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Today’s genetic composition is the result of continual refinement processes on primordial heterocycles present in prebiotic Earth and at least partially regulated by ultraviolet radiation. Femtosecond transient absorption spectroscopy and state-of-the-art ab initio calculations are combined to unravel the electronic relaxation mechanism of pyrimidine―the common chromophore of the nucleobases. Excitation of pyrimidine at 268 nm populates the S1(nπ*) state directly. A fraction of the population intersystem crosses to the triplet manifold within 7.8 ps, partially decaying within 1.5 ns, while another fraction recovers the ground state in >3 ns. The pyrimidine chromophore is not responsible for the photostability of the nucleobases. Instead, C2 and C4 amino and/or carbonyl functionalization is essential for shaping the topography of pyrimidine’s potential energy surfaces, which present accessible conical intersections between the initially populated electronic excited state and the ground state. 

Sulfur-substituted DNA and RNA nucleobase derivatives (a.k.a., thiobases) are an important family of biomolecules. They are used as prodrugs and as chemotherapeutic agents in medical settings, and as photocrosslinker molecules in structural-biology applications. Remarkably, excitation of thiobases with ultraviolet to near-visible light results in the population of long-lived and reactive triplet states on a time scale of hundreds of femtoseconds and with near-unity yields. This efficient nonradiative decay pathway explains the vanishingly small fluorescence yields reported for the thiobases and the scarcity of fluorescence lifetimes in the literature. In this study, we report fluorescence lifetimes for twelve thiobase derivatives, both in aqueous solution at physiological pH and in acetonitrile. Excitation is performed at 267 and 362 nm, while fluorescence emission is detected at 380, 425, 450, 525, or 532 nm. All the investigated thiobases reveal fluorescence lifetimes that decay in a few hundreds of femtoseconds and with magnitudes that depend and are sensitive to the position and degree of sulfur-atom substitution and on the solvent environment. Interestingly, however, three thiopyrimidine derivatives (i.e., 2-thiocytidine, 2-thiouridine, and 4-thiothymidine) also exhibit a small amplitude fluorescence component of a few picoseconds in aqueous solution. Furthermore, the N-glycosylation of thiobases to form DNA or RNA nucleoside analogues is demonstrated as affecting their fluorescence lifetimes. In aqueous solution, the fluorescence decay signals exciting at 267 nm are equal or slower than those collected exciting at 362 nm. In acetonitrile, however, the fluorescence decay signals recorded upon 267 nm excitation are, in all cases, faster than those measured exciting at 362 nm. A comparison to the literature values show that, while both the DNA and RNA nucleobase and thiobase derivatives exhibit sub-picosecond fluorescence lifetimes, the 1ππ* excited-state population in the nucleobase monomers primarily decay back to the ground state, whereas it predominantly populates long-lived and reactive triplet states in thiobase monomers.