Ultrafast Excited State Dynamics of Allopurinol, a Modified DNA Base

Abstract

The decay of electronically excited allopurinol riboside was studied through the fluorescence up-conversion technique and high level ab initio calculations. For the allopurinol system with a pyrazolic five-membered ring, we observed an ultrafast decay of the fluorescence signal in water (τ < 0.2 ps), similar to what has been observed for hypoxanthine and inosine (with an imidazolic five-membered ring). These results show that the S(1) dynamics in this type of heterocyclic systems are general and dominated by the distortion in the pyrimidinic six-membered ring with a negligible influence of the rest of the heterocycle. The measurements are consistent with the presence of a highly accessible conical intersection between the S(1) (π-π*) excited state and S(0), as calculated by MR-CIS/CASSCF computations. Our calculations show that the loss of planarity of the six-membered ring is responsible for direct access to the S(1)-S(0) degeneracy region without requiring distortions in the rest of the molecule.

Electronic and Structural Elements That Regulate the Excited-State Dynamics in Purine Nucleobase Derivatives

“…This idea is consistent with recent ultrafast measurements reported for hypoxanthine and inosine, 10 , 11 2-aminopurine, 81 and allopurinol. 80 It might also be consistent with recent transient absorption measurements for the methylxanthine derivatives, 12 although these compounds lack the C2=N3 and N1=C6 double bonds, which are available in the other purine derivatives and play an important role in their puckering deformation pathways.…”

“…As previously reported for adenine, guanine, and other purine derivatives, 3 , 8 , 9 , 39 , 75 − 80 excitation at 266 nm takes the ground-state population primarily to the lowest-energy ππ* state (S 2 ), whereas a small fraction also populates higher-energy singlet states directly, both in the purine free base and 9-methylpurine (Table 2 and Figure 6 ). Assignment of the transient absorption bands for the purine free base is further complicated by the presence of both N7H and N9H purine tautomers in solution, as summarized in the previous section and discussed in more detail in the SI .…”

Ab Initio Study on Ultrafast Excited-State Decay of Allopurinol Keto-N9H Tautomer from Gas Phase to Aqueous Solution

“…The multireference second-order Møller−Plesset (MR-MP2) approach has been used to calculate its vertical excitation energies of the optically bright ππ* and optically dark nπ* excited singlet states, and 1 (nπ*) is predicted to be 0.24 eV higher in energy than 1 (ππ*) in the Franck−Condon region. 18 Furthermore, considering that the 1 (nπ*) state is generally less accessible optically, it could be not expected in the following nonradiative decay process. The multireference configuration interaction with single excitations (MR-CIS) predicts an easily accessible conical intersection between the S 1 ( 1 ππ*) and S 0 electronic states, exhibiting an envelope-like structure characterized by a puckering of a ring CH group.…”

“…The multireference configuration interaction with single excitations (MR-CIS) predicts an easily accessible conical intersection between the S 1 ( 1 ππ*) and S 0 electronic states, exhibiting an envelope-like structure characterized by a puckering of a ring CH group. 18 Despite much effort, excited-state dynamics simulations in the gas phase, especially in aqueous solution, have never been carried out for allopurinol to the best of our knowledge. In the previous studies, the hybrid quantum-mechanical/molecularmechanical (QM/MM) method was successfully applied to gain insights into the radiationless deactivations of the nucleic acid bases such as adenine and guanine, in which the QM part was treated at a semiempirical orthogonalization model (OM2) level.…”

Theoretical studies on excited states of biorelated systems from gas phase to aqueous solution

“…Allopurinol is a modified nucleic acid base, which is structurally similar to hypoxanthine. Fluorescence upconversion experiment showed that allopurinol riboside exhibits ultrafast excited‐state relaxation in aqueous solution ( τ < 200 fs) . Our dynamics simulations indicated that allopurinol also has similar excited‐state behavior with its riboside derivative .…”