UC Santa Barbara

The photochemistry of nucleobases upon UV exposure has been studied explicitly. Adenine, Guanine, Cytosine, and Thymine are the canonical nucleobases chosen as molecular building blocks by nature presumably under chemical selection pressure that preceded biology. Canonical nucleobases tend to have short excited-state lifetimes by removing the excited state population back to the electronic ground state via internal conversion (IC) in picoseconds or less. This fast and safe decay pathway is one of the intrinsic properties of the canonical bases and protects them against photodamage. This property is preserved since prebiotic times which makes these bases molecular fossils in modern days. The canonical bases and their derivates have been studied in the spectroscopy community, including the de Vries Lab. While canonical nucleobases are usually UV protected by having short excited-state lifetimes, a lot of derivatives and alternative compounds tend to have longer lifetimes which makes them susceptible to possibly destructive photochemical processes. Gas phase studies in the de Vries lab examine intrinsic properties of these molecules devoid of the molecular environment with isomer selectivity and supported by high level theoretical computations. Laser Desorption Jet-Cooling Mass Spectroscopy is used. Laser desorption prevents fragmentation or thermal degradation of molecules, and the desorbed neutral molecules are entrained in a jet cooled molecular beam through supersonic expansion. This supersonic expansion lowers the internal temperature resulting in vibrational and rotational cooling. Jet cooled molecules are excited and ionized downstream, and the ions are detected with a time-of-flight mass spectrometer. Resonance enhanced multi-photon ionization (REMPI) is used. Double resonance or hole burning spectroscopy employs a burn laser and a probe laser. The burn laser, either UV or IR, is fired prior to the probe laser and is scanned while the probe laser is parked at a single resonant wavelength. Whenever the burn laser is resonant with a transition, it depletes the ground state population, and a decrease in ion signal is observed. When the burn laser is in UV range, this hole burning technique provides REMPI spectra of individual tuatomers present in the experimental set up. When the burn laser is in IR range, IR spectra of selected tautomers can be obtained and used for tautomer identification. Once the tautomers are identified, peaks from the REMPI spectrum can be selected for tautomer specific pump-probe spectroscopy that provides excited lifetime data. Nir et al. (1999) presented the first vibronic spectroscopy of guanine in the gas phase, and they were able to assign a 0-0 origin transition at 32, 878 cm-11 The excited lifetime appeared to be exceeding 10 μs. Nir et al. (2001) presented an analysis based on double resonance spectroscopy, showing that the R2PI spectra included overlapping spectra of three different tautomers in Guanine.2 IR-UV and UV-UV double resonance spectroscopy led to an initial assignment of three tautomers, as 9H-enol, and 9H- and 7H- keto tautomers respectively.2 Brister et al. (2016) investigated promising nucleobase alternatives in solution, barbituric acid and 2,4,6-triaminopyrimidine.3 They found both of the compounds to possess efficient electronic relaxation mechanisms for eliminating excess amounts of absorbed energy into their aqueous molecular environment as heat within hundreds of femtoseconds.3 On the other hand, Gengeliczki et al (2010) studied the excited state dynamics of pyrimidine derivatives such as 2,4 diaminopyrimidine and 2,6-diaminopurine, and they were able to observe the most stable tautomers of each compound respectively.4 2,4 diaminopyrimidine had only the diamino tautomer with its excited state lifetime bracketed between experimental limits of 10 ps and 1ns.4 2,6- aminopurine presented in two tautomeric forms, 9H- and 7H diamino forms with lifetimes of 6.36.3±0.4 ns and 8.7±0.8 ns respectively.4 Lastly a pair of alternative nucleobases, isocytosine and isoguanine were studied.5,6 They illustrate possible photochemical reasons why cytosine and guanine emerged as the building blocks of life rather than the alternatives, isocytosine and guanine.5,6 Isocytosine presented similar dynamics to guanine while isoguanine presented similarities to cytosine.5,6 These findings suggested that the lower photostability of the biologically relevant keto form of isoguanine can be a reason why isocytosine and isoguanine were not chosen by the nature under prebiotic conditions.5,6