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-the formation of hydrogen bonds between bases located opposite each other on the two strands. DNA replication and transcription involve the breaking and re-forming of these hydrogen bonds, but it is difficult to probe these processes directly. For example, conventional DNA spectroscopy(1-3) is dominated by solvent interactions, crystal modes and collective modes of the DNA backbone; gas-phase studies, in contrast, can in principle measure interactions between individual molecules in the absence of external effects, but require the vaporization of the interacting species without thermal degradation(4-9). Here we report the generation of gas-phase complexes comprising paired bases, and the spectroscopic characterization of the hydrogen bonding in isolated guanine-cytosine (G-C) and guanine-guanine (G-G) base pairs. We 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, and we see evidence for proton transfer in the G-C pair, an important step in radiation-induced DNA damage pathways(10). Interactions between different bases and between bases and water molecules can also be characterized by our approach, providing stringent tests for high-level ab initio computations that aim to elucidate the fundamental aspects of nucleotide interactions(11-13).