Pairing of isolated nucleic-acid bases in the absence of the DNA backbone

The two intertwined strands of DNA are held together through base pairing-t he formation of hydrogen bonds between bases located opposite each other on the two strands. DNA replication and transcription involve the breaking an d re-forming of these hydrogen bonds, but it is difficult to probe these pr ocesses directly. For example, conventional DNA spectroscopy(1-3) is domina ted by solvent interactions, crystal modes and collective modes of the DNA backbone; gas-phase studies, in contrast, can in principle measure interact ions between individual molecules in the absence of external effects, but r equire the vaporization of the interacting species without thermal degradat ion(4-9). Here we report the generation of gas-phase complexes comprising p aired bases, and the spectroscopic characterization of the hydrogen bonding in isolated guanine-cytosine (G-C) and guanine-guanine (G-G) base pairs. W e rnd that the gas-phase G-C base pair adopts a single configuration, which may be Watson-Crick, whereas G-G exists in two different configurations, a nd we see evidence for proton transfer in the G-C pair, an important step i n radiation-induced DNA damage pathways(10). Interactions between different bases and between bases and water molecules can also be characterized by o ur approach, providing stringent tests for high-level ab initio computation s that aim to elucidate the fundamental aspects of nucleotide interactions( 11-13).