After discovering and documenting the gas phase spectroscopy of the individual bases in various tautomeric forms, we have investigated their H-bonded binary clusters with each other (both homo- and hetero-diners) and their clusters with one and two water molecules. We produce these clusters by laser desorption and jet-cooling. We measure their spectroscopy by resonant two-photon ionization (R2PI) and double resonance techniques. We have determined structures and obtained structure selective vibrational frequencies from IR-UV double resonance spectroscopy.
One of the most intriguing and unexpected findings of this work is that the excited state lifetime depends markedly on structure. The biologically most relevant bases and their tautomer and cluster structures appear to have the shortest lifetimes. This may be part of nature's defense mechanism against potentially harmful photochemistry. In the next few paragraphs we summarize some highlights of this work.
GC base pairs in the gas phase shed light on prebiotic chemistry
We have studied the structures of isolated base pairs by studying clusters of guanine and cytosine. There are at least 50 different ways to form hydrogen bonded clusters out of these two bases, only one of which – the Watson-Crick (WC) structure – predominates in DNA. Is this the most stable of these structures or are other biological factors required for this structure to stand out? As discussed above, we found this structure to exhibit significantly different photochemistry compared to non-Watson-Crick structures
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.
We have performed experiments on a series of guanosines at the free electron laser FELIX in the Netherlands  . This allowed us to perform IR-UV hole burning in the mid-IR range of 400-2000 cm -1, which complements the range of 2000-4000 cm -1 available in our own laboratory. The results confirm the presence of the enol form in the gas phase.
Funding: This material is based upon work supported by the National Science Foundation under Grant No. CHE-0615401.