Double Proton Transfer (DPT) is one of the fundamental reaction types that occur in biologically relevant systems. Important questions concern: is DPT sequential or concerted, do quantum effect influence reaction rates, and are there possibilities for reversible DPT. The latter one is of paritulcar interest to DPT in DNA since this process would expose DNA to mutations.
Using semiempirical SCC-DFTB calculations and molecular dynamics simulations the energetics and dynamics of double proton transfer in 2-Pyridone 2-Hydroxypyridine was investigated. The infrared spectrum calculated from the dipole-dipole autocorrelation function establishes that otherwise dark regions in the spectrum (between 2500 and 3000 cm-1) show spectral features if hydrogen motion is highly excited. This is in agreement with experiment that found signal in the relevant spectral region upon irradiation with laser light.Experiment has suggested that double proton transfer in the electronic ground state in DNA base pairs embedded in the helix is quite improbable. However, isolated base pairs may show DPT but experimental detection is difficult due to issues such as low or no fluorescence of the base pairs themselves. We also investigated the DPT reaction in an isolated and DNA-embedded GC pair using semiempirical DFT using both, static and dynamic approaches. While for the isolated base pair a barrier of 14 kcal/mol does not in principle rule out DPT in the electronic ground state 30 kcal/mol are required to drive the proton over the barrier for the DNA-embedded dimer.