Photophysics of 5,6,7,8-tetrahydrobiopterin on a femtosecond time-scale

Pterins are naturally occurring compounds widespread in living organisms. 5,6,7,8-Tetrahydrobiopterin (H₄Bip) is a cofactor of several key enzymes, including NO-synthases and phenylalanine hydroxylase, whereas tetrahydrocyanopterin is a photoreceptor molecule in cyanobacteria. In this regard, tetrahydropterins (H₄pterins) photochemistry and photophysics have been attracting our attention. H₄pterins photodegrade in presence of molecular oxygen yielding dihydropterins (H₂pterins) and oxidized pterins. Meanwhile, the excited states dynamics of H₄pterins on a femto- and picosecond time-scale remains unclear. To shed light on this area, we perform time-resolved spectroscopy of H₄Bip using fluorescence up-conversion as well as transient absorption spectroscopy techniques along with TD-DFT non-adiabatic molecular dynamics. We show that the lowest H₄Bip exited state has a lifetime of ca. 200 fs. Using the BHandHLYP functional and multireference spin-flip (MRSF) method we demonstrate that starting from the S₄ state, H₄Bip passes to the S₁ state within 50 fs, and after 200 fs a conical intersection with the ground S₀ state is achieved. As a whole, the excited state behavior of H₄Bip is similar to DNA nucleobases, in particular guanine. These findings allow us to make some speculations about the biochemical role of H₄pterins photophysics.