Quantum mechanics/molecular mechanics studies on excited state decay pathways of 5-azacytosine in aqueous solution

Abstract

In this work, we have employed the QM(CASPT2//CASSCF)/MM approach to study the photophysical properties and relaxation mechanism of 5‑azacytosine (5-AC) in aqueous solution. Based on the relevant minimum-energy structures and...

Quantum mechanics/molecular mechanics studies on mechanistic photophysics of cytosine aza-analogues: 2,4-diamino-1,3,5-triazine and 2-amino-1,3,5-triazine in aqueous solution

“…This half-boat shaped structure is similar to the well-known S 1 /S 0 conical intersection of benzene 77 and has also been found in many similar compounds such as 5-azacytosine, 6-azauracil, 4-aminopyrimidine, 9 H -8-azaguanine, etc. 27,33,37–40,43,44 The relative energies of this S 1 /S 0 conical intersection are estimated to be 91.3/89.8 kcal mol −1 , which locates 5.1 kcal mol −1 above the S1-MIN (86.2 kcal mol −1 ) and far below the Franck–Condon region (109.7 kcal mol −1 ). Therefore, this accessible S 1 /S 0 conical intersection would be expected to be responsible for the ultrafast decay dynamics of 2,4-DT in aqueous solution.…”

“…This strategy for system setup was adopted in many previous similar works. 33,[50][51][52][53][54][55] In MM minimizations and MD simulations, 2,4-DT and 2-AT are treated with the modified CHARMM22 force field and water molecules are described by the TIP3P model, respectively. [56][57][58] All MM computations are executed using the DL_POLY module of the CHEMSHELL3.5 package.…”

“…The state-average complete active space self-consistent field (SA-CASSCF) method over five roots with equal weights is taken as the QM electronic structure method to optimize the minima, minimum energy crossing points (MECPs) in and between the lowest singlet and triplet states, i.e., S 1 , S 2 , T 1 , T 2 , T 3 , and S 0 states. Note that MECPs involving electronic states of the same spin multiplicity are denoted as conical intersections, and those involving singlet and triplet states are singlet-triplet crossings (referred to as crossing points hereinafter, see our recent work 33,[50][51][52][53][54] ). The active space used for 2-AT is composed of three lone pair (n) orbitals localized on the nitrogen atoms of the triazine ring, one p orbital localized on the NH 2 group, and three p/p* orbitals delocalized over the triazine ring (referred to as CAS(14e,10o)).…”

“…, S 1 , S 2 , T 1 , T 2 , T 3 , and S 0 states. Note that MECPs involving electronic states of the same spin multiplicity are denoted as conical intersections, and those involving singlet and triplet states are singlet–triplet crossings (referred to as crossing points hereinafter, see our recent work 33,50–54 ). The active space used for 2-AT is composed of three lone pair (n) orbitals localized on the nitrogen atoms of the triazine ring, one π orbital localized on the NH 2 group, and three π/π* orbitals delocalized over the triazine ring (referred to as CAS(14e,10o)).…”

“…In the past few years, the excited-state properties and photophysics of aza-nucleobases have been studied both experimentally and theoretically. [21][22][23][24][25][26][27][28][29][30][31][32][33] In 2008, Kobayashi et al first detected particular excited-state dynamics of 6-azauracil and then systematically explored photophysics of 5-azacytosine, 8-azaadenine, and 8-azaguanine in solution by using a series of spectroscopic technology and time-dependent density functional theory (TD-DFT). 20,34,35 They found that the dark 1 np* state below the initially populated bright 1 pp* state is a bridge state in the intersystem crossing (ISC) process and those azanucleobases can be divided into two groups.…”