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Microhydration of Gas Phase Nucleobases


The gas phase dynamics of the canonical nucleobses are well understood at this point, being highly characterized in both gas phase and solution. Gas phase offers tuatomer specific information that can be easily modeled to theory but isn't as representative of the conditions on an early earth as solution phase. Microhydration allows for the benefits of gas phase while looking at the effects of solvation in a way that can also be modeled computationally. This is important to understand because dynamics and lifetimes can change dramatically due to solvation for alternative and canonical nucleobses. Solvation changes the potential energy surface, modifying the available relaxation pathways and thus changing the lifetimes. Adding a single water molecule to 2-AP can determine if it fluoresceces, dramatically increasing its excited state lifetime. It is also likely that solvent might introduce new decay mechanisms by interacting through intermoleclar proton transfer. 

We have previously studied clusters of guanine monomers and dimers with one and two water molecules with ns laser pulses without measuring excited state lifetimes. In that work, we identified structures and found that we did not observe all theoretically predicted structures. We will revisit these clusters with our new ps capabilities. Since we already know the REMPI spectra and the formation conditions for these clusters, these will serve as our first target for studying the role of hydrogen bonding by water in the excited state dynamics.

Guanine microhydration: One H2O molecule does not selectively stabilize keto tautomer

We obtained the IR-UV double resonance spectrum of guanine monohydrate in the region 3200 cm -1 to 3800 cm -1 along with the energies and frequencies of these structures calculated at the non-empirical correlated ab initio RI-MP2/cc-pVDZ level. We observed three different conformers of guanine monohydrate in the gas phase. The three structures include two enol hydrates, which differ in the position of the water molecule.

Guanine base pair microhydration: Selective structure stabilization

We recorded the vibronic spectra of the mass selected GG(H 2O) and GG(H 2O) 2 clusters using resonant two photon ionization (R2PI) . We used IR-UV double resonance spectroscopy to obtain IR spectra of the ground state in the region 3150 – 3850 cm -1. Shifts in these frequencies as a result of H-bonding allow us to assign cluster structures. The primary conclusion from these data is that a single H 2O suffices to stabilize one specific base pair structure, relative to one that in the absence of solvent is close in energy.

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