A concerted effort : proton transfer in a chemical reaction

Chemical reactions are processes in which one substance is transformed into another and involve the motion of atoms and electrons. Because these processes occur on short time-scales that are measured in femtoseconds (millionths of a billionth of a second), it is difficult to study what actually happens during a chemical reaction.
Of particular interest are reactions that involve the transfer of a hydrogen nucleus (a proton) between two molecules—an important process in biological systems. Tahei Tahara from RIKEN’s Discovery Research Institute in Wako has been studying proton transfer reactions for many years and views them as a challenge at the limits of science. “Because hydrogen is the lightest atomic species, it usually moves very quickly and is difficult to catch,” comments Tahara.
A model system in which proton transfer has been extensively studied is 7-azaindole. In solution, this compound exists in two different forms; discrete individual molecules (monomers), and pairs known as dimers. The dimers can be pushed into a higher energy ‘excited’ state by shining ultraviolet light on them, and subsequently undergo a double proton transfer reaction to form a structure known as a tautomer.
When Tahara published his first results on this system ten years ago, he says that, “the work triggered very intense world-wide debate.” The controversy stemmed from whether the two proton-transfer steps occurred sequentially in a step-wise reaction, or simultaneously in a ‘concerted’ process. Tahara has always argued that the concerted process is the correct one, a hypothesis that is further supported by his recent findings published in the Proceedings of the National Academy of Sciences of the USA (1).
By exciting the 7-azaindole dimer with different wavelengths of ultraviolet light and monitoring the fluorescence, Tahara and colleague, Satoshi Takeuchi, show conclusively that no intermediate structure is formed, thereby ruling out the possibility of a step-wise process. Significantly, their experiments demonstrate that a feature of the fluorescence decay that was attributed to a separate proton transfer actually corresponds to the conversion of the dimer from one excited state to another.
Because the 7-azaindole dimer is very similar in structure to the base pairs found in DNA, Tahara expects that this work may help to understand the chemical mechanism of how ultraviolet light affects DNA. In addition, Tahara and co-workers are now intending to observe nuclear motion in real-time using sub-10-femtosecond pulses of light, which he suggests, “may offer new opportunities for using light to control chemical reactions.”