of these molecules or hot valence isomers.…”

“…As is well known, the 1 pp* excited states of the six-membered heterocycles pyridine, pyrazine, pyrimidine and triazine are prone to ultrafast internal conversion to the electronic ground state via 1 pp*-S 0 conical intersections which are accessible by out-of-plane puckering of the aromatic ring. [69][70][71] This ultrafast photochemistry yields the hot ground state of these molecules or hot valence isomers. These highly excited species are able to react with water molecules of the aqueous environment, yielding the respective photohydrates.…”

Solar Energy Harvesting with Carbon Nitrides: Do We Understand the Mechanism?

“…This photoproduct was identified as the hydrate of pyridine. As is well known, the 1 ππ* excited states of the six-membered heterocycles pyridine, pyrazine, pyrimidine and triazine are prone to ultrafast internal conversion to the electronic ground state via 1 ππ*-S 0 conical intersections which are accessible by out-of-plane puckering of the aromatic ring [66][67][68]. This ultrafast photochemistry yields the hot ground state of these molecules or hot valence isomers.…”