Photo-induced Dissociation of the N1–H Bond in the Imino Tautomers of Isocytosine in Water Medium

Abstract

The imino tautomers of isocytosine were objects of investigation at the TD-DFT level of theory-TD BLYP/6-311++G(d,p). We studied the mechanisms of the H1-N detachment in these tautomers through excited-state reaction paths. It was proposed that these transformations occur through the 1 πσ* excited-state reaction paths of the imino tautomers. The mechanisms involve dissociations of the N1-H bonds in the tautomers and lead to crossings between the reaction paths of 1 πσ* and S0 electronic states. One can suppose that such processes would facilitate the tautomerizations of the imino tautomers if further mechanisms have been found.

Ultrafast excited state dynamics of isocytosine unveiled by femtosecond broadband time‐resolved spectroscopy combined with density functional theoretical study

“…The notable lack of transient signal due to other intermediate species indicates the insignificance of the possible nπ* state from either keto-N(3) H or keto-N(1)H in the deactivation dynamics. The results also show no participation of PIDA involving phototautomerization from keto-N(3)H to enol, 9,12,14,21 which is verified by the measurement observing that the sample displayed identical absorption spectra before and after the time-resolved experiments (Figure S1, see Supporting Information). Furthermore, the data reveal that the triplet state of keto-N(3)H, if involved, must play only a minimal part (~4%, Table S10, see Supporting Information), which is in opposite to the case reported in the earlier theoretical study for the deactivation in gas-phase condition.…”

“…This process occurs via a repulsive πσ* state, and the estimated energy barrier is ~0.8 eV at the CASSCF level. 9,12,14,21 Similar long-term UV radiation-induced phototautomerization has also been observed for isocytosine in solid argon matrix conditions. 11,21 Despite the invaluable information available regarding the tautomeric forms and the excited states of isocytosine, further investigation is necessary to address important unanswered questions.…”

“… 18–20 Furthermore, it has been reported that long‐term UV radiation of isocytosine in solvents such as acetonitrile (CH 3 CN) 12 and water 14 facilitates the tautomerization of the keto‐N(3)H form into the enol form through a mechanism called photo‐induced dissociation–association (PIDA). This process occurs via a repulsive πσ* state, and the estimated energy barrier is ~0.8 eV at the CASSCF level 9,12,14,21 . Similar long‐term UV radiation‐induced phototautomerization has also been observed for isocytosine in solid argon matrix conditions 11,21 …”

“…[3][4][5] Recent research has shown that isocytosine is one of the building blocks for Hachimoji DNAs and RNAs, a genetic system that may support life in the cosmos. 6 The molecule has attracted attention due to its various properties, and the photo-induced excited state dynamics of isocytosine have been investigated both theoretically [7][8][9][10] and experimentally. [11][12][13][14] Previous studies have demonstrated that isocytosine can exist in different tautomeric forms depending on the environmental conditions, as shown in Scheme 1.…”